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rfc:rfc4120

Network Working Group C. Neuman Request for Comments: 4120 USC-ISI Obsoletes: 1510 T. Yu Category: Standards Track S. Hartman

                                                            K. Raeburn
                                                                   MIT
                                                             July 2005
          The Kerberos Network Authentication Service (V5)

Status of This Memo

 This document specifies an Internet standards track protocol for the
 Internet community, and requests discussion and suggestions for
 improvements.  Please refer to the current edition of the Internet
 Official Protocol Standards" (STD 1) for the standardization state
 and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

 Copyright (C) The Internet Society (2005).

Abstract

 This document provides an overview and specification of Version 5 of
 the Kerberos protocol, and it obsoletes RFC 1510 to clarify aspects
 of the protocol and its intended use that require more detailed or
 clearer explanation than was provided in RFC 1510.  This document is
 intended to provide a detailed description of the protocol, suitable
 for implementation, together with descriptions of the appropriate use
 of protocol messages and fields within those messages.

Neuman, et al. Standards Track [Page 1] RFC 4120 Kerberos V5 July 2005

Table of Contents

 1. Introduction ....................................................5
    1.1. The Kerberos Protocol ......................................6
    1.2. Cross-Realm Operation ......................................8
    1.3. Choosing a Principal with Which to Communicate .............9
    1.4. Authorization .............................................10
    1.5. Extending Kerberos without Breaking Interoperability ......11
         1.5.1. Compatibility with RFC 1510 ........................11
         1.5.2. Sending Extensible Messages ........................12
    1.6. Environmental Assumptions .................................12
    1.7. Glossary of Terms .........................................13
 2. Ticket Flag Uses and Requests ..................................16
    2.1. Initial, Pre-authenticated, and
         Hardware-Authenticated Tickets ............................17
    2.2. Invalid Tickets ...........................................17
    2.3. Renewable Tickets .........................................17
    2.4. Postdated Tickets .........................................18
    2.5. Proxiable and Proxy Tickets ...............................19
    2.6. Forwardable Tickets .......................................19
    2.7. Transited Policy Checking .................................20
    2.8. OK as Delegate ............................................21
    2.9. Other KDC Options .........................................21
         2.9.1. Renewable-OK .......................................21
         2.9.2. ENC-TKT-IN-SKEY ....................................22
         2.9.3. Passwordless Hardware Authentication ...............22
 3. Message Exchanges ..............................................22
    3.1. The Authentication Service Exchange .......................22
         3.1.1. Generation of KRB_AS_REQ Message ...................24
         3.1.2. Receipt of KRB_AS_REQ Message ......................24
         3.1.3. Generation of KRB_AS_REP Message ...................24
         3.1.4. Generation of KRB_ERROR Message ....................27
         3.1.5. Receipt of KRB_AS_REP Message ......................27
         3.1.6. Receipt of KRB_ERROR Message .......................28
    3.2. The Client/Server Authentication Exchange .................29
         3.2.1. The KRB_AP_REQ Message .............................29
         3.2.2. Generation of a KRB_AP_REQ Message .................29
         3.2.3. Receipt of KRB_AP_REQ Message ......................30
         3.2.4. Generation of a KRB_AP_REP Message .................33
         3.2.5. Receipt of KRB_AP_REP Message ......................33
         3.2.6. Using the Encryption Key ...........................33
    3.3. The Ticket-Granting Service (TGS) Exchange ................34
         3.3.1. Generation of KRB_TGS_REQ Message ..................35
         3.3.2. Receipt of KRB_TGS_REQ Message .....................37
         3.3.3. Generation of KRB_TGS_REP Message ..................38
         3.3.4. Receipt of KRB_TGS_REP Message .....................42

Neuman, et al. Standards Track [Page 2] RFC 4120 Kerberos V5 July 2005

    3.4. The KRB_SAFE Exchange .....................................42
         3.4.1. Generation of a KRB_SAFE Message ...................42
         3.4.2. Receipt of KRB_SAFE Message ........................43
    3.5. The KRB_PRIV Exchange .....................................44
         3.5.1. Generation of a KRB_PRIV Message ...................44
         3.5.2. Receipt of KRB_PRIV Message ........................44
    3.6. The KRB_CRED Exchange .....................................45
         3.6.1. Generation of a KRB_CRED Message ...................45
         3.6.2. Receipt of KRB_CRED Message ........................46
    3.7. User-to-User Authentication Exchanges .....................47
 4. Encryption and Checksum Specifications .........................48
 5. Message Specifications .........................................50
    5.1. Specific Compatibility Notes on ASN.1 .....................51
         5.1.1. ASN.1 Distinguished Encoding Rules .................51
         5.1.2. Optional Integer Fields ............................52
         5.1.3. Empty SEQUENCE OF Types ............................52
         5.1.4. Unrecognized Tag Numbers ...........................52
         5.1.5. Tag Numbers Greater Than 30 ........................53
    5.2. Basic Kerberos Types ......................................53
         5.2.1. KerberosString .....................................53
         5.2.2. Realm and PrincipalName ............................55
         5.2.3. KerberosTime .......................................55
         5.2.4. Constrained Integer Types ..........................55
         5.2.5. HostAddress and HostAddresses ......................56
         5.2.6. AuthorizationData ..................................57
         5.2.7. PA-DATA ............................................60
         5.2.8. KerberosFlags ......................................64
         5.2.9. Cryptosystem-Related Types .........................65
    5.3. Tickets ...................................................66
    5.4. Specifications for the AS and TGS Exchanges ...............73
         5.4.1. KRB_KDC_REQ Definition .............................73
         5.4.2. KRB_KDC_REP Definition .............................81
    5.5. Client/Server (CS) Message Specifications .................84
         5.5.1. KRB_AP_REQ Definition ..............................84
         5.5.2. KRB_AP_REP Definition ..............................88
         5.5.3. Error Message Reply ................................89
    5.6. KRB_SAFE Message Specification ............................89
         5.6.1. KRB_SAFE definition ................................89
    5.7. KRB_PRIV Message Specification ............................91
         5.7.1. KRB_PRIV Definition ................................91
    5.8. KRB_CRED Message Specification ............................92
         5.8.1. KRB_CRED Definition ................................92
    5.9. Error Message Specification ...............................94
         5.9.1. KRB_ERROR Definition ...............................94
    5.10. Application Tag Numbers ..................................96

Neuman, et al. Standards Track [Page 3] RFC 4120 Kerberos V5 July 2005

 6. Naming Constraints .............................................97
    6.1. Realm Names ...............................................97
    6.2. Principal Names .......................................... 99
         6.2.1. Name of Server Principals .........................100
 7. Constants and Other Defined Values ............................101
    7.1. Host Address Types .......................................101
    7.2. KDC Messaging: IP Transports .............................102
         7.2.1. UDP/IP transport ..................................102
         7.2.2. TCP/IP Transport ..................................103
         7.2.3. KDC Discovery on IP Networks ......................104
    7.3. Name of the TGS ..........................................105
    7.4. OID Arc for KerberosV5 ...................................106
    7.5. Protocol Constants and Associated Values .................106
         7.5.1. Key Usage Numbers .................................106
         7.5.2. PreAuthentication Data Types ......................108
         7.5.3. Address Types .....................................109
         7.5.4. Authorization Data Types ..........................109
         7.5.5. Transited Encoding Types ..........................109
         7.5.6. Protocol Version Number ...........................109
         7.5.7. Kerberos Message Types ............................110
         7.5.8. Name Types ........................................110
         7.5.9. Error Codes .......................................110
 8. Interoperability Requirements .................................113
    8.1. Specification 2 ..........................................113
    8.2. Recommended KDC Values ...................................116
 9. IANA Considerations ...........................................116
 10. Security Considerations ......................................117
 11. Acknowledgements .............................................121
 A. ASN.1 Module ..................................................123
 B. Changes since RFC 1510 ........................................131
 Normative References .............................................134
 Informative References ...........................................135

Neuman, et al. Standards Track [Page 4] RFC 4120 Kerberos V5 July 2005

1. Introduction

 This document describes the concepts and model upon which the
 Kerberos network authentication system is based.  It also specifies
 Version 5 of the Kerberos protocol.  The motivations, goals,
 assumptions, and rationale behind most design decisions are treated
 cursorily; they are more fully described in a paper available in IEEE
 communications [NT94] and earlier in the Kerberos portion of the
 Athena Technical Plan [MNSS87].
 This document is not intended to describe Kerberos to the end user,
 system administrator, or application developer.  Higher-level papers
 describing Version 5 of the Kerberos system [NT94] and documenting
 version 4 [SNS88] are available elsewhere.
 The Kerberos model is based in part on Needham and Schroeder's
 trusted third-party authentication protocol [NS78] and on
 modifications suggested by Denning and Sacco [DS81].  The original
 design and implementation of Kerberos Versions 1 through 4 was the
 work of two former Project Athena staff members, Steve Miller of
 Digital Equipment Corporation and Clifford Neuman (now at the
 Information Sciences Institute of the University of Southern
 California), along with Jerome Saltzer, Technical Director of Project
 Athena, and Jeffrey Schiller, MIT Campus Network Manager.  Many other
 members of Project Athena have also contributed to the work on
 Kerberos.
 Version 5 of the Kerberos protocol (described in this document) has
 evolved because of new requirements and desires for features not
 available in Version 4.  The design of Version 5 was led by Clifford
 Neuman and John Kohl with much input from the community.  The
 development of the MIT reference implementation was led at MIT by
 John Kohl and Theodore Ts'o, with help and contributed code from many
 others.  Since RFC 1510 was issued, many individuals have proposed
 extensions and revisions to the protocol.  This document reflects
 some of these proposals.  Where such changes involved significant
 effort, the document cites the contribution of the proposer.
 Reference implementations of both Version 4 and Version 5 of Kerberos
 are publicly available, and commercial implementations have been
 developed and are widely used.  Details on the differences between
 Versions 4 and 5 can be found in [KNT94].
 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
 document are to be interpreted as described in [RFC2119].

Neuman, et al. Standards Track [Page 5] RFC 4120 Kerberos V5 July 2005

1.1. The Kerberos Protocol

 Kerberos provides a means of verifying the identities of principals,
 (e.g., a workstation user or a network server) on an open
 (unprotected) network.  This is accomplished without relying on
 assertions by the host operating system, without basing trust on host
 addresses, without requiring physical security of all the hosts on
 the network, and under the assumption that packets traveling along
 the network can be read, modified, and inserted at will.  Kerberos
 performs authentication under these conditions as a trusted third-
 party authentication service by using conventional (shared secret
 key) cryptography.  Extensions to Kerberos (outside the scope of this
 document) can provide for the use of public key cryptography during
 certain phases of the authentication protocol.  Such extensions
 support Kerberos authentication for users registered with public key
 certification authorities and provide certain benefits of public key
 cryptography in situations where they are needed.
 The basic Kerberos authentication process proceeds as follows: A
 client sends a request to the authentication server (AS) for
 "credentials" for a given server.  The AS responds with these
 credentials, encrypted in the client's key.  The credentials consist
 of a "ticket" for the server and a temporary encryption key (often
 called a "session key").  The client transmits the ticket (which
 contains the client's identity and a copy of the session key, all
 encrypted in the server's key) to the server.  The session key (now
 shared by the client and server) is used to authenticate the client
 and may optionally be used to authenticate the server.  It may also
 be used to encrypt further communication between the two parties or
 to exchange a separate sub-session key to be used to encrypt further
 communication.  Note that many applications use Kerberos' functions
 only upon the initiation of a stream-based network connection.
 Unless an application performs encryption or integrity protection for
 the data stream, the identity verification applies only to the
 initiation of the connection, and it does not guarantee that
 subsequent messages on the connection originate from the same
 principal.
 Implementation of the basic protocol consists of one or more
 authentication servers running on physically secure hosts.  The
 authentication servers maintain a database of principals (i.e., users
 and servers) and their secret keys.  Code libraries provide
 encryption and implement the Kerberos protocol.  In order to add
 authentication to its transactions, a typical network application
 adds calls to the Kerberos library directly or through the Generic
 Security Services Application Programming Interface (GSS-API)
 described in a separate document [RFC4121].  These calls result in
 the transmission of the messages necessary to achieve authentication.

Neuman, et al. Standards Track [Page 6] RFC 4120 Kerberos V5 July 2005

 The Kerberos protocol consists of several sub-protocols (or
 exchanges).  There are two basic methods by which a client can ask a
 Kerberos server for credentials.  In the first approach, the client
 sends a cleartext request for a ticket for the desired server to the
 AS.  The reply is sent encrypted in the client's secret key.  Usually
 this request is for a ticket-granting ticket (TGT), which can later
 be used with the ticket-granting server (TGS).  In the second method,
 the client sends a request to the TGS.  The client uses the TGT to
 authenticate itself to the TGS in the same manner as if it were
 contacting any other application server that requires Kerberos
 authentication.  The reply is encrypted in the session key from the
 TGT.  Though the protocol specification describes the AS and the TGS
 as separate servers, in practice they are implemented as different
 protocol entry points within a single Kerberos server.
 Once obtained, credentials may be used to verify the identity of the
 principals in a transaction, to ensure the integrity of messages
 exchanged between them, or to preserve privacy of the messages.  The
 application is free to choose whatever protection may be necessary.
 To verify the identities of the principals in a transaction, the
 client transmits the ticket to the application server.  Because the
 ticket is sent "in the clear" (parts of it are encrypted, but this
 encryption doesn't thwart replay) and might be intercepted and reused
 by an attacker, additional information is sent to prove that the
 message originated with the principal to whom the ticket was issued.
 This information (called the authenticator) is encrypted in the
 session key and includes a timestamp.  The timestamp proves that the
 message was recently generated and is not a replay.  Encrypting the
 authenticator in the session key proves that it was generated by a
 party possessing the session key.  Since no one except the requesting
 principal and the server know the session key (it is never sent over
 the network in the clear), this guarantees the identity of the
 client.
 The integrity of the messages exchanged between principals can also
 be guaranteed by using the session key (passed in the ticket and
 contained in the credentials).  This approach provides detection of
 both replay attacks and message stream modification attacks.  It is
 accomplished by generating and transmitting a collision-proof
 checksum (elsewhere called a hash or digest function) of the client's
 message, keyed with the session key.  Privacy and integrity of the
 messages exchanged between principals can be secured by encrypting
 the data to be passed by using the session key contained in the
 ticket or the sub-session key found in the authenticator.

Neuman, et al. Standards Track [Page 7] RFC 4120 Kerberos V5 July 2005

 The authentication exchanges mentioned above require read-only access
 to the Kerberos database.  Sometimes, however, the entries in the
 database must be modified, such as when adding new principals or
 changing a principal's key.  This is done using a protocol between a
 client and a third Kerberos server, the Kerberos Administration
 Server (KADM).  There is also a protocol for maintaining multiple
 copies of the Kerberos database.  Neither of these protocols are
 described in this document.

1.2. Cross-Realm Operation

 The Kerberos protocol is designed to operate across organizational
 boundaries.  A client in one organization can be authenticated to a
 server in another.  Each organization wishing to run a Kerberos
 server establishes its own "realm".  The name of the realm in which a
 client is registered is part of the client's name and can be used by
 the end-service to decide whether to honor a request.
 By establishing "inter-realm" keys, the administrators of two realms
 can allow a client authenticated in the local realm to prove its
 identity to servers in other realms.  The exchange of inter-realm
 keys (a separate key may be used for each direction) registers the
 ticket-granting service of each realm as a principal in the other
 realm.  A client is then able to obtain a TGT for the remote realm's
 ticket-granting service from its local realm.  When that TGT is used,
 the remote ticket-granting service uses the inter-realm key (which
 usually differs from its own normal TGS key) to decrypt the TGT; thus
 it is certain that the ticket was issued by the client's own TGS.
 Tickets issued by the remote ticket-granting service will indicate to
 the end-service that the client was authenticated from another realm.
 Without cross-realm operation, and with appropriate permission, the
 client can arrange registration of a separately-named principal in a
 remote realm and engage in normal exchanges with that realm's
 services.  However, for even small numbers of clients this becomes
 cumbersome, and more automatic methods as described here are
 necessary.
 A realm is said to communicate with another realm if the two realms
 share an inter-realm key, or if the local realm shares an inter-realm
 key with an intermediate realm that communicates with the remote
 realm.  An authentication path is the sequence of intermediate realms
 that are transited in communicating from one realm to another.
 Realms may be organized hierarchically.  Each realm shares a key with
 its parent and a different key with each child.  If an inter-realm
 key is not directly shared by two realms, the hierarchical
 organization allows an authentication path to be easily constructed.

Neuman, et al. Standards Track [Page 8] RFC 4120 Kerberos V5 July 2005

 If a hierarchical organization is not used, it may be necessary to
 consult a database in order to construct an authentication path
 between realms.
 Although realms are typically hierarchical, intermediate realms may
 be bypassed to achieve cross-realm authentication through alternate
 authentication paths.  (These might be established to make
 communication between two realms more efficient.)  It is important
 for the end-service to know which realms were transited when deciding
 how much faith to place in the authentication process.  To facilitate
 this decision, a field in each ticket contains the names of the
 realms that were involved in authenticating the client.
 The application server is ultimately responsible for accepting or
 rejecting authentication and SHOULD check the transited field.  The
 application server may choose to rely on the Key Distribution Center
 (KDC) for the application server's realm to check the transited
 field.  The application server's KDC will set the
 TRANSITED-POLICY-CHECKED flag in this case.  The KDCs for
 intermediate realms may also check the transited field as they issue
 TGTs for other realms, but they are encouraged not to do so.  A
 client may request that the KDCs not check the transited field by
 setting the DISABLE-TRANSITED-CHECK flag.  KDCs SHOULD honor this
 flag.

1.3. Choosing a Principal with Which to Communicate

 The Kerberos protocol provides the means for verifying (subject to
 the assumptions in Section 1.6) that the entity with which one
 communicates is the same entity that was registered with the KDC
 using the claimed identity (principal name).  It is still necessary
 to determine whether that identity corresponds to the entity with
 which one intends to communicate.
 When appropriate data has been exchanged in advance, the application
 may perform this determination syntactically based on the application
 protocol specification, information provided by the user, and
 configuration files.  For example, the server principal name
 (including realm) for a telnet server might be derived from the
 user-specified host name (from the telnet command line), the "host/"
 prefix specified in the application protocol specification, and a
 mapping to a Kerberos realm derived syntactically from the domain
 part of the specified hostname and information from the local
 Kerberos realms database.
 One can also rely on trusted third parties to make this
 determination, but only when the data obtained from the third party
 is suitably integrity-protected while resident on the third-party

Neuman, et al. Standards Track [Page 9] RFC 4120 Kerberos V5 July 2005

 server and when transmitted.  Thus, for example, one should not rely
 on an unprotected DNS record to map a host alias to the primary name
 of a server, accepting the primary name as the party that one intends
 to contact, since an attacker can modify the mapping and impersonate
 the party.
 Implementations of Kerberos and protocols based on Kerberos MUST NOT
 use insecure DNS queries to canonicalize the hostname components of
 the service principal names (i.e., they MUST NOT use insecure DNS
 queries to map one name to another to determine the host part of the
 principal name with which one is to communicate).  In an environment
 without secure name service, application authors MAY append a
 statically configured domain name to unqualified hostnames before
 passing the name to the security mechanisms, but they should do no
 more than that.  Secure name service facilities, if available, might
 be trusted for hostname canonicalization, but such canonicalization
 by the client SHOULD NOT be required by KDC implementations.
 Implementation note: Many current implementations do some degree of
 canonicalization of the provided service name, often using DNS even
 though it creates security problems.  However, there is no
 consistency among implementations as to whether the service name is
 case folded to lowercase or whether reverse resolution is used.  To
 maximize interoperability and security, applications SHOULD provide
 security mechanisms with names that result from folding the user-
 entered name to lowercase without performing any other modifications
 or canonicalization.

1.4. Authorization

 As an authentication service, Kerberos provides a means of verifying
 the identity of principals on a network.  Authentication is usually
 useful primarily as a first step in the process of authorization,
 determining whether a client may use a service, which objects the
 client is allowed to access, and the type of access allowed for each.
 Kerberos does not, by itself, provide authorization.  Possession of a
 client ticket for a service provides only for authentication of the
 client to that service, and in the absence of a separate
 authorization procedure, an application should not consider it to
 authorize the use of that service.
 Separate authorization methods MAY be implemented as application-
 specific access control functions and may utilize files on the
 application server, on separately issued authorization credentials
 such as those based on proxies [Neu93], or on other authorization
 services.  Separately authenticated authorization credentials MAY be
 embedded in a ticket's authorization data when encapsulated by the
 KDC-issued authorization data element.

Neuman, et al. Standards Track [Page 10] RFC 4120 Kerberos V5 July 2005

 Applications should not accept the mere issuance of a service ticket
 by the Kerberos server (even by a modified Kerberos server) as
 granting authority to use the service, since such applications may
 become vulnerable to the bypass of this authorization check in an
 environment where other options for application authentication are
 provided, or if they interoperate with other KDCs.

1.5. Extending Kerberos without Breaking Interoperability

 As the deployed base of Kerberos implementations grows, extending
 Kerberos becomes more important.  Unfortunately, some extensions to
 the existing Kerberos protocol create interoperability issues because
 of uncertainty regarding the treatment of certain extensibility
 options by some implementations.  This section includes guidelines
 that will enable future implementations to maintain interoperability.
 Kerberos provides a general mechanism for protocol extensibility.
 Some protocol messages contain typed holes -- sub-messages that
 contain an octet-string along with an integer that defines how to
 interpret the octet-string.  The integer types are registered
 centrally, but they can be used both for vendor extensions and for
 extensions standardized through the IETF.
 In this document, the word "extension" refers to extension by
 defining a new type to insert into an existing typed hole in a
 protocol message.  It does not refer to extension by addition of new
 fields to ASN.1 types, unless the text explicitly indicates
 otherwise.

1.5.1. Compatibility with RFC 1510

 Note that existing Kerberos message formats cannot readily be
 extended by adding fields to the ASN.1 types.  Sending additional
 fields often results in the entire message being discarded without an
 error indication.  Future versions of this specification will provide
 guidelines to ensure that ASN.1 fields can be added without creating
 an interoperability problem.
 In the meantime, all new or modified implementations of Kerberos that
 receive an unknown message extension SHOULD preserve the encoding of
 the extension but otherwise ignore its presence.  Recipients MUST NOT
 decline a request simply because an extension is present.
 There is one exception to this rule.  If an unknown authorization
 data element type is received by a server other than the ticket-
 granting service either in an AP-REQ or in a ticket contained in an
 AP-REQ, then authentication MUST fail.  One of the primary uses of
 authorization data is to restrict the use of the ticket.  If the

Neuman, et al. Standards Track [Page 11] RFC 4120 Kerberos V5 July 2005

 service cannot determine whether the restriction applies to that
 service, then a security weakness may result if the ticket can be
 used for that service.  Authorization elements that are optional
 SHOULD be enclosed in the AD-IF-RELEVANT element.
 The ticket-granting service MUST ignore but propagate to derivative
 tickets any unknown authorization data types, unless those data types
 are embedded in a MANDATORY-FOR-KDC element, in which case the
 request will be rejected.  This behavior is appropriate because
 requiring that the ticket-granting service understand unknown
 authorization data types would require that KDC software be upgraded
 to understand new application-level restrictions before applications
 used these restrictions, decreasing the utility of authorization data
 as a mechanism for restricting the use of tickets.  No security
 problem is created because services to which the tickets are issued
 will verify the authorization data.
 Implementation note: Many RFC 1510 implementations ignore unknown
 authorization data elements.  Depending on these implementations to
 honor authorization data restrictions may create a security weakness.

1.5.2. Sending Extensible Messages

 Care must be taken to ensure that old implementations can understand
 messages sent to them, even if they do not understand an extension
 that is used.  Unless the sender knows that an extension is
 supported, the extension cannot change the semantics of the core
 message or previously defined extensions.
 For example, an extension including key information necessary to
 decrypt the encrypted part of a KDC-REP could only be used in
 situations where the recipient was known to support the extension.
 Thus when designing such extensions it is important to provide a way
 for the recipient to notify the sender of support for the extension.
 For example in the case of an extension that changes the KDC-REP
 reply key, the client could indicate support for the extension by
 including a padata element in the AS-REQ sequence.  The KDC should
 only use the extension if this padata element is present in the
 AS-REQ.  Even if policy requires the use of the extension, it is
 better to return an error indicating that the extension is required
 than to use the extension when the recipient may not support it.
 Debugging implementations that do not interoperate is easier when
 errors are returned.

1.6. Environmental Assumptions

 Kerberos imposes a few assumptions on the environment in which it can
 properly function, including the following:

Neuman, et al. Standards Track [Page 12] RFC 4120 Kerberos V5 July 2005

  • "Denial of service" attacks are not solved with Kerberos. There

are places in the protocols where an intruder can prevent an

    application from participating in the proper authentication steps.
    Detection and solution of such attacks (some of which can appear
    to be not-uncommon "normal" failure modes for the system) are
    usually best left to the human administrators and users.
  • Principals MUST keep their secret keys secret. If an intruder

somehow steals a principal's key, it will be able to masquerade as

    that principal or to impersonate any server to the legitimate
    principal.
  • "Password guessing" attacks are not solved by Kerberos. If a user

chooses a poor password, it is possible for an attacker to

    successfully mount an offline dictionary attack by repeatedly
    attempting to decrypt, with successive entries from a dictionary,
    messages obtained which are encrypted under a key derived from the
    user's password.
  • Each host on the network MUST have a clock which is "loosely

synchronized" to the time of the other hosts; this synchronization

    is used to reduce the bookkeeping needs of application servers
    when they do replay detection.  The degree of "looseness" can be
    configured on a per-server basis, but it is typically on the order
    of 5 minutes.  If the clocks are synchronized over the network,
    the clock synchronization protocol MUST itself be secured from
    network attackers.
  • Principal identifiers are not recycled on a short-term basis. A

typical mode of access control will use access control lists

    (ACLs) to grant permissions to particular principals.  If a stale
    ACL entry remains for a deleted principal and the principal
    identifier is reused, the new principal will inherit rights
    specified in the stale ACL entry.  By not re-using principal
    identifiers, the danger of inadvertent access is removed.

1.7. Glossary of Terms

 Below is a list of terms used throughout this document.
 Authentication
    Verifying the claimed identity of a principal.
 Authentication header
    A record containing a Ticket and an Authenticator to be presented
    to a server as part of the authentication process.

Neuman, et al. Standards Track [Page 13] RFC 4120 Kerberos V5 July 2005

 Authentication path
    A sequence of intermediate realms transited in the authentication
    process when communicating from one realm to another.
 Authenticator
    A record containing information that can be shown to have been
    recently generated using the session key known only by the client
    and server.
 Authorization
    The process of determining whether a client may use a service,
    which objects the client is allowed to access, and the type of
    access allowed for each.
 Capability
    A token that grants the bearer permission to access an object or
    service.  In Kerberos, this might be a ticket whose use is
    restricted by the contents of the authorization data field, but
    which lists no network addresses, together with the session key
    necessary to use the ticket.
 Ciphertext
    The output of an encryption function.  Encryption transforms
    plaintext into ciphertext.
 Client
    A process that makes use of a network service on behalf of a user.
    Note that in some cases a Server may itself be a client of some
    other server (e.g., a print server may be a client of a file
    server).
 Credentials
    A ticket plus the secret session key necessary to use that ticket
    successfully in an authentication exchange.
 Encryption Type (etype)
    When associated with encrypted data, an encryption type identifies
    the algorithm used to encrypt the data and is used to select the
    appropriate algorithm for decrypting the data.  Encryption type
    tags are communicated in other messages to enumerate algorithms
    that are desired, supported, preferred, or allowed to be used for
    encryption of data between parties.  This preference is combined
    with local information and policy to select an algorithm to be
    used.
 KDC
    Key Distribution Center.  A network service that supplies tickets
    and temporary session keys; or an instance of that service or the

Neuman, et al. Standards Track [Page 14] RFC 4120 Kerberos V5 July 2005

    host on which it runs.  The KDC services both initial ticket and
    ticket-granting ticket requests.  The initial ticket portion is
    sometimes referred to as the Authentication Server (or service).
    The ticket-granting ticket portion is sometimes referred to as the
    ticket-granting server (or service).
 Kerberos
    The name given to the Project Athena's authentication service, the
    protocol used by that service, or the code used to implement the
    authentication service.  The name is adopted from the three-headed
    dog that guards Hades.
 Key Version Number (kvno)
    A tag associated with encrypted data identifies which key was used
    for encryption when a long-lived key associated with a principal
    changes over time.  It is used during the transition to a new key
    so that the party decrypting a message can tell whether the data
    was encrypted with the old or the new key.
 Plaintext
    The input to an encryption function or the output of a decryption
    function.  Decryption transforms ciphertext into plaintext.
 Principal
    A named client or server entity that participates in a network
    communication, with one name that is considered canonical.
 Principal identifier
    The canonical name used to identify each different principal
    uniquely.
 Seal
    To encipher a record containing several fields in such a way that
    the fields cannot be individually replaced without knowledge of
    the encryption key or leaving evidence of tampering.
 Secret key
    An encryption key shared by a principal and the KDC, distributed
    outside the bounds of the system, with a long lifetime.  In the
    case of a human user's principal, the secret key MAY be derived
    from a password.
 Server
    A particular Principal that provides a resource to network
    clients.  The server is sometimes referred to as the Application
    Server.

Neuman, et al. Standards Track [Page 15] RFC 4120 Kerberos V5 July 2005

 Service
    A resource provided to network clients; often provided by more
    than one server (for example, remote file service).
 Session key
    A temporary encryption key used between two principals, with a
    lifetime limited to the duration of a single login "session".  In
    the Kerberos system, a session key is generated by the KDC.  The
    session key is distinct from the sub-session key, described next.
 Sub-session key
    A temporary encryption key used between two principals, selected
    and exchanged by the principals using the session key, and with a
    lifetime limited to the duration of a single association.  The
    sub-session key is also referred to as the subkey.
 Ticket
    A record that helps a client authenticate itself to a server; it
    contains the client's identity, a session key, a timestamp, and
    other information, all sealed using the server's secret key.  It
    only serves to authenticate a client when presented along with a
    fresh Authenticator.

2. Ticket Flag Uses and Requests

 Each Kerberos ticket contains a set of flags that are used to
 indicate attributes of that ticket.  Most flags may be requested by a
 client when the ticket is obtained; some are automatically turned on
 and off by a Kerberos server as required.  The following sections
 explain what the various flags mean and give examples of reasons to
 use them.  With the exception of the INVALID flag, clients MUST
 ignore ticket flags that are not recognized.  KDCs MUST ignore KDC
 options that are not recognized.  Some implementations of RFC 1510
 are known to reject unknown KDC options, so clients may need to
 resend a request without new KDC options if the request was rejected
 when sent with options added since RFC 1510.  Because new KDCs will
 ignore unknown options, clients MUST confirm that the ticket returned
 by the KDC meets their needs.
 Note that it is not, in general, possible to determine whether an
 option was not honored because it was not understood or because it
 was rejected through either configuration or policy.  When adding a
 new option to the Kerberos protocol, designers should consider
 whether the distinction is important for their option.  If it is, a
 mechanism for the KDC to return an indication that the option was
 understood but rejected needs to be provided in the specification of
 the option.  Often in such cases, the mechanism needs to be broad
 enough to permit an error or reason to be returned.

Neuman, et al. Standards Track [Page 16] RFC 4120 Kerberos V5 July 2005

2.1. Initial, Pre-authenticated, and Hardware-Authenticated Tickets

 The INITIAL flag indicates that a ticket was issued using the AS
 protocol, rather than issued based on a TGT.  Application servers
 that want to require the demonstrated knowledge of a client's secret
 key (e.g., a password-changing program) can insist that this flag be
 set in any tickets they accept, and can thus be assured that the
 client's key was recently presented to the authentication server.
 The PRE-AUTHENT and HW-AUTHENT flags provide additional information
 about the initial authentication, regardless of whether the current
 ticket was issued directly (in which case INITIAL will also be set)
 or issued on the basis of a TGT (in which case the INITIAL flag is
 clear, but the PRE-AUTHENT and HW-AUTHENT flags are carried forward
 from the TGT).

2.2. Invalid Tickets

 The INVALID flag indicates that a ticket is invalid.  Application
 servers MUST reject tickets that have this flag set.  A postdated
 ticket will be issued in this form.  Invalid tickets MUST be
 validated by the KDC before use, by being presented to the KDC in a
 TGS request with the VALIDATE option specified.  The KDC will only
 validate tickets after their starttime has passed.  The validation is
 required so that postdated tickets that have been stolen before their
 starttime can be rendered permanently invalid (through a hot-list
 mechanism) (see Section 3.3.3.1).

2.3. Renewable Tickets

 Applications may desire to hold tickets that can be valid for long
 periods of time.  However, this can expose their credentials to
 potential theft for equally long periods, and those stolen
 credentials would be valid until the expiration time of the
 ticket(s).  Simply using short-lived tickets and obtaining new ones
 periodically would require the client to have long-term access to its
 secret key, an even greater risk.  Renewable tickets can be used to
 mitigate the consequences of theft.  Renewable tickets have two
 "expiration times": the first is when the current instance of the
 ticket expires, and the second is the latest permissible value for an
 individual expiration time.  An application client must periodically
 (i.e., before it expires) present a renewable ticket to the KDC, with
 the RENEW option set in the KDC request.  The KDC will issue a new
 ticket with a new session key and a later expiration time.  All other
 fields of the ticket are left unmodified by the renewal process.
 When the latest permissible expiration time arrives, the ticket
 expires permanently.  At each renewal, the KDC MAY consult a hot-list
 to determine whether the ticket had been reported stolen since its

Neuman, et al. Standards Track [Page 17] RFC 4120 Kerberos V5 July 2005

 last renewal; it will refuse to renew stolen tickets, and thus the
 usable lifetime of stolen tickets is reduced.
 The RENEWABLE flag in a ticket is normally only interpreted by the
 ticket-granting service (discussed below in Section 3.3).  It can
 usually be ignored by application servers.  However, some
 particularly careful application servers MAY disallow renewable
 tickets.
 If a renewable ticket is not renewed by its expiration time, the KDC
 will not renew the ticket.  The RENEWABLE flag is reset by default,
 but a client MAY request it be set by setting the RENEWABLE option in
 the KRB_AS_REQ message.  If it is set, then the renew-till field in
 the ticket contains the time after which the ticket may not be
 renewed.

2.4. Postdated Tickets

 Applications may occasionally need to obtain tickets for use much
 later; e.g., a batch submission system would need tickets to be valid
 at the time the batch job is serviced.  However, it is dangerous to
 hold valid tickets in a batch queue, since they will be on-line
 longer and more prone to theft.  Postdated tickets provide a way to
 obtain these tickets from the KDC at job submission time, but to
 leave them "dormant" until they are activated and validated by a
 further request of the KDC.  If a ticket theft were reported in the
 interim, the KDC would refuse to validate the ticket, and the thief
 would be foiled.
 The MAY-POSTDATE flag in a ticket is normally only interpreted by the
 ticket-granting service.  It can be ignored by application servers.
 This flag MUST be set in a TGT in order to issue a postdated ticket
 based on the presented ticket.  It is reset by default; a client MAY
 request it by setting the ALLOW-POSTDATE option in the KRB_AS_REQ
 message.  This flag does not allow a client to obtain a postdated
 TGT; postdated TGTs can only be obtained by requesting the postdating
 in the KRB_AS_REQ message.  The life (endtime-starttime) of a
 postdated ticket will be the remaining life of the TGT at the time of
 the request, unless the RENEWABLE option is also set, in which case
 it can be the full life (endtime-starttime) of the TGT.  The KDC MAY
 limit how far in the future a ticket may be postdated.
 The POSTDATED flag indicates that a ticket has been postdated.  The
 application server can check the authtime field in the ticket to see
 when the original authentication occurred.  Some services MAY choose
 to reject postdated tickets, or they may only accept them within a
 certain period after the original authentication.  When the KDC
 issues a POSTDATED ticket, it will also be marked as INVALID, so that

Neuman, et al. Standards Track [Page 18] RFC 4120 Kerberos V5 July 2005

 the application client MUST present the ticket to the KDC to be
 validated before use.

2.5. Proxiable and Proxy Tickets

 At times it may be necessary for a principal to allow a service to
 perform an operation on its behalf.  The service must be able to take
 on the identity of the client, but only for a particular purpose.  A
 principal can allow a service to do this by granting it a proxy.
 The process of granting a proxy by using the proxy and proxiable
 flags is used to provide credentials for use with specific services.
 Though conceptually also a proxy, users wishing to delegate their
 identity in a form usable for all purposes MUST use the ticket
 forwarding mechanism described in the next section to forward a TGT.
 The PROXIABLE flag in a ticket is normally only interpreted by the
 ticket-granting service.  It can be ignored by application servers.
 When set, this flag tells the ticket-granting server that it is OK to
 issue a new ticket (but not a TGT) with a different network address
 based on this ticket.  This flag is set if requested by the client on
 initial authentication.  By default, the client will request that it
 be set when requesting a TGT, and that it be reset when requesting
 any other ticket.
 This flag allows a client to pass a proxy to a server to perform a
 remote request on its behalf (e.g., a print service client can give
 the print server a proxy to access the client's files on a particular
 file server in order to satisfy a print request).
 In order to complicate the use of stolen credentials, Kerberos
 tickets are often valid only from those network addresses
 specifically included in the ticket, but it is permissible as a
 policy option to allow requests and to issue tickets with no network
 addresses specified.  When granting a proxy, the client MUST specify
 the new network address from which the proxy is to be used or
 indicate that the proxy is to be issued for use from any address.
 The PROXY flag is set in a ticket by the TGS when it issues a proxy
 ticket.  Application servers MAY check this flag; and at their option
 they MAY require additional authentication from the agent presenting
 the proxy in order to provide an audit trail.

2.6. Forwardable Tickets

 Authentication forwarding is an instance of a proxy where the service
 that is granted is complete use of the client's identity.  An example
 of where it might be used is when a user logs in to a remote system

Neuman, et al. Standards Track [Page 19] RFC 4120 Kerberos V5 July 2005

 and wants authentication to work from that system as if the login
 were local.
 The FORWARDABLE flag in a ticket is normally only interpreted by the
 ticket-granting service.  It can be ignored by application servers.
 The FORWARDABLE flag has an interpretation similar to that of the
 PROXIABLE flag, except TGTs may also be issued with different network
 addresses.  This flag is reset by default, but users MAY request that
 it be set by setting the FORWARDABLE option in the AS request when
 they request their initial TGT.
 This flag allows for authentication forwarding without requiring the
 user to enter a password again.  If the flag is not set, then
 authentication forwarding is not permitted, but the same result can
 still be achieved if the user engages in the AS exchange, specifies
 the requested network addresses, and supplies a password.
 The FORWARDED flag is set by the TGS when a client presents a ticket
 with the FORWARDABLE flag set and requests a forwarded ticket by
 specifying the FORWARDED KDC option and supplying a set of addresses
 for the new ticket.  It is also set in all tickets issued based on
 tickets with the FORWARDED flag set.  Application servers may choose
 to process FORWARDED tickets differently than non-FORWARDED tickets.
 If addressless tickets are forwarded from one system to another,
 clients SHOULD still use this option to obtain a new TGT in order to
 have different session keys on the different systems.

2.7. Transited Policy Checking

 In Kerberos, the application server is ultimately responsible for
 accepting or rejecting authentication, and it SHOULD check that only
 suitably trusted KDCs are relied upon to authenticate a principal.
 The transited field in the ticket identifies which realms (and thus
 which KDCs) were involved in the authentication process, and an
 application server would normally check this field.  If any of these
 are untrusted to authenticate the indicated client principal
 (probably determined by a realm-based policy), the authentication
 attempt MUST be rejected.  The presence of trusted KDCs in this list
 does not provide any guarantee; an untrusted KDC may have fabricated
 the list.
 Although the end server ultimately decides whether authentication is
 valid, the KDC for the end server's realm MAY apply a realm-specific
 policy for validating the transited field and accepting credentials
 for cross-realm authentication.  When the KDC applies such checks and
 accepts such cross-realm authentication, it will set the
 TRANSITED-POLICY-CHECKED flag in the service tickets it issues based

Neuman, et al. Standards Track [Page 20] RFC 4120 Kerberos V5 July 2005

 on the cross-realm TGT.  A client MAY request that the KDCs not check
 the transited field by setting the DISABLE-TRANSITED-CHECK flag.
 KDCs are encouraged but not required to honor this flag.
 Application servers MUST either do the transited-realm checks
 themselves or reject cross-realm tickets without
 TRANSITED-POLICY-CHECKED set.

2.8. OK as Delegate

 For some applications, a client may need to delegate authority to a
 server to act on its behalf in contacting other services.  This
 requires that the client forward credentials to an intermediate
 server.  The ability for a client to obtain a service ticket to a
 server conveys no information to the client about whether the server
 should be trusted to accept delegated credentials.  The
 OK-AS-DELEGATE provides a way for a KDC to communicate local realm
 policy to a client regarding whether an intermediate server is
 trusted to accept such credentials.
 The copy of the ticket flags in the encrypted part of the KDC reply
 may have the OK-AS-DELEGATE flag set to indicate to the client that
 the server specified in the ticket has been determined by the policy
 of the realm to be a suitable recipient of delegation.  A client can
 use the presence of this flag to help it decide whether to delegate
 credentials (grant either a proxy or a forwarded TGT) to this server.
 It is acceptable to ignore the value of this flag.  When setting this
 flag, an administrator should consider the security and placement of
 the server on which the service will run, as well as whether the
 service requires the use of delegated credentials.

2.9. Other KDC Options

 There are three additional options that MAY be set in a client's
 request of the KDC.

2.9.1. Renewable-OK

 The RENEWABLE-OK option indicates that the client will accept a
 renewable ticket if a ticket with the requested life cannot otherwise
 be provided.  If a ticket with the requested life cannot be provided,
 then the KDC MAY issue a renewable ticket with a renew-till equal to
 the requested endtime.  The value of the renew-till field MAY still
 be adjusted by site-determined limits or limits imposed by the
 individual principal or server.

Neuman, et al. Standards Track [Page 21] RFC 4120 Kerberos V5 July 2005

2.9.2. ENC-TKT-IN-SKEY

 In its basic form, the Kerberos protocol supports authentication in a
 client-server setting and is not well suited to authentication in a
 peer-to-peer environment because the long-term key of the user does
 not remain on the workstation after initial login.  Authentication of
 such peers may be supported by Kerberos in its user-to-user variant.
 The ENC-TKT-IN-SKEY option supports user-to-user authentication by
 allowing the KDC to issue a service ticket encrypted using the
 session key from another TGT issued to another user.  The
 ENC-TKT-IN-SKEY option is honored only by the ticket-granting
 service.  It indicates that the ticket to be issued for the end
 server is to be encrypted in the session key from the additional
 second TGT provided with the request.  See Section 3.3.3 for specific
 details.

2.9.3. Passwordless Hardware Authentication

 The OPT-HARDWARE-AUTH option indicates that the client wishes to use
 some form of hardware authentication instead of or in addition to the
 client's password or other long-lived encryption key.
 OPT-HARDWARE-AUTH is honored only by the authentication service.  If
 supported and allowed by policy, the KDC will return an error code of
 KDC_ERR_PREAUTH_REQUIRED and include the required METHOD-DATA to
 perform such authentication.

3. Message Exchanges

 The following sections describe the interactions between network
 clients and servers and the messages involved in those exchanges.

3.1. The Authentication Service Exchange

                           Summary
       Message direction       Message type    Section
       1. Client to Kerberos   KRB_AS_REQ      5.4.1
       2. Kerberos to client   KRB_AS_REP or   5.4.2
                               KRB_ERROR       5.9.1
 The Authentication Service (AS) Exchange between the client and the
 Kerberos Authentication Server is initiated by a client when it
 wishes to obtain authentication credentials for a given server but
 currently holds no credentials.  In its basic form, the client's
 secret key is used for encryption and decryption.  This exchange is
 typically used at the initiation of a login session to obtain
 credentials for a Ticket-Granting Server, which will subsequently be
 used to obtain credentials for other servers (see Section 3.3)

Neuman, et al. Standards Track [Page 22] RFC 4120 Kerberos V5 July 2005

 without requiring further use of the client's secret key.  This
 exchange is also used to request credentials for services that must
 not be mediated through the Ticket-Granting Service, but rather
 require knowledge of a principal's secret key, such as the password-
 changing service (the password-changing service denies requests
 unless the requester can demonstrate knowledge of the user's old
 password; requiring this knowledge prevents unauthorized password
 changes by someone walking up to an unattended session).
 This exchange does not by itself provide any assurance of the
 identity of the user.  To authenticate a user logging on to a local
 system, the credentials obtained in the AS exchange may first be used
 in a TGS exchange to obtain credentials for a local server; those
 credentials must then be verified by a local server through
 successful completion of the Client/Server exchange.
 The AS exchange consists of two messages: KRB_AS_REQ from the client
 to Kerberos, and KRB_AS_REP or KRB_ERROR in reply.  The formats for
 these messages are described in Sections 5.4.1, 5.4.2, and 5.9.1.
 In the request, the client sends (in cleartext) its own identity and
 the identity of the server for which it is requesting credentials,
 other information about the credentials it is requesting, and a
 randomly generated nonce, which can be used to detect replays and to
 associate replies with the matching requests.  This nonce MUST be
 generated randomly by the client and remembered for checking against
 the nonce in the expected reply.  The response, KRB_AS_REP, contains
 a ticket for the client to present to the server, and a session key
 that will be shared by the client and the server.  The session key
 and additional information are encrypted in the client's secret key.
 The encrypted part of the KRB_AS_REP message also contains the nonce
 that MUST be matched with the nonce from the KRB_AS_REQ message.
 Without pre-authentication, the authentication server does not know
 whether the client is actually the principal named in the request.
 It simply sends a reply without knowing or caring whether they are
 the same.  This is acceptable because nobody but the principal whose
 identity was given in the request will be able to use the reply.  Its
 critical information is encrypted in that principal's key.  However,
 an attacker can send a KRB_AS_REQ message to get known plaintext in
 order to attack the principal's key.  Especially if the key is based
 on a password, this may create a security exposure.  So the initial
 request supports an optional field that can be used to pass
 additional information that might be needed for the initial exchange.
 This field SHOULD be used for pre-authentication as described in
 sections 3.1.1 and 5.2.7.

Neuman, et al. Standards Track [Page 23] RFC 4120 Kerberos V5 July 2005

 Various errors can occur; these are indicated by an error response
 (KRB_ERROR) instead of the KRB_AS_REP response.  The error message is
 not encrypted.  The KRB_ERROR message contains information that can
 be used to associate it with the message to which it replies.  The
 contents of the KRB_ERROR message are not integrity-protected.  As
 such, the client cannot detect replays, fabrications, or
 modifications.  A solution to this problem will be included in a
 future version of the protocol.

3.1.1. Generation of KRB_AS_REQ Message

 The client may specify a number of options in the initial request.
 Among these options are whether pre-authentication is to be
 performed; whether the requested ticket is to be renewable,
 proxiable, or forwardable; whether it should be postdated or allow
 postdating of derivative tickets; and whether a renewable ticket will
 be accepted in lieu of a non-renewable ticket if the requested ticket
 expiration date cannot be satisfied by a non-renewable ticket (due to
 configuration constraints).
 The client prepares the KRB_AS_REQ message and sends it to the KDC.

3.1.2. Receipt of KRB_AS_REQ Message

 If all goes well, processing the KRB_AS_REQ message will result in
 the creation of a ticket for the client to present to the server.
 The format for the ticket is described in Section 5.3.
 Because Kerberos can run over unreliable transports such as UDP, the
 KDC MUST be prepared to retransmit responses in case they are lost.
 If a KDC receives a request identical to one it has recently
 processed successfully, the KDC MUST respond with a KRB_AS_REP
 message rather than a replay error.  In order to reduce ciphertext
 given to a potential attacker, KDCs MAY send the same response
 generated when the request was first handled.  KDCs MUST obey this
 replay behavior even if the actual transport in use is reliable.

3.1.3. Generation of KRB_AS_REP Message

 The authentication server looks up the client and server principals
 named in the KRB_AS_REQ in its database, extracting their respective
 keys.  If the requested client principal named in the request is
 unknown because it doesn't exist in the KDC's principal database,
 then an error message with a KDC_ERR_C_PRINCIPAL_UNKNOWN is returned.
 If required to do so, the server pre-authenticates the request, and
 if the pre-authentication check fails, an error message with the code
 KDC_ERR_PREAUTH_FAILED is returned.  If pre-authentication is

Neuman, et al. Standards Track [Page 24] RFC 4120 Kerberos V5 July 2005

 required, but was not present in the request, an error message with
 the code KDC_ERR_PREAUTH_REQUIRED is returned, and a METHOD-DATA
 object will be stored in the e-data field of the KRB-ERROR message to
 specify which pre-authentication mechanisms are acceptable.  Usually
 this will include PA-ETYPE-INFO and/or PA-ETYPE-INFO2 elements as
 described below.  If the server cannot accommodate any encryption
 type requested by the client, an error message with code
 KDC_ERR_ETYPE_NOSUPP is returned.  Otherwise, the KDC generates a
 'random' session key, meaning that, among other things, it should be
 impossible to guess the next session key based on knowledge of past
 session keys.  Although this can be achieved in a pseudo-random
 number generator if it is based on cryptographic principles, it is
 more desirable to use a truly random number generator, such as one
 based on measurements of random physical phenomena.  See [RFC4086]
 for an in-depth discussion of randomness.
 In response to an AS request, if there are multiple encryption keys
 registered for a client in the Kerberos database, then the etype
 field from the AS request is used by the KDC to select the encryption
 method to be used to protect the encrypted part of the KRB_AS_REP
 message that is sent to the client.  If there is more than one
 supported strong encryption type in the etype list, the KDC SHOULD
 use the first valid strong etype for which an encryption key is
 available.
 When the user's key is generated from a password or pass phrase, the
 string-to-key function for the particular encryption key type is
 used, as specified in [RFC3961].  The salt value and additional
 parameters for the string-to-key function have default values
 (specified by Section 4 and by the encryption mechanism
 specification, respectively) that may be overridden by
 pre-authentication data (PA-PW-SALT, PA-AFS3-SALT, PA-ETYPE-INFO,
 PA-ETYPE-INFO2, etc).  Since the KDC is presumed to store a copy of
 the resulting key only, these values should not be changed for
 password-based keys except when changing the principal's key.
 When the AS server is to include pre-authentication data in a
 KRB-ERROR or in an AS-REP, it MUST use PA-ETYPE-INFO2, not PA-ETYPE-
 INFO, if the etype field of the client's AS-REQ lists at least one
 "newer" encryption type.  Otherwise (when the etype field of the
 client's AS-REQ does not list any "newer" encryption types), it MUST
 send both PA-ETYPE-INFO2 and PA-ETYPE-INFO (both with an entry for
 each enctype).  A "newer" enctype is any enctype first officially
 specified concurrently with or subsequent to the issue of this RFC.
 The enctypes DES, 3DES, or RC4 and any defined in [RFC1510] are not
 "newer" enctypes.

Neuman, et al. Standards Track [Page 25] RFC 4120 Kerberos V5 July 2005

 It is not possible to generate a user's key reliably given a pass
 phrase without contacting the KDC, since it will not be known whether
 alternate salt or parameter values are required.
 The KDC will attempt to assign the type of the random session key
 from the list of methods in the etype field.  The KDC will select the
 appropriate type using the list of methods provided and information
 from the Kerberos database indicating acceptable encryption methods
 for the application server.  The KDC will not issue tickets with a
 weak session key encryption type.
 If the requested starttime is absent, indicates a time in the past,
 or is within the window of acceptable clock skew for the KDC and the
 POSTDATE option has not been specified, then the starttime of the
 ticket is set to the authentication server's current time.  If it
 indicates a time in the future beyond the acceptable clock skew, but
 the POSTDATED option has not been specified, then the error
 KDC_ERR_CANNOT_POSTDATE is returned.  Otherwise the requested
 starttime is checked against the policy of the local realm (the
 administrator might decide to prohibit certain types or ranges of
 postdated tickets), and if the ticket's starttime is acceptable, it
 is set as requested, and the INVALID flag is set in the new ticket.
 The postdated ticket MUST be validated before use by presenting it to
 the KDC after the starttime has been reached.
 The expiration time of the ticket will be set to the earlier of the
 requested endtime and a time determined by local policy, possibly by
 using realm- or principal-specific factors.  For example, the
 expiration time MAY be set to the earliest of the following:
  • The expiration time (endtime) requested in the KRB_AS_REQ

message.

  • The ticket's starttime plus the maximum allowable lifetime

associated with the client principal from the authentication

       server's database.
  • The ticket's starttime plus the maximum allowable lifetime

associated with the server principal.

  • The ticket's starttime plus the maximum lifetime set by the

policy of the local realm.

 If the requested expiration time minus the starttime (as determined
 above) is less than a site-determined minimum lifetime, an error
 message with code KDC_ERR_NEVER_VALID is returned.  If the requested
 expiration time for the ticket exceeds what was determined as above,
 and if the 'RENEWABLE-OK' option was requested, then the 'RENEWABLE'

Neuman, et al. Standards Track [Page 26] RFC 4120 Kerberos V5 July 2005

 flag is set in the new ticket, and the renew-till value is set as if
 the 'RENEWABLE' option were requested (the field and option names are
 described fully in Section 5.4.1).
 If the RENEWABLE option has been requested or if the RENEWABLE-OK
 option has been set and a renewable ticket is to be issued, then the
 renew-till field MAY be set to the earliest of:
  • Its requested value.
  • The starttime of the ticket plus the minimum of the two maximum

renewable lifetimes associated with the principals' database

       entries.
  • The starttime of the ticket plus the maximum renewable lifetime

set by the policy of the local realm.

 The flags field of the new ticket will have the following options set
 if they have been requested and if the policy of the local realm
 allows:  FORWARDABLE, MAY-POSTDATE, POSTDATED, PROXIABLE, RENEWABLE.
 If the new ticket is postdated (the starttime is in the future), its
 INVALID flag will also be set.
 If all of the above succeed, the server will encrypt the ciphertext
 part of the ticket using the encryption key extracted from the server
 principal's record in the Kerberos database using the encryption type
 associated with the server principal's key.  (This choice is NOT
 affected by the etype field in the request.)  It then formats a
 KRB_AS_REP message (see Section 5.4.2), copying the addresses in the
 request into the caddr of the response, placing any required pre-
 authentication data into the padata of the response, and encrypts the
 ciphertext part in the client's key using an acceptable encryption
 method requested in the etype field of the request, or in some key
 specified by pre-authentication mechanisms being used.

3.1.4. Generation of KRB_ERROR Message

 Several errors can occur, and the Authentication Server responds by
 returning an error message, KRB_ERROR, to the client, with the
 error-code and e-text fields set to appropriate values.  The error
 message contents and details are described in Section 5.9.1.

3.1.5. Receipt of KRB_AS_REP Message

 If the reply message type is KRB_AS_REP, then the client verifies
 that the cname and crealm fields in the cleartext portion of the
 reply match what it requested.  If any padata fields are present,
 they may be used to derive the proper secret key to decrypt the

Neuman, et al. Standards Track [Page 27] RFC 4120 Kerberos V5 July 2005

 message.  The client decrypts the encrypted part of the response
 using its secret key and verifies that the nonce in the encrypted
 part matches the nonce it supplied in its request (to detect
 replays).  It also verifies that the sname and srealm in the response
 match those in the request (or are otherwise expected values), and
 that the host address field is also correct.  It then stores the
 ticket, session key, start and expiration times, and other
 information for later use.  The last-req field (and the deprecated
 key-expiration field) from the encrypted part of the response MAY be
 checked to notify the user of impending key expiration.  This enables
 the client program to suggest remedial action, such as a password
 change.
 Upon validation of the KRB_AS_REP message (by checking the returned
 nonce against that sent in the KRB_AS_REQ message), the client knows
 that the current time on the KDC is that read from the authtime field
 of the encrypted part of the reply.  The client can optionally use
 this value for clock synchronization in subsequent messages by
 recording with the ticket the difference (offset) between the
 authtime value and the local clock.  This offset can then be used by
 the same user to adjust the time read from the system clock when
 generating messages [DGT96].
 This technique MUST be used when adjusting for clock skew instead of
 directly changing the system clock, because the KDC reply is only
 authenticated to the user whose secret key was used, but not to the
 system or workstation.  If the clock were adjusted, an attacker
 colluding with a user logging into a workstation could agree on a
 password, resulting in a KDC reply that would be correctly validated
 even though it did not originate from a KDC trusted by the
 workstation.
 Proper decryption of the KRB_AS_REP message is not sufficient for the
 host to verify the identity of the user; the user and an attacker
 could cooperate to generate a KRB_AS_REP format message that decrypts
 properly but is not from the proper KDC.  If the host wishes to
 verify the identity of the user, it MUST require the user to present
 application credentials that can be verified using a securely-stored
 secret key for the host.  If those credentials can be verified, then
 the identity of the user can be assured.

3.1.6. Receipt of KRB_ERROR Message

 If the reply message type is KRB_ERROR, then the client interprets it
 as an error and performs whatever application-specific tasks are
 necessary for recovery.

Neuman, et al. Standards Track [Page 28] RFC 4120 Kerberos V5 July 2005

3.2. The Client/Server Authentication Exchange

                              Summary
 Message direction                         Message type    Section
 Client to Application server              KRB_AP_REQ      5.5.1
 [optional] Application server to client   KRB_AP_REP or   5.5.2
                                           KRB_ERROR       5.9.1
 The client/server authentication (CS) exchange is used by network
 applications to authenticate the client to the server and vice versa.
 The client MUST have already acquired credentials for the server
 using the AS or TGS exchange.

3.2.1. The KRB_AP_REQ Message

 The KRB_AP_REQ contains authentication information that SHOULD be
 part of the first message in an authenticated transaction.  It
 contains a ticket, an authenticator, and some additional bookkeeping
 information (see Section 5.5.1 for the exact format).  The ticket by
 itself is insufficient to authenticate a client, since tickets are
 passed across the network in cleartext (tickets contain both an
 encrypted and unencrypted portion, so cleartext here refers to the
 entire unit, which can be copied from one message and replayed in
 another without any cryptographic skill).  The authenticator is used
 to prevent invalid replay of tickets by proving to the server that
 the client knows the session key of the ticket and thus is entitled
 to use the ticket.  The KRB_AP_REQ message is referred to elsewhere
 as the 'authentication header'.

3.2.2. Generation of a KRB_AP_REQ Message

 When a client wishes to initiate authentication to a server, it
 obtains (either through a credentials cache, the AS exchange, or the
 TGS exchange) a ticket and session key for the desired service.  The
 client MAY re-use any tickets it holds until they expire.  To use a
 ticket, the client constructs a new Authenticator from the system
 time and its name, and optionally from an application-specific
 checksum, an initial sequence number to be used in KRB_SAFE or
 KRB_PRIV messages, and/or a session subkey to be used in negotiations
 for a session key unique to this particular session.  Authenticators
 MUST NOT be re-used and SHOULD be rejected if replayed to a server.
 Note that this can make applications based on unreliable transports
 difficult to code correctly.  If the transport might deliver
 duplicated messages, either a new authenticator MUST be generated for
 each retry, or the application server MUST match requests and replies
 and replay the first reply in response to a detected duplicate.

Neuman, et al. Standards Track [Page 29] RFC 4120 Kerberos V5 July 2005

 If a sequence number is to be included, it SHOULD be randomly chosen
 so that even after many messages have been exchanged it is not likely
 to collide with other sequence numbers in use.
 The client MAY indicate a requirement of mutual authentication or the
 use of a session-key based ticket (for user-to-user authentication,
 see section 3.7) by setting the appropriate flag(s) in the ap-options
 field of the message.
 The Authenticator is encrypted in the session key and combined with
 the ticket to form the KRB_AP_REQ message, which is then sent to the
 end server along with any additional application-specific
 information.

3.2.3. Receipt of KRB_AP_REQ Message

 Authentication is based on the server's current time of day (clocks
 MUST be loosely synchronized), the authenticator, and the ticket.
 Several errors are possible.  If an error occurs, the server is
 expected to reply to the client with a KRB_ERROR message.  This
 message MAY be encapsulated in the application protocol if its raw
 form is not acceptable to the protocol.  The format of error messages
 is described in Section 5.9.1.
 The algorithm for verifying authentication information is as follows.
 If the message type is not KRB_AP_REQ, the server returns the
 KRB_AP_ERR_MSG_TYPE error.  If the key version indicated by the
 Ticket in the KRB_AP_REQ is not one the server can use (e.g., it
 indicates an old key, and the server no longer possesses a copy of
 the old key), the KRB_AP_ERR_BADKEYVER error is returned.  If the
 USE-SESSION-KEY flag is set in the ap-options field, it indicates to
 the server that user-to-user authentication is in use, and that the
 ticket is encrypted in the session key from the server's TGT rather
 than in the server's secret key.  See Section 3.7 for a more complete
 description of the effect of user-to-user authentication on all
 messages in the Kerberos protocol.
 Because it is possible for the server to be registered in multiple
 realms, with different keys in each, the srealm field in the
 unencrypted portion of the ticket in the KRB_AP_REQ is used to
 specify which secret key the server should use to decrypt that
 ticket.  The KRB_AP_ERR_NOKEY error code is returned if the server
 doesn't have the proper key to decipher the ticket.
 The ticket is decrypted using the version of the server's key
 specified by the ticket.  If the decryption routines detect a
 modification of the ticket (each encryption system MUST provide
 safeguards to detect modified ciphertext), the

Neuman, et al. Standards Track [Page 30] RFC 4120 Kerberos V5 July 2005

 KRB_AP_ERR_BAD_INTEGRITY error is returned (chances are good that
 different keys were used to encrypt and decrypt).
 The authenticator is decrypted using the session key extracted from
 the decrypted ticket.  If decryption shows that is has been modified,
 the KRB_AP_ERR_BAD_INTEGRITY error is returned.  The name and realm
 of the client from the ticket are compared against the same fields in
 the authenticator.  If they don't match, the KRB_AP_ERR_BADMATCH
 error is returned; normally this is caused by a client error or an
 attempted attack.  The addresses in the ticket (if any) are then
 searched for an address matching the operating-system reported
 address of the client.  If no match is found or the server insists on
 ticket addresses but none are present in the ticket, the
 KRB_AP_ERR_BADADDR error is returned.  If the local (server) time and
 the client time in the authenticator differ by more than the
 allowable clock skew (e.g., 5 minutes), the KRB_AP_ERR_SKEW error is
 returned.
 Unless the application server provides its own suitable means to
 protect against replay (for example, a challenge-response sequence
 initiated by the server after authentication, or use of a server-
 generated encryption subkey), the server MUST utilize a replay cache
 to remember any authenticator presented within the allowable clock
 skew.  Careful analysis of the application protocol and
 implementation is recommended before eliminating this cache.  The
 replay cache will store at least the server name, along with the
 client name, time, and microsecond fields from the recently-seen
 authenticators, and if a matching tuple is found, the
 KRB_AP_ERR_REPEAT error is returned.  Note that the rejection here is
 restricted to authenticators from the same principal to the same
 server.  Other client principals communicating with the same server
 principal should not have their authenticators rejected if the time
 and microsecond fields happen to match some other client's
 authenticator.
 If a server loses track of authenticators presented within the
 allowable clock skew, it MUST reject all requests until the clock
 skew interval has passed, providing assurance that any lost or
 replayed authenticators will fall outside the allowable clock skew
 and can no longer be successfully replayed.  If this were not done,
 an attacker could subvert the authentication by recording the ticket
 and authenticator sent over the network to a server and replaying
 them following an event that caused the server to lose track of
 recently seen authenticators.
 Implementation note: If a client generates multiple requests to the
 KDC with the same timestamp, including the microsecond field, all but
 the first of the requests received will be rejected as replays.  This

Neuman, et al. Standards Track [Page 31] RFC 4120 Kerberos V5 July 2005

 might happen, for example, if the resolution of the client's clock is
 too coarse.  Client implementations SHOULD ensure that the timestamps
 are not reused, possibly by incrementing the microseconds field in
 the time stamp when the clock returns the same time for multiple
 requests.
 If multiple servers (for example, different services on one machine,
 or a single service implemented on multiple machines) share a service
 principal (a practice that we do not recommend in general, but that
 we acknowledge will be used in some cases), either they MUST share
 this replay cache, or the application protocol MUST be designed so as
 to eliminate the need for it.  Note that this applies to all of the
 services.  If any of the application protocols does not have replay
 protection built in, an authenticator used with such a service could
 later be replayed to a different service with the same service
 principal but no replay protection, if the former doesn't record the
 authenticator information in the common replay cache.
 If a sequence number is provided in the authenticator, the server
 saves it for later use in processing KRB_SAFE and/or KRB_PRIV
 messages.  If a subkey is present, the server either saves it for
 later use or uses it to help generate its own choice for a subkey to
 be returned in a KRB_AP_REP message.
 The server computes the age of the ticket: local (server) time minus
 the starttime inside the Ticket.  If the starttime is later than the
 current time by more than the allowable clock skew, or if the INVALID
 flag is set in the ticket, the KRB_AP_ERR_TKT_NYV error is returned.
 Otherwise, if the current time is later than end time by more than
 the allowable clock skew, the KRB_AP_ERR_TKT_EXPIRED error is
 returned.
 If all these checks succeed without an error, the server is assured
 that the client possesses the credentials of the principal named in
 the ticket, and thus, that the client has been authenticated to the
 server.
 Passing these checks provides only authentication of the named
 principal; it does not imply authorization to use the named service.
 Applications MUST make a separate authorization decision based upon
 the authenticated name of the user, the requested operation, local
 access control information such as that contained in a .k5login or
 .k5users file, and possibly a separate distributed authorization
 service.

Neuman, et al. Standards Track [Page 32] RFC 4120 Kerberos V5 July 2005

3.2.4. Generation of a KRB_AP_REP Message

 Typically, a client's request will include both the authentication
 information and its initial request in the same message, and the
 server need not explicitly reply to the KRB_AP_REQ.  However, if
 mutual authentication (authenticating not only the client to the
 server, but also the server to the client) is being performed, the
 KRB_AP_REQ message will have MUTUAL-REQUIRED set in its ap-options
 field, and a KRB_AP_REP message is required in response.  As with the
 error message, this message MAY be encapsulated in the application
 protocol if its "raw" form is not acceptable to the application's
 protocol.  The timestamp and microsecond field used in the reply MUST
 be the client's timestamp and microsecond field (as provided in the
 authenticator).  If a sequence number is to be included, it SHOULD be
 randomly chosen as described above for the authenticator.  A subkey
 MAY be included if the server desires to negotiate a different
 subkey.  The KRB_AP_REP message is encrypted in the session key
 extracted from the ticket.
 Note that in the Kerberos Version 4 protocol, the timestamp in the
 reply was the client's timestamp plus one.  This is not necessary in
 Version 5 because Version 5 messages are formatted in such a way that
 it is not possible to create the reply by judicious message surgery
 (even in encrypted form) without knowledge of the appropriate
 encryption keys.

3.2.5. Receipt of KRB_AP_REP Message

 If a KRB_AP_REP message is returned, the client uses the session key
 from the credentials obtained for the server to decrypt the message
 and verifies that the timestamp and microsecond fields match those in
 the Authenticator it sent to the server.  If they match, then the
 client is assured that the server is genuine.  The sequence number
 and subkey (if present) are retained for later use.  (Note that for
 encrypting the KRB_AP_REP message, the sub-session key is not used,
 even if it is present in the Authentication.)

3.2.6. Using the Encryption Key

 After the KRB_AP_REQ/KRB_AP_REP exchange has occurred, the client and
 server share an encryption key that can be used by the application.
 In some cases, the use of this session key will be implicit in the
 protocol; in others the method of use must be chosen from several
 alternatives.  The application MAY choose the actual encryption key
 to be used for KRB_PRIV, KRB_SAFE, or other application-specific uses
 based on the session key from the ticket and subkeys in the
 KRB_AP_REP message and the authenticator.  Implementations of the
 protocol MAY provide routines to choose subkeys based on session keys

Neuman, et al. Standards Track [Page 33] RFC 4120 Kerberos V5 July 2005

 and random numbers and to generate a negotiated key to be returned in
 the KRB_AP_REP message.
 To mitigate the effect of failures in random number generation on the
 client, it is strongly encouraged that any key derived by an
 application for subsequent use include the full key entropy derived
 from the KDC-generated session key carried in the ticket.  We leave
 the protocol negotiations of how to use the key (e.g., for selecting
 an encryption or checksum type) to the application programmer.  The
 Kerberos protocol does not constrain the implementation options, but
 an example of how this might be done follows.
 One way that an application may choose to negotiate a key to be used
 for subsequent integrity and privacy protection is for the client to
 propose a key in the subkey field of the authenticator.  The server
 can then choose a key using the key proposed by the client as input,
 returning the new subkey in the subkey field of the application
 reply.  This key could then be used for subsequent communication.
 With both the one-way and mutual authentication exchanges, the peers
 should take care not to send sensitive information to each other
 without proper assurances.  In particular, applications that require
 privacy or integrity SHOULD use the KRB_AP_REP response from the
 server to the client to assure both client and server of their peer's
 identity.  If an application protocol requires privacy of its
 messages, it can use the KRB_PRIV message (section 3.5).  The
 KRB_SAFE message (Section 3.4) can be used to ensure integrity.

3.3. The Ticket-Granting Service (TGS) Exchange

                           Summary
       Message direction       Message type     Section
       1. Client to Kerberos   KRB_TGS_REQ      5.4.1
       2. Kerberos to client   KRB_TGS_REP or   5.4.2
                               KRB_ERROR        5.9.1
 The TGS exchange between a client and the Kerberos TGS is initiated
 by a client when it seeks to obtain authentication credentials for a
 given server (which might be registered in a remote realm), when it
 seeks to renew or validate an existing ticket, or when it seeks to
 obtain a proxy ticket.  In the first case, the client must already
 have acquired a ticket for the Ticket-Granting Service using the AS
 exchange (the TGT is usually obtained when a client initially
 authenticates to the system, such as when a user logs in).  The
 message format for the TGS exchange is almost identical to that for
 the AS exchange.  The primary difference is that encryption and
 decryption in the TGS exchange does not take place under the client's

Neuman, et al. Standards Track [Page 34] RFC 4120 Kerberos V5 July 2005

 key.  Instead, the session key from the TGT or renewable ticket, or
 sub-session key from an Authenticator is used.  As is the case for
 all application servers, expired tickets are not accepted by the TGS,
 so once a renewable or TGT expires, the client must use a separate
 exchange to obtain valid tickets.
 The TGS exchange consists of two messages: a request (KRB_TGS_REQ)
 from the client to the Kerberos Ticket-Granting Server, and a reply
 (KRB_TGS_REP or KRB_ERROR).  The KRB_TGS_REQ message includes
 information authenticating the client plus a request for credentials.
 The authentication information consists of the authentication header
 (KRB_AP_REQ), which includes the client's previously obtained
 ticket-granting, renewable, or invalid ticket.  In the TGT and proxy
 cases, the request MAY include one or more of the following: a list
 of network addresses, a collection of typed authorization data to be
 sealed in the ticket for authorization use by the application server,
 or additional tickets (the use of which are described later).  The
 TGS reply (KRB_TGS_REP) contains the requested credentials, encrypted
 in the session key from the TGT or renewable ticket, or, if present,
 in the sub-session key from the Authenticator (part of the
 authentication header).  The KRB_ERROR message contains an error code
 and text explaining what went wrong.  The KRB_ERROR message is not
 encrypted.  The KRB_TGS_REP message contains information that can be
 used to detect replays, and to associate it with the message to which
 it replies.  The KRB_ERROR message also contains information that can
 be used to associate it with the message to which it replies.  The
 same comments about integrity protection of KRB_ERROR messages
 mentioned in Section 3.1 apply to the TGS exchange.

3.3.1. Generation of KRB_TGS_REQ Message

 Before sending a request to the ticket-granting service, the client
 MUST determine in which realm the application server is believed to
 be registered.  This can be accomplished in several ways.  It might
 be known beforehand (since the realm is part of the principal
 identifier), it might be stored in a nameserver, or it might be
 obtained from a configuration file.  If the realm to be used is
 obtained from a nameserver, there is a danger of being spoofed if the
 nameservice providing the realm name is not authenticated.  This
 might result in the use of a realm that has been compromised, which
 would result in an attacker's ability to compromise the
 authentication of the application server to the client.
 If the client knows the service principal name and realm and it does
 not already possess a TGT for the appropriate realm, then one must be
 obtained.  This is first attempted by requesting a TGT for the
 destination realm from a Kerberos server for which the client
 possesses a TGT (by using the KRB_TGS_REQ message recursively).  The

Neuman, et al. Standards Track [Page 35] RFC 4120 Kerberos V5 July 2005

 Kerberos server MAY return a TGT for the desired realm, in which case
 one can proceed.  Alternatively, the Kerberos server MAY return a TGT
 for a realm that is 'closer' to the desired realm (further along the
 standard hierarchical path between the client's realm and the
 requested realm server's realm).  Note that in this case
 misconfiguration of the Kerberos servers may cause loops in the
 resulting authentication path, which the client should be careful to
 detect and avoid.
 If the Kerberos server returns a TGT for a realm 'closer' than the
 desired realm, the client MAY use local policy configuration to
 verify that the authentication path used is an acceptable one.
 Alternatively, a client MAY choose its own authentication path,
 rather than rely on the Kerberos server to select one.  In either
 case, any policy or configuration information used to choose or
 validate authentication paths, whether by the Kerberos server or by
 the client, MUST be obtained from a trusted source.
 When a client obtains a TGT that is 'closer' to the destination
 realm, the client MAY cache this ticket and reuse it in future
 KRB-TGS exchanges with services in the 'closer' realm.  However, if
 the client were to obtain a TGT for the 'closer' realm by starting at
 the initial KDC rather than as part of obtaining another ticket, then
 a shorter path to the 'closer' realm might be used.  This shorter
 path may be desirable because fewer intermediate KDCs would know the
 session key of the ticket involved.  For this reason, clients SHOULD
 evaluate whether they trust the realms transited in obtaining the
 'closer' ticket when making a decision to use the ticket in future.
 Once the client obtains a TGT for the appropriate realm, it
 determines which Kerberos servers serve that realm and contacts one
 of them.  The list might be obtained through a configuration file or
 network service, or it MAY be generated from the name of the realm.
 As long as the secret keys exchanged by realms are kept secret, only
 denial of service results from using a false Kerberos server.
 As in the AS exchange, the client MAY specify a number of options in
 the KRB_TGS_REQ message.  One of these options is the ENC-TKT-IN-SKEY
 option used for user-to-user authentication.  An overview of user-
 to-user authentication can be found in Section 3.7.  When generating
 the KRB_TGS_REQ message, this option indicates that the client is
 including a TGT obtained from the application server in the
 additional tickets field of the request and that the KDC SHOULD
 encrypt the ticket for the application server using the session key
 from this additional ticket, instead of a server key from the
 principal database.

Neuman, et al. Standards Track [Page 36] RFC 4120 Kerberos V5 July 2005

 The client prepares the KRB_TGS_REQ message, providing an
 authentication header as an element of the padata field, and
 including the same fields as used in the KRB_AS_REQ message along
 with several optional fields: the enc-authorizatfion-data field for
 application server use and additional tickets required by some
 options.
 In preparing the authentication header, the client can select a sub-
 session key under which the response from the Kerberos server will be
 encrypted.  If the client selects a sub-session key, care must be
 taken to ensure the randomness of the selected sub-session key.
 If the sub-session key is not specified, the session key from the TGT
 will be used.  If the enc-authorization-data is present, it MUST be
 encrypted in the sub-session key, if present, from the authenticator
 portion of the authentication header, or, if not present, by using
 the session key from the TGT.
 Once prepared, the message is sent to a Kerberos server for the
 destination realm.

3.3.2. Receipt of KRB_TGS_REQ Message

 The KRB_TGS_REQ message is processed in a manner similar to the
 KRB_AS_REQ message, but there are many additional checks to be
 performed.  First, the Kerberos server MUST determine which server
 the accompanying ticket is for, and it MUST select the appropriate
 key to decrypt it.  For a normal KRB_TGS_REQ message, it will be for
 the ticket-granting service, and the TGS's key will be used.  If the
 TGT was issued by another realm, then the appropriate inter-realm key
 MUST be used.  If (a) the accompanying ticket is not a TGT for the
 current realm, but is for an application server in the current realm,
 (b) the RENEW, VALIDATE, or PROXY options are specified in the
 request, and (c) the server for which a ticket is requested is the
 server named in the accompanying ticket, then the KDC will decrypt
 the ticket in the authentication header using the key of the server
 for which it was issued.  If no ticket can be found in the padata
 field, the KDC_ERR_PADATA_TYPE_NOSUPP error is returned.
 Once the accompanying ticket has been decrypted, the user-supplied
 checksum in the Authenticator MUST be verified against the contents
 of the request, and the message MUST be rejected if the checksums do
 not match (with an error code of KRB_AP_ERR_MODIFIED) or if the
 checksum is not collision-proof (with an error code of
 KRB_AP_ERR_INAPP_CKSUM).  If the checksum type is not supported, the
 KDC_ERR_SUMTYPE_NOSUPP error is returned.  If the authorization-data
 are present, they are decrypted using the sub-session key from the
 Authenticator.

Neuman, et al. Standards Track [Page 37] RFC 4120 Kerberos V5 July 2005

 If any of the decryptions indicate failed integrity checks, the
 KRB_AP_ERR_BAD_INTEGRITY error is returned.
 As discussed in Section 3.1.2, the KDC MUST send a valid KRB_TGS_REP
 message if it receives a KRB_TGS_REQ message identical to one it has
 recently processed.  However, if the authenticator is a replay, but
 the rest of the request is not identical, then the KDC SHOULD return
 KRB_AP_ERR_REPEAT.

3.3.3. Generation of KRB_TGS_REP Message

 The KRB_TGS_REP message shares its format with the KRB_AS_REP
 (KRB_KDC_REP), but with its type field set to KRB_TGS_REP.  The
 detailed specification is in Section 5.4.2.
 The response will include a ticket for the requested server or for a
 ticket granting server of an intermediate KDC to be contacted to
 obtain the requested ticket.  The Kerberos database is queried to
 retrieve the record for the appropriate server (including the key
 with which the ticket will be encrypted).  If the request is for a
 TGT for a remote realm, and if no key is shared with the requested
 realm, then the Kerberos server will select the realm 'closest' to
 the requested realm with which it does share a key and use that realm
 instead.  This is the only case where the response for the KDC will
 be for a different server than that requested by the client.
 By default, the address field, the client's name and realm, the list
 of transited realms, the time of initial authentication, the
 expiration time, and the authorization data of the newly-issued
 ticket will be copied from the TGT or renewable ticket.  If the
 transited field needs to be updated, but the transited type is not
 supported, the KDC_ERR_TRTYPE_NOSUPP error is returned.
 If the request specifies an endtime, then the endtime of the new
 ticket is set to the minimum of (a) that request, (b) the endtime
 from the TGT, and (c) the starttime of the TGT plus the minimum of
 the maximum life for the application server and the maximum life for
 the local realm (the maximum life for the requesting principal was
 already applied when the TGT was issued).  If the new ticket is to be
 a renewal, then the endtime above is replaced by the minimum of (a)
 the value of the renew_till field of the ticket and (b) the starttime
 for the new ticket plus the life (endtime-starttime) of the old
 ticket.
 If the FORWARDED option has been requested, then the resulting ticket
 will contain the addresses specified by the client.  This option will
 only be honored if the FORWARDABLE flag is set in the TGT.  The PROXY
 option is similar; the resulting ticket will contain the addresses

Neuman, et al. Standards Track [Page 38] RFC 4120 Kerberos V5 July 2005

 specified by the client.  It will be honored only if the PROXIABLE
 flag in the TGT is set.  The PROXY option will not be honored on
 requests for additional TGTs.
 If the requested starttime is absent, indicates a time in the past,
 or is within the window of acceptable clock skew for the KDC and the
 POSTDATE option has not been specified, then the starttime of the
 ticket is set to the authentication server's current time.  If it
 indicates a time in the future beyond the acceptable clock skew, but
 the POSTDATED option has not been specified or the MAY-POSTDATE flag
 is not set in the TGT, then the error KDC_ERR_CANNOT_POSTDATE is
 returned.  Otherwise, if the TGT has the MAY-POSTDATE flag set, then
 the resulting ticket will be postdated, and the requested starttime
 is checked against the policy of the local realm.  If acceptable, the
 ticket's starttime is set as requested, and the INVALID flag is set.
 The postdated ticket MUST be validated before use by presenting it to
 the KDC after the starttime has been reached.  However, in no case
 may the starttime, endtime, or renew-till time of a newly-issued
 postdated ticket extend beyond the renew-till time of the TGT.
 If the ENC-TKT-IN-SKEY option has been specified and an additional
 ticket has been included in the request, it indicates that the client
 is using user-to-user authentication to prove its identity to a
 server that does not have access to a persistent key.  Section 3.7
 describes the effect of this option on the entire Kerberos protocol.
 When generating the KRB_TGS_REP message, this option in the
 KRB_TGS_REQ message tells the KDC to decrypt the additional ticket
 using the key for the server to which the additional ticket was
 issued and to verify that it is a TGT.  If the name of the requested
 server is missing from the request, the name of the client in the
 additional ticket will be used.  Otherwise, the name of the requested
 server will be compared to the name of the client in the additional
 ticket.  If it is different, the request will be rejected.  If the
 request succeeds, the session key from the additional ticket will be
 used to encrypt the new ticket that is issued instead of using the
 key of the server for which the new ticket will be used.
 If (a) the name of the server in the ticket that is presented to the
 KDC as part of the authentication header is not that of the TGS
 itself, (b) the server is registered in the realm of the KDC, and (c)
 the RENEW option is requested, then the KDC will verify that the
 RENEWABLE flag is set in the ticket, that the INVALID flag is not set
 in the ticket, and that the renew_till time is still in the future.
 If the VALIDATE option is requested, the KDC will check that the
 starttime has passed and that the INVALID flag is set.  If the PROXY
 option is requested, then the KDC will check that the PROXIABLE flag

Neuman, et al. Standards Track [Page 39] RFC 4120 Kerberos V5 July 2005

 is set in the ticket.  If the tests succeed and the ticket passes the
 hotlist check described in the next section, the KDC will issue the
 appropriate new ticket.
 The ciphertext part of the response in the KRB_TGS_REP message is
 encrypted in the sub-session key from the Authenticator, if present,
 or in the session key from the TGT.  It is not encrypted using the
 client's secret key.  Furthermore, the client's key's expiration date
 and the key version number fields are left out since these values are
 stored along with the client's database record, and that record is
 not needed to satisfy a request based on a TGT.

3.3.3.1. Checking for Revoked Tickets

 Whenever a request is made to the ticket-granting server, the
 presented ticket(s) is (are) checked against a hot-list of tickets
 that have been canceled.  This hot-list might be implemented by
 storing a range of issue timestamps for 'suspect tickets'; if a
 presented ticket had an authtime in that range, it would be rejected.
 In this way, a stolen TGT or renewable ticket cannot be used to gain
 additional tickets (renewals or otherwise) once the theft has been
 reported to the KDC for the realm in which the server resides.  Any
 normal ticket obtained before it was reported stolen will still be
 valid (because tickets require no interaction with the KDC), but only
 until its normal expiration time.  If TGTs have been issued for
 cross-realm authentication, use of the cross-realm TGT will not be
 affected unless the hot-list is propagated to the KDCs for the realms
 for which such cross-realm tickets were issued.

3.3.3.2. Encoding the Transited Field

 If the identity of the server in the TGT that is presented to the KDC
 as part of the authentication header is that of the ticket-granting
 service, but the TGT was issued from another realm, the KDC will look
 up the inter-realm key shared with that realm and use that key to
 decrypt the ticket.  If the ticket is valid, then the KDC will honor
 the request, subject to the constraints outlined above in the section
 describing the AS exchange.  The realm part of the client's identity
 will be taken from the TGT.  The name of the realm that issued the
 TGT, if it is not the realm of the client principal, will be added to
 the transited field of the ticket to be issued.  This is accomplished
 by reading the transited field from the TGT (which is treated as an
 unordered set of realm names), adding the new realm to the set, and
 then constructing and writing out its encoded (shorthand) form (this
 may involve a rearrangement of the existing encoding).
 Note that the ticket-granting service does not add the name of its
 own realm.  Instead, its responsibility is to add the name of the

Neuman, et al. Standards Track [Page 40] RFC 4120 Kerberos V5 July 2005

 previous realm.  This prevents a malicious Kerberos server from
 intentionally leaving out its own name (it could, however, omit other
 realms' names).
 The names of neither the local realm nor the principal's realm are to
 be included in the transited field.  They appear elsewhere in the
 ticket and both are known to have taken part in authenticating the
 principal.  Because the endpoints are not included, both local and
 single-hop inter-realm authentication result in a transited field
 that is empty.
 Because this field has the name of each transited realm added to it,
 it might potentially be very long.  To decrease the length of this
 field, its contents are encoded.  The initially supported encoding is
 optimized for the normal case of inter-realm communication: a
 hierarchical arrangement of realms using either domain or X.500 style
 realm names.  This encoding (called DOMAIN-X500-COMPRESS) is now
 described.
 Realm names in the transited field are separated by a ",".  The ",",
 "\", trailing "."s, and leading spaces (" ") are special characters,
 and if they are part of a realm name, they MUST be quoted in the
 transited field by preceding them with a "\".
 A realm name ending with a "." is interpreted as being prepended to
 the previous realm.  For example, we can encode traversal of EDU,
 MIT.EDU, ATHENA.MIT.EDU, WASHINGTON.EDU, and CS.WASHINGTON.EDU as:
    "EDU,MIT.,ATHENA.,WASHINGTON.EDU,CS.".
 Note that if either ATHENA.MIT.EDU, or CS.WASHINGTON.EDU were
 endpoints, they would not be included in this field, and we would
 have:
    "EDU,MIT.,WASHINGTON.EDU"
 A realm name beginning with a "/" is interpreted as being appended to
 the previous realm.  For the purpose of appending, the realm
 preceding the first listed realm is considered the null realm ("").
 If a realm name beginning with a "/" is to stand by itself, then it
 SHOULD be preceded by a space (" ").  For example, we can encode
 traversal of /COM/HP/APOLLO, /COM/HP, /COM, and /COM/DEC as:
    "/COM,/HP,/APOLLO, /COM/DEC".
 As in the example above, if /COM/HP/APOLLO and /COM/DEC were
 endpoints, they would not be included in this field, and we would
 have:

Neuman, et al. Standards Track [Page 41] RFC 4120 Kerberos V5 July 2005

    "/COM,/HP"
 A null subfield preceding or following a "," indicates that all
 realms between the previous realm and the next realm have been
 traversed.  For the purpose of interpreting null subfields, the
 client's realm is considered to precede those in the transited field,
 and the server's realm is considered to follow them.  Thus, "," means
 that all realms along the path between the client and the server have
 been traversed.  ",EDU, /COM," means that all realms from the
 client's realm up to EDU (in a domain style hierarchy) have been
 traversed, and that everything from /COM down to the server's realm
 in an X.500 style has also been traversed.  This could occur if the
 EDU realm in one hierarchy shares an inter-realm key directly with
 the /COM realm in another hierarchy.

3.3.4. Receipt of KRB_TGS_REP Message

 When the KRB_TGS_REP is received by the client, it is processed in
 the same manner as the KRB_AS_REP processing described above.  The
 primary difference is that the ciphertext part of the response must
 be decrypted using the sub-session key from the Authenticator, if it
 was specified in the request, or the session key from the TGT, rather
 than the client's secret key.  The server name returned in the reply
 is the true principal name of the service.

3.4. The KRB_SAFE Exchange

 The KRB_SAFE message MAY be used by clients requiring the ability to
 detect modifications of messages they exchange.  It achieves this by
 including a keyed collision-proof checksum of the user data and some
 control information.  The checksum is keyed with an encryption key
 (usually the last key negotiated via subkeys, or the session key if
 no negotiation has occurred).

3.4.1. Generation of a KRB_SAFE Message

 When an application wishes to send a KRB_SAFE message, it collects
 its data and the appropriate control information and computes a
 checksum over them.  The checksum algorithm should be the keyed
 checksum mandated to be implemented along with the crypto system used
 for the sub-session or session key.  The checksum is generated using
 the sub-session key, if present, or the session key.  Some
 implementations use a different checksum algorithm for the KRB_SAFE
 messages, but doing so in an interoperable manner is not always
 possible.
 The control information for the KRB_SAFE message includes both a
 timestamp and a sequence number.  The designer of an application

Neuman, et al. Standards Track [Page 42] RFC 4120 Kerberos V5 July 2005

 using the KRB_SAFE message MUST choose at least one of the two
 mechanisms.  This choice SHOULD be based on the needs of the
 application protocol.
 Sequence numbers are useful when all messages sent will be received
 by one's peer.  Connection state is presently required to maintain
 the session key, so maintaining the next sequence number should not
 present an additional problem.
 If the application protocol is expected to tolerate lost messages
 without their being resent, the use of the timestamp is the
 appropriate replay detection mechanism.  Using timestamps is also the
 appropriate mechanism for multi-cast protocols in which all of one's
 peers share a common sub-session key, but some messages will be sent
 to a subset of one's peers.
 After computing the checksum, the client then transmits the
 information and checksum to the recipient in the message format
 specified in Section 5.6.1.

3.4.2. Receipt of KRB_SAFE Message

 When an application receives a KRB_SAFE message, it verifies it as
 follows.  If any error occurs, an error code is reported for use by
 the application.
 The message is first checked by verifying that the protocol version
 and type fields match the current version and KRB_SAFE, respectively.
 A mismatch generates a KRB_AP_ERR_BADVERSION or KRB_AP_ERR_MSG_TYPE
 error.  The application verifies that the checksum used is a
 collision-proof keyed checksum that uses keys compatible with the
 sub-session or session key as appropriate (or with the application
 key derived from the session or sub-session keys).  If it is not, a
 KRB_AP_ERR_INAPP_CKSUM error is generated.  The sender's address MUST
 be included in the control information; the recipient verifies that
 the operating system's report of the sender's address matches the
 sender's address in the message, and (if a recipient address is
 specified or the recipient requires an address) that one of the
 recipient's addresses appears as the recipient's address in the
 message.  To work with network address translation, senders MAY use
 the directional address type specified in Section 8.1 for the sender
 address and not include recipient addresses.  A failed match for
 either case generates a KRB_AP_ERR_BADADDR error.  Then the timestamp
 and usec and/or the sequence number fields are checked.  If timestamp
 and usec are expected and not present, or if they are present but not
 current, the KRB_AP_ERR_SKEW error is generated.  Timestamps are not
 required to be strictly ordered; they are only required to be in the
 skew window.  If the server name, along with the client name, time,

Neuman, et al. Standards Track [Page 43] RFC 4120 Kerberos V5 July 2005

 and microsecond fields from the Authenticator match any recently-seen
 (sent or received) such tuples, the KRB_AP_ERR_REPEAT error is
 generated.  If an incorrect sequence number is included, or if a
 sequence number is expected but not present, the KRB_AP_ERR_BADORDER
 error is generated.  If neither a time-stamp and usec nor a sequence
 number is present, a KRB_AP_ERR_MODIFIED error is generated.
 Finally, the checksum is computed over the data and control
 information, and if it doesn't match the received checksum, a
 KRB_AP_ERR_MODIFIED error is generated.
 If all the checks succeed, the application is assured that the
 message was generated by its peer and was not modified in transit.
 Implementations SHOULD accept any checksum algorithm they implement
 that has both adequate security and keys compatible with the sub-
 session or session key.  Unkeyed or non-collision-proof checksums are
 not suitable for this use.

3.5. The KRB_PRIV Exchange

 The KRB_PRIV message MAY be used by clients requiring confidentiality
 and the ability to detect modifications of exchanged messages.  It
 achieves this by encrypting the messages and adding control
 information.

3.5.1. Generation of a KRB_PRIV Message

 When an application wishes to send a KRB_PRIV message, it collects
 its data and the appropriate control information (specified in
 Section 5.7.1) and encrypts them under an encryption key (usually the
 last key negotiated via subkeys, or the session key if no negotiation
 has occurred).  As part of the control information, the client MUST
 choose to use either a timestamp or a sequence number (or both); see
 the discussion in Section 3.4.1 for guidelines on which to use.
 After the user data and control information are encrypted, the client
 transmits the ciphertext and some 'envelope' information to the
 recipient.

3.5.2. Receipt of KRB_PRIV Message

 When an application receives a KRB_PRIV message, it verifies it as
 follows.  If any error occurs, an error code is reported for use by
 the application.
 The message is first checked by verifying that the protocol version
 and type fields match the current version and KRB_PRIV, respectively.
 A mismatch generates a KRB_AP_ERR_BADVERSION or KRB_AP_ERR_MSG_TYPE
 error.  The application then decrypts the ciphertext and processes

Neuman, et al. Standards Track [Page 44] RFC 4120 Kerberos V5 July 2005

 the resultant plaintext.  If decryption shows that the data has been
 modified, a KRB_AP_ERR_BAD_INTEGRITY error is generated.
 The sender's address MUST be included in the control information; the
 recipient verifies that the operating system's report of the sender's
 address matches the sender's address in the message.  If a recipient
 address is specified or the recipient requires an address, then one
 of the recipient's addresses MUST also appear as the recipient's
 address in the message.  Where a sender's or receiver's address might
 not otherwise match the address in a message because of network
 address translation, an application MAY be written to use addresses
 of the directional address type in place of the actual network
 address.
 A failed match for either case generates a KRB_AP_ERR_BADADDR error.
 To work with network address translation, implementations MAY use the
 directional address type defined in Section 7.1 for the sender
 address and include no recipient address.
 Next the timestamp and usec and/or the sequence number fields are
 checked.  If timestamp and usec are expected and not present, or if
 they are present but not current, the KRB_AP_ERR_SKEW error is
 generated.  If the server name, along with the client name, time, and
 microsecond fields from the Authenticator match any such recently-
 seen tuples, the KRB_AP_ERR_REPEAT error is generated.  If an
 incorrect sequence number is included, or if a sequence number is
 expected but not present, the KRB_AP_ERR_BADORDER error is generated.
 If neither a time-stamp and usec nor a sequence number is present, a
 KRB_AP_ERR_MODIFIED error is generated.
 If all the checks succeed, the application can assume the message was
 generated by its peer and was securely transmitted (without intruders
 seeing the unencrypted contents).

3.6. The KRB_CRED Exchange

 The KRB_CRED message MAY be used by clients requiring the ability to
 send Kerberos credentials from one host to another.  It achieves this
 by sending the tickets together with encrypted data containing the
 session keys and other information associated with the tickets.

3.6.1. Generation of a KRB_CRED Message

 When an application wishes to send a KRB_CRED message, it first
 (using the KRB_TGS exchange) obtains credentials to be sent to the
 remote host.  It then constructs a KRB_CRED message using the ticket
 or tickets so obtained, placing the session key needed to use each

Neuman, et al. Standards Track [Page 45] RFC 4120 Kerberos V5 July 2005

 ticket in the key field of the corresponding KrbCredInfo sequence of
 the encrypted part of the KRB_CRED message.
 Other information associated with each ticket and obtained during the
 KRB_TGS exchange is also placed in the corresponding KrbCredInfo
 sequence in the encrypted part of the KRB_CRED message.  The current
 time and, if they are specifically required by the application, the
 nonce, s-address, and r-address fields are placed in the encrypted
 part of the KRB_CRED message, which is then encrypted under an
 encryption key previously exchanged in the KRB_AP exchange (usually
 the last key negotiated via subkeys, or the session key if no
 negotiation has occurred).
 Implementation note: When constructing a KRB_CRED message for
 inclusion in a GSSAPI initial context token, the MIT implementation
 of Kerberos will not encrypt the KRB_CRED message if the session key
 is a DES or triple DES key.  For interoperability with MIT, the
 Microsoft implementation will not encrypt the KRB_CRED in a GSSAPI
 token if it is using a DES session key.  Starting at version 1.2.5,
 MIT Kerberos can receive and decode either encrypted or unencrypted
 KRB_CRED tokens in the GSSAPI exchange.  The Heimdal implementation
 of Kerberos can also accept either encrypted or unencrypted KRB_CRED
 messages.  Since the KRB_CRED message in a GSSAPI token is encrypted
 in the authenticator, the MIT behavior does not present a security
 problem, although it is a violation of the Kerberos specification.

3.6.2. Receipt of KRB_CRED Message

 When an application receives a KRB_CRED message, it verifies it.  If
 any error occurs, an error code is reported for use by the
 application.  The message is verified by checking that the protocol
 version and type fields match the current version and KRB_CRED,
 respectively.  A mismatch generates a KRB_AP_ERR_BADVERSION or
 KRB_AP_ERR_MSG_TYPE error.  The application then decrypts the
 ciphertext and processes the resultant plaintext.  If decryption
 shows the data to have been modified, a KRB_AP_ERR_BAD_INTEGRITY
 error is generated.
 If present or required, the recipient MAY verify that the operating
 system's report of the sender's address matches the sender's address
 in the message, and that one of the recipient's addresses appears as
 the recipient's address in the message.  The address check does not
 provide any added security, since the address, if present, has
 already been checked in the KRB_AP_REQ message and there is not any
 benefit to be gained by an attacker in reflecting a KRB_CRED message
 back to its originator.  Thus, the recipient MAY ignore the address
 even if it is present in order to work better in Network Address
 Translation (NAT) environments.  A failed match for either case

Neuman, et al. Standards Track [Page 46] RFC 4120 Kerberos V5 July 2005

 generates a KRB_AP_ERR_BADADDR error.  Recipients MAY skip the
 address check, as the KRB_CRED message cannot generally be reflected
 back to the originator.  The timestamp and usec fields (and the nonce
 field, if required) are checked next.  If the timestamp and usec are
 not present, or if they are present but not current, the
 KRB_AP_ERR_SKEW error is generated.
 If all the checks succeed, the application stores each of the new
 tickets in its credentials cache together with the session key and
 other information in the corresponding KrbCredInfo sequence from the
 encrypted part of the KRB_CRED message.

3.7. User-to-User Authentication Exchanges

 User-to-User authentication provides a method to perform
 authentication when the verifier does not have a access to long-term
 service key.  This might be the case when running a server (for
 example, a window server) as a user on a workstation.  In such cases,
 the server may have access to the TGT obtained when the user logged
 in to the workstation, but because the server is running as an
 unprivileged user, it might not have access to system keys.  Similar
 situations may arise when running peer-to-peer applications.
                           Summary
     Message direction                    Message type     Sections
     0. Message from application server   Not specified
     1. Client to Kerberos                KRB_TGS_REQ      3.3 & 5.4.1
     2. Kerberos to client                KRB_TGS_REP or   3.3 & 5.4.2
                                          KRB_ERROR        5.9.1
     3. Client to application server      KRB_AP_REQ       3.2 & 5.5.1
 To address this problem, the Kerberos protocol allows the client to
 request that the ticket issued by the KDC be encrypted using a
 session key from a TGT issued to the party that will verify the
 authentication.  This TGT must be obtained from the verifier by means
 of an exchange external to the Kerberos protocol, usually as part of
 the application protocol.  This message is shown in the summary above
 as message 0.  Note that because the TGT is encrypted in the KDC's
 secret key, it cannot be used for authentication without possession
 of the corresponding secret key.  Furthermore, because the verifier
 does not reveal the corresponding secret key, providing a copy of the
 verifier's TGT does not allow impersonation of the verifier.
 Message 0 in the table above represents an application-specific
 negotiation between the client and server, at the end of which both
 have determined that they will use user-to-user authentication, and
 the client has obtained the server's TGT.

Neuman, et al. Standards Track [Page 47] RFC 4120 Kerberos V5 July 2005

 Next, the client includes the server's TGT as an additional ticket in
 its KRB_TGS_REQ request to the KDC (message 1 in the table above) and
 specifies the ENC-TKT-IN-SKEY option in its request.
 If validated according to the instructions in Section 3.3.3, the
 application ticket returned to the client (message 2 in the table
 above) will be encrypted using the session key from the additional
 ticket and the client will note this when it uses or stores the
 application ticket.
 When contacting the server using a ticket obtained for user-to-user
 authentication (message 3 in the table above), the client MUST
 specify the USE-SESSION-KEY flag in the ap-options field.  This tells
 the application server to use the session key associated with its TGT
 to decrypt the server ticket provided in the application request.

4. Encryption and Checksum Specifications

 The Kerberos protocols described in this document are designed to
 encrypt messages of arbitrary sizes, using stream or block encryption
 ciphers.  Encryption is used to prove the identities of the network
 entities participating in message exchanges.  The Key Distribution
 Center for each realm is trusted by all principals registered in that
 realm to store a secret key in confidence.  Proof of knowledge of
 this secret key is used to verify the authenticity of a principal.
 The KDC uses the principal's secret key (in the AS exchange) or a
 shared session key (in the TGS exchange) to encrypt responses to
 ticket requests; the ability to obtain the secret key or session key
 implies the knowledge of the appropriate keys and the identity of the
 KDC.  The ability of a principal to decrypt the KDC response and to
 present a Ticket and a properly formed Authenticator (generated with
 the session key from the KDC response) to a service verifies the
 identity of the principal; likewise the ability of the service to
 extract the session key from the Ticket and to prove its knowledge
 thereof in a response verifies the identity of the service.
 [RFC3961] defines a framework for defining encryption and checksum
 mechanisms for use with Kerberos.  It also defines several such
 mechanisms, and more may be added in future updates to that document.
 The string-to-key operation provided by [RFC3961] is used to produce
 a long-term key for a principal (generally for a user).  The default
 salt string, if none is provided via pre-authentication data, is the
 concatenation of the principal's realm and name components, in order,
 with no separators.  Unless it is indicated otherwise, the default
 string-to-key opaque parameter set as defined in [RFC3961] is used.

Neuman, et al. Standards Track [Page 48] RFC 4120 Kerberos V5 July 2005

 Encrypted data, keys, and checksums are transmitted using the
 EncryptedData, EncryptionKey, and Checksum data objects defined in
 Section 5.2.9.  The encryption, decryption, and checksum operations
 described in this document use the corresponding encryption,
 decryption, and get_mic operations described in [RFC3961], with
 implicit "specific key" generation using the "key usage" values
 specified in the description of each EncryptedData or Checksum object
 to vary the key for each operation.  Note that in some cases, the
 value to be used is dependent on the method of choosing the key or
 the context of the message.
 Key usages are unsigned 32-bit integers; zero is not permitted.  The
 key usage values for encrypting or checksumming Kerberos messages are
 indicated in Section 5 along with the message definitions.  The key
 usage values 512-1023 are reserved for uses internal to a Kerberos
 implementation.  (For example, seeding a pseudo-random number
 generator with a value produced by encrypting something with a
 session key and a key usage value not used for any other purpose.)
 Key usage values between 1024 and 2047 (inclusive) are reserved for
 application use; applications SHOULD use even values for encryption
 and odd values for checksums within this range.  Key usage values are
 also summarized in a table in Section 7.5.1.
 There might exist other documents that define protocols in terms of
 the RFC 1510 encryption types or checksum types.  These documents
 would not know about key usages.  In order that these specifications
 continue to be meaningful until they are updated, if no key usage
 values are specified, then key usages 1024 and 1025 must be used to
 derive keys for encryption and checksums, respectively.  (This does
 not apply to protocols that do their own encryption independent of
 this framework, by directly using the key resulting from the Kerberos
 authentication exchange.)  New protocols defined in terms of the
 Kerberos encryption and checksum types SHOULD use their own key usage
 values.
 Unless it is indicated otherwise, no cipher state chaining is done
 from one encryption operation to another.
 Implementation note: Although it is not recommended, some application
 protocols will continue to use the key data directly, even if only in
 currently existing protocol specifications.  An implementation
 intended to support general Kerberos applications may therefore need
 to make key data available, as well as the attributes and operations
 described in [RFC3961].  One of the more common reasons for directly
 performing encryption is direct control over negotiation and
 selection of a "sufficiently strong" encryption algorithm (in the
 context of a given application).  Although Kerberos does not directly
 provide a facility for negotiating encryption types between the

Neuman, et al. Standards Track [Page 49] RFC 4120 Kerberos V5 July 2005

 application client and server, there are approaches for using
 Kerberos to facilitate this negotiation.  For example, a client may
 request only "sufficiently strong" session key types from the KDC and
 expect that any type returned by the KDC will be understood and
 supported by the application server.

5. Message Specifications

 The ASN.1 collected here should be identical to the contents of
 Appendix A.  In the case of a conflict, the contents of Appendix A
 shall take precedence.
 The Kerberos protocol is defined here in terms of Abstract Syntax
 Notation One (ASN.1) [X680], which provides a syntax for specifying
 both the abstract layout of protocol messages as well as their
 encodings.  Implementors not utilizing an existing ASN.1 compiler or
 support library are cautioned to understand the actual ASN.1
 specification thoroughly in order to ensure correct implementation
 behavior.  There is more complexity in the notation than is
 immediately obvious, and some tutorials and guides to ASN.1 are
 misleading or erroneous.
 Note that in several places, changes to abstract types from RFC 1510
 have been made.  This is in part to address widespread assumptions
 that various implementors have made, in some cases resulting in
 unintentional violations of the ASN.1 standard.  These are clearly
 flagged where they occur.  The differences between the abstract types
 in RFC 1510 and abstract types in this document can cause
 incompatible encodings to be emitted when certain encoding rules,
 e.g., the Packed Encoding Rules (PER), are used.  This theoretical
 incompatibility should not be relevant for Kerberos, since Kerberos
 explicitly specifies the use of the Distinguished Encoding Rules
 (DER).  It might be an issue for protocols seeking to use Kerberos
 types with other encoding rules.  (This practice is not recommended.)
 With very few exceptions (most notably the usages of BIT STRING), the
 encodings resulting from using the DER remain identical between the
 types defined in RFC 1510 and the types defined in this document.
 The type definitions in this section assume an ASN.1 module
 definition of the following form:

Neuman, et al. Standards Track [Page 50] RFC 4120 Kerberos V5 July 2005

 KerberosV5Spec2 {
         iso(1) identified-organization(3) dod(6) internet(1)
         security(5) kerberosV5(2) modules(4) krb5spec2(2)
 } DEFINITIONS EXPLICIT TAGS ::= BEGIN
  1. - rest of definitions here
 END
 This specifies that the tagging context for the module will be
 explicit and non-automatic.
 Note that in some other publications (such as [RFC1510] and
 [RFC1964]), the "dod" portion of the object identifier is erroneously
 specified as having the value "5".  In the case of RFC 1964, use of
 the "correct" OID value would result in a change in the wire
 protocol; therefore, it remains unchanged for now.
 Note that elsewhere in this document, nomenclature for various
 message types is inconsistent, but it largely follows C language
 conventions, including use of underscore (_) characters and all-caps
 spelling of names intended to be numeric constants.  Also, in some
 places, identifiers (especially those referring to constants) are
 written in all-caps in order to distinguish them from surrounding
 explanatory text.
 The ASN.1 notation does not permit underscores in identifiers, so in
 actual ASN.1 definitions, underscores are replaced with hyphens (-).
 Additionally, structure member names and defined values in ASN.1 MUST
 begin with a lowercase letter, whereas type names MUST begin with an
 uppercase letter.

5.1. Specific Compatibility Notes on ASN.1

 For compatibility purposes, implementors should heed the following
 specific notes regarding the use of ASN.1 in Kerberos.  These notes
 do not describe deviations from standard usage of ASN.1.  The purpose
 of these notes is instead to describe some historical quirks and
 non-compliance of various implementations, as well as historical
 ambiguities, which, although they are valid ASN.1, can lead to
 confusion during implementation.

5.1.1. ASN.1 Distinguished Encoding Rules

 The encoding of Kerberos protocol messages shall obey the
 Distinguished Encoding Rules (DER) of ASN.1 as described in [X690].
 Some implementations (believed primarily to be those derived from DCE
 1.1 and earlier) are known to use the more general Basic Encoding

Neuman, et al. Standards Track [Page 51] RFC 4120 Kerberos V5 July 2005

 Rules (BER); in particular, these implementations send indefinite
 encodings of lengths.  Implementations MAY accept such encodings in
 the interest of backward compatibility, though implementors are
 warned that decoding fully-general BER is fraught with peril.

5.1.2. Optional Integer Fields

 Some implementations do not internally distinguish between an omitted
 optional integer value and a transmitted value of zero.  The places
 in the protocol where this is relevant include various microseconds
 fields, nonces, and sequence numbers.  Implementations SHOULD treat
 omitted optional integer values as having been transmitted with a
 value of zero, if the application is expecting this.

5.1.3. Empty SEQUENCE OF Types

 There are places in the protocol where a message contains a SEQUENCE
 OF type as an optional member.  This can result in an encoding that
 contains an empty SEQUENCE OF encoding.  The Kerberos protocol does
 not semantically distinguish between an absent optional SEQUENCE OF
 type and a present optional but empty SEQUENCE OF type.
 Implementations SHOULD NOT send empty SEQUENCE OF encodings that are
 marked OPTIONAL, but SHOULD accept them as being equivalent to an
 omitted OPTIONAL type.  In the ASN.1 syntax describing Kerberos
 messages, instances of these problematic optional SEQUENCE OF types
 are indicated with a comment.

5.1.4. Unrecognized Tag Numbers

 Future revisions to this protocol may include new message types with
 different APPLICATION class tag numbers.  Such revisions should
 protect older implementations by only sending the message types to
 parties that are known to understand them; e.g., by means of a flag
 bit set by the receiver in a preceding request.  In the interest of
 robust error handling, implementations SHOULD gracefully handle
 receiving a message with an unrecognized tag anyway, and return an
 error message, if appropriate.
 In particular, KDCs SHOULD return KRB_AP_ERR_MSG_TYPE if the
 incorrect tag is sent over a TCP transport.  The KDCs SHOULD NOT
 respond to messages received with an unknown tag over UDP transport
 in order to avoid denial of service attacks.  For non-KDC
 applications, the Kerberos implementation typically indicates an
 error to the application which takes appropriate steps based on the
 application protocol.

Neuman, et al. Standards Track [Page 52] RFC 4120 Kerberos V5 July 2005

5.1.5. Tag Numbers Greater Than 30

 A naive implementation of a DER ASN.1 decoder may experience problems
 with ASN.1 tag numbers greater than 30, due to such tag numbers being
 encoded using more than one byte.  Future revisions of this protocol
 may utilize tag numbers greater than 30, and implementations SHOULD
 be prepared to gracefully return an error, if appropriate, when they
 do not recognize the tag.

5.2. Basic Kerberos Types

 This section defines a number of basic types that are potentially
 used in multiple Kerberos protocol messages.

5.2.1. KerberosString

 The original specification of the Kerberos protocol in RFC 1510 uses
 GeneralString in numerous places for human-readable string data.
 Historical implementations of Kerberos cannot utilize the full power
 of GeneralString.  This ASN.1 type requires the use of designation
 and invocation escape sequences as specified in ISO-2022/ECMA-35
 [ISO-2022/ECMA-35] to switch character sets, and the default
 character set that is designated as G0 is the ISO-646/ECMA-6
 [ISO-646/ECMA-6] International Reference Version (IRV) (a.k.a. U.S.
 ASCII), which mostly works.
 ISO-2022/ECMA-35 defines four character-set code elements (G0..G3)
 and two Control-function code elements (C0..C1).  DER prohibits the
 designation of character sets as any but the G0 and C0 sets.
 Unfortunately, this seems to have the side effect of prohibiting the
 use of ISO-8859 (ISO Latin) [ISO-8859] character sets or any other
 character sets that utilize a 96-character set, as ISO-2022/ECMA-35
 prohibits designating them as the G0 code element.  This side effect
 is being investigated in the ASN.1 standards community.
 In practice, many implementations treat GeneralStrings as if they
 were 8-bit strings of whichever character set the implementation
 defaults to, without regard to correct usage of character-set
 designation escape sequences.  The default character set is often
 determined by the current user's operating system-dependent locale.
 At least one major implementation places unescaped UTF-8 encoded
 Unicode characters in the GeneralString.  This failure to adhere to
 the GeneralString specifications results in interoperability issues
 when conflicting character encodings are utilized by the Kerberos
 clients, services, and KDC.

Neuman, et al. Standards Track [Page 53] RFC 4120 Kerberos V5 July 2005

 This unfortunate situation is the result of improper documentation of
 the restrictions of the ASN.1 GeneralString type in prior Kerberos
 specifications.
 The new (post-RFC 1510) type KerberosString, defined below, is a
 GeneralString that is constrained to contain only characters in
 IA5String.
    KerberosString  ::= GeneralString (IA5String)
 In general, US-ASCII control characters should not be used in
 KerberosString.  Control characters SHOULD NOT be used in principal
 names or realm names.
 For compatibility, implementations MAY choose to accept GeneralString
 values that contain characters other than those permitted by
 IA5String, but they should be aware that character set designation
 codes will likely be absent, and that the encoding should probably be
 treated as locale-specific in almost every way.  Implementations MAY
 also choose to emit GeneralString values that are beyond those
 permitted by IA5String, but they should be aware that doing so is
 extraordinarily risky from an interoperability perspective.
 Some existing implementations use GeneralString to encode unescaped
 locale-specific characters.  This is a violation of the ASN.1
 standard.  Most of these implementations encode US-ASCII in the
 left-hand half, so as long as the implementation transmits only
 US-ASCII, the ASN.1 standard is not violated in this regard.  As soon
 as such an implementation encodes unescaped locale-specific
 characters with the high bit set, it violates the ASN.1 standard.
 Other implementations have been known to use GeneralString to contain
 a UTF-8 encoding.  This also violates the ASN.1 standard, since UTF-8
 is a different encoding, not a 94 or 96 character "G" set as defined
 by ISO 2022.  It is believed that these implementations do not even
 use the ISO 2022 escape sequence to change the character encoding.
 Even if implementations were to announce the encoding change by using
 that escape sequence, the ASN.1 standard prohibits the use of any
 escape sequences other than those used to designate/invoke "G" or "C"
 sets allowed by GeneralString.
 Future revisions to this protocol will almost certainly allow for a
 more interoperable representation of principal names, probably
 including UTF8String.
 Note that applying a new constraint to a previously unconstrained
 type constitutes creation of a new ASN.1 type.  In this particular
 case, the change does not result in a changed encoding under DER.

Neuman, et al. Standards Track [Page 54] RFC 4120 Kerberos V5 July 2005

5.2.2. Realm and PrincipalName

 Realm           ::= KerberosString
 PrincipalName   ::= SEQUENCE {
         name-type       [0] Int32,
         name-string     [1] SEQUENCE OF KerberosString
 }
 Kerberos realm names are encoded as KerberosStrings.  Realms shall
 not contain a character with the code 0 (the US-ASCII NUL).  Most
 realms will usually consist of several components separated by
 periods (.), in the style of Internet Domain Names, or separated by
 slashes (/), in the style of X.500 names.  Acceptable forms for realm
 names are specified in Section 6.1.  A PrincipalName is a typed
 sequence of components consisting of the following subfields:
 name-type
    This field specifies the type of name that follows.  Pre-defined
    values for this field are specified in Section 6.2.  The name-type
    SHOULD be treated as a hint.  Ignoring the name type, no two names
    can be the same (i.e., at least one of the components, or the
    realm, must be different).
 name-string
    This field encodes a sequence of components that form a name, each
    component encoded as a KerberosString.  Taken together, a
    PrincipalName and a Realm form a principal identifier.  Most
    PrincipalNames will have only a few components (typically one or
    two).

5.2.3. KerberosTime

 KerberosTime    ::= GeneralizedTime -- with no fractional seconds
 The timestamps used in Kerberos are encoded as GeneralizedTimes.  A
 KerberosTime value shall not include any fractional portions of the
 seconds.  As required by the DER, it further shall not include any
 separators, and it shall specify the UTC time zone (Z).  Example: The
 only valid format for UTC time 6 minutes, 27 seconds after 9 pm on 6
 November 1985 is 19851106210627Z.

5.2.4. Constrained Integer Types

 Some integer members of types SHOULD be constrained to values
 representable in 32 bits, for compatibility with reasonable
 implementation limits.

Neuman, et al. Standards Track [Page 55] RFC 4120 Kerberos V5 July 2005

 Int32           ::= INTEGER (-2147483648..2147483647)
                     -- signed values representable in 32 bits
 UInt32          ::= INTEGER (0..4294967295)
                     -- unsigned 32 bit values
 Microseconds    ::= INTEGER (0..999999)
                     -- microseconds
 Although this results in changes to the abstract types from the RFC
 1510 version, the encoding in DER should be unaltered.  Historical
 implementations were typically limited to 32-bit integer values
 anyway, and assigned numbers SHOULD fall in the space of integer
 values representable in 32 bits in order to promote interoperability
 anyway.
 Several integer fields in messages are constrained to fixed values.
 pvno
    also TKT-VNO or AUTHENTICATOR-VNO, this recurring field is always
    the constant integer 5.  There is no easy way to make this field
    into a useful protocol version number, so its value is fixed.
 msg-type
    this integer field is usually identical to the application tag
    number of the containing message type.

5.2.5. HostAddress and HostAddresses

 HostAddress     ::= SEQUENCE  {
         addr-type       [0] Int32,
         address         [1] OCTET STRING
 }
  1. - NOTE: HostAddresses is always used as an OPTIONAL field and
  2. - should not be empty.

HostAddresses – NOTE: subtly different from rfc1510,

  1. - but has a value mapping and encodes the same

::= SEQUENCE OF HostAddress

 The host address encodings consist of two fields:
 addr-type
    This field specifies the type of address that follows.  Pre-
    defined values for this field are specified in Section 7.5.3.
 address
    This field encodes a single address of type addr-type.

Neuman, et al. Standards Track [Page 56] RFC 4120 Kerberos V5 July 2005

5.2.6. AuthorizationData

  1. - NOTE: AuthorizationData is always used as an OPTIONAL field and
  2. - should not be empty.

AuthorizationData ::= SEQUENCE OF SEQUENCE {

            ad-type         [0] Int32,
            ad-data         [1] OCTET STRING
    }
 ad-data
    This field contains authorization data to be interpreted according
    to the value of the corresponding ad-type field.
 ad-type
    This field specifies the format for the ad-data subfield.  All
    negative values are reserved for local use.  Non-negative values
    are reserved for registered use.
 Each sequence of type and data is referred to as an authorization
 element.  Elements MAY be application specific; however, there is a
 common set of recursive elements that should be understood by all
 implementations.  These elements contain other elements embedded
 within them, and the interpretation of the encapsulating element
 determines which of the embedded elements must be interpreted, and
 which may be ignored.
 These common authorization data elements are recursively defined,
 meaning that the ad-data for these types will itself contain a
 sequence of authorization data whose interpretation is affected by
 the encapsulating element.  Depending on the meaning of the
 encapsulating element, the encapsulated elements may be ignored,
 might be interpreted as issued directly by the KDC, or might be
 stored in a separate plaintext part of the ticket.  The types of the
 encapsulating elements are specified as part of the Kerberos
 specification because the behavior based on these values should be
 understood across implementations, whereas other elements need only
 be understood by the applications that they affect.
 Authorization data elements are considered critical if present in a
 ticket or authenticator.  If an unknown authorization data element
 type is received by a server either in an AP-REQ or in a ticket
 contained in an AP-REQ, then, unless it is encapsulated in a known
 authorization data element amending the criticality of the elements
 it contains, authentication MUST fail.  Authorization data is
 intended to restrict the use of a ticket.  If the service cannot
 determine whether the restriction applies to that service, then a

Neuman, et al. Standards Track [Page 57] RFC 4120 Kerberos V5 July 2005

 security weakness may result if the ticket can be used for that
 service.  Authorization elements that are optional can be enclosed in
 an AD-IF-RELEVANT element.
 In the definitions that follow, the value of the ad-type for the
 element will be specified as the least significant part of the
 subsection number, and the value of the ad-data will be as shown in
 the ASN.1 structure that follows the subsection heading.
 Contents of ad-data                ad-type
 DER encoding of AD-IF-RELEVANT        1
 DER encoding of AD-KDCIssued          4
 DER encoding of AD-AND-OR             5
 DER encoding of AD-MANDATORY-FOR-KDC  8

5.2.6.1. IF-RELEVANT

 AD-IF-RELEVANT          ::= AuthorizationData
 AD elements encapsulated within the if-relevant element are intended
 for interpretation only by application servers that understand the
 particular ad-type of the embedded element.  Application servers that
 do not understand the type of an element embedded within the
 if-relevant element MAY ignore the uninterpretable element.  This
 element promotes interoperability across implementations that may
 have local extensions for authorization.  The ad-type for
 AD-IF-RELEVANT is (1).

5.2.6.2. KDCIssued

 AD-KDCIssued            ::= SEQUENCE {
         ad-checksum     [0] Checksum,
         i-realm         [1] Realm OPTIONAL,
         i-sname         [2] PrincipalName OPTIONAL,
         elements        [3] AuthorizationData
 }
 ad-checksum
    A cryptographic checksum computed over the DER encoding of the
    AuthorizationData in the "elements" field, keyed with the session
    key.  Its checksumtype is the mandatory checksum type for the
    encryption type of the session key, and its key usage value is 19.

Neuman, et al. Standards Track [Page 58] RFC 4120 Kerberos V5 July 2005

 i-realm, i-sname
    The name of the issuing principal if different from that of the
    KDC itself.  This field would be used when the KDC can verify the
    authenticity of elements signed by the issuing principal, and it
    allows this KDC to notify the application server of the validity
    of those elements.
 elements
    A sequence of authorization data elements issued by the KDC.
 The KDC-issued ad-data field is intended to provide a means for
 Kerberos principal credentials to embed within themselves privilege
 attributes and other mechanisms for positive authorization,
 amplifying the privileges of the principal beyond what can be done
 using credentials without such an a-data element.
 The above means cannot be provided without this element because the
 definition of the authorization-data field allows elements to be
 added at will by the bearer of a TGT at the time when they request
 service tickets, and elements may also be added to a delegated ticket
 by inclusion in the authenticator.
 For KDC-issued elements, this is prevented because the elements are
 signed by the KDC by including a checksum encrypted using the
 server's key (the same key used to encrypt the ticket or a key
 derived from that key).  Elements encapsulated with in the KDC-issued
 element MUST be ignored by the application server if this "signature"
 is not present.  Further, elements encapsulated within this element
 from a TGT MAY be interpreted by the KDC, and used as a basis
 according to policy for including new signed elements within
 derivative tickets, but they will not be copied to a derivative
 ticket directly.  If they are copied directly to a derivative ticket
 by a KDC that is not aware of this element, the signature will not be
 correct for the application ticket elements, and the field will be
 ignored by the application server.
 This element and the elements it encapsulates MAY safely be ignored
 by applications, application servers, and KDCs that do not implement
 this element.
 The ad-type for AD-KDC-ISSUED is (4).

5.2.6.3. AND-OR

 AD-AND-OR               ::= SEQUENCE {
         condition-count [0] Int32,
         elements        [1] AuthorizationData
 }

Neuman, et al. Standards Track [Page 59] RFC 4120 Kerberos V5 July 2005

 When restrictive AD elements are encapsulated within the and-or
 element, the and-or element is considered satisfied if and only if at
 least the number of encapsulated elements specified in condition-
 count are satisfied.  Therefore, this element MAY be used to
 implement an "or" operation by setting the condition-count field to
 1, and it MAY specify an "and" operation by setting the condition
 count to the number of embedded elements.  Application servers that
 do not implement this element MUST reject tickets that contain
 authorization data elements of this type.
 The ad-type for AD-AND-OR is (5).

5.2.6.4. MANDATORY-FOR-KDC

 AD-MANDATORY-FOR-KDC    ::= AuthorizationData
 AD elements encapsulated within the mandatory-for-kdc element are to
 be interpreted by the KDC.  KDCs that do not understand the type of
 an element embedded within the mandatory-for-kdc element MUST reject
 the request.
 The ad-type for AD-MANDATORY-FOR-KDC is (8).

5.2.7. PA-DATA

 Historically, PA-DATA have been known as "pre-authentication data",
 meaning that they were used to augment the initial authentication
 with the KDC.  Since that time, they have also been used as a typed
 hole with which to extend protocol exchanges with the KDC.
 PA-DATA         ::= SEQUENCE {
         -- NOTE: first tag is [1], not [0]
         padata-type     [1] Int32,
         padata-value    [2] OCTET STRING -- might be encoded AP-REQ
 }
 padata-type
    Indicates the way that the padata-value element is to be
    interpreted.  Negative values of padata-type are reserved for
    unregistered use; non-negative values are used for a registered
    interpretation of the element type.
 padata-value
    Usually contains the DER encoding of another type; the padata-type
    field identifies which type is encoded here.

Neuman, et al. Standards Track [Page 60] RFC 4120 Kerberos V5 July 2005

    padata-type  Name             Contents of padata-value
    1            pa-tgs-req       DER encoding of AP-REQ
    2            pa-enc-timestamp DER encoding of PA-ENC-TIMESTAMP
    3            pa-pw-salt       salt (not ASN.1 encoded)
    11           pa-etype-info    DER encoding of ETYPE-INFO
    19           pa-etype-info2   DER encoding of ETYPE-INFO2
    This field MAY also contain information needed by certain
    extensions to the Kerberos protocol.  For example, it might be
    used to verify the identity of a client initially before any
    response is returned.
    The padata field can also contain information needed to help the
    KDC or the client select the key needed for generating or
    decrypting the response.  This form of the padata is useful for
    supporting the use of certain token cards with Kerberos.  The
    details of such extensions are specified in separate documents.
    See [Pat92] for additional uses of this field.

5.2.7.1. PA-TGS-REQ

 In the case of requests for additional tickets (KRB_TGS_REQ),
 padata-value will contain an encoded AP-REQ.  The checksum in the
 authenticator (which MUST be collision-proof) is to be computed over
 the KDC-REQ-BODY encoding.

5.2.7.2. Encrypted Timestamp Pre-authentication

 There are pre-authentication types that may be used to pre-
 authenticate a client by means of an encrypted timestamp.
 PA-ENC-TIMESTAMP        ::= EncryptedData -- PA-ENC-TS-ENC
 PA-ENC-TS-ENC           ::= SEQUENCE {
         patimestamp     [0] KerberosTime -- client's time --,
         pausec          [1] Microseconds OPTIONAL
 }
 Patimestamp contains the client's time, and pausec contains the
 microseconds, which MAY be omitted if a client will not generate more
 than one request per second.  The ciphertext (padata-value) consists
 of the PA-ENC-TS-ENC encoding, encrypted using the client's secret
 key and a key usage value of 1.

Neuman, et al. Standards Track [Page 61] RFC 4120 Kerberos V5 July 2005

 This pre-authentication type was not present in RFC 1510, but many
 implementations support it.

5.2.7.3. PA-PW-SALT

 The padata-value for this pre-authentication type contains the salt
 for the string-to-key to be used by the client to obtain the key for
 decrypting the encrypted part of an AS-REP message.  Unfortunately,
 for historical reasons, the character set to be used is unspecified
 and probably locale-specific.
 This pre-authentication type was not present in RFC 1510, but many
 implementations support it.  It is necessary in any case where the
 salt for the string-to-key algorithm is not the default.
 In the trivial example, a zero-length salt string is very commonplace
 for realms that have converted their principal databases from
 Kerberos Version 4.
 A KDC SHOULD NOT send PA-PW-SALT when issuing a KRB-ERROR message
 that requests additional pre-authentication.  Implementation note:
 Some KDC implementations issue an erroneous PA-PW-SALT when issuing a
 KRB-ERROR message that requests additional pre-authentication.
 Therefore, clients SHOULD ignore a PA-PW-SALT accompanying a
 KRB-ERROR message that requests additional pre-authentication.  As
 noted in section 3.1.3, a KDC MUST NOT send PA-PW-SALT when the
 client's AS-REQ includes at least one "newer" etype.

5.2.7.4. PA-ETYPE-INFO

 The ETYPE-INFO pre-authentication type is sent by the KDC in a
 KRB-ERROR indicating a requirement for additional pre-authentication.
 It is usually used to notify a client of which key to use for the
 encryption of an encrypted timestamp for the purposes of sending a
 PA-ENC-TIMESTAMP pre-authentication value.  It MAY also be sent in an
 AS-REP to provide information to the client about which key salt to
 use for the string-to-key to be used by the client to obtain the key
 for decrypting the encrypted part the AS-REP.
 ETYPE-INFO-ENTRY        ::= SEQUENCE {
         etype           [0] Int32,
         salt            [1] OCTET STRING OPTIONAL
 }
 ETYPE-INFO              ::= SEQUENCE OF ETYPE-INFO-ENTRY
 The salt, like that of PA-PW-SALT, is also completely unspecified
 with respect to character set and is probably locale-specific.

Neuman, et al. Standards Track [Page 62] RFC 4120 Kerberos V5 July 2005

 If ETYPE-INFO is sent in an AS-REP, there shall be exactly one
 ETYPE-INFO-ENTRY, and its etype shall match that of the enc-part in
 the AS-REP.
 This pre-authentication type was not present in RFC 1510, but many
 implementations that support encrypted timestamps for pre-
 authentication need to support ETYPE-INFO as well.  As noted in
 Section 3.1.3, a KDC MUST NOT send PA-ETYPE-INFO when the client's
 AS-REQ includes at least one "newer" etype.

5.2.7.5. PA-ETYPE-INFO2

 The ETYPE-INFO2 pre-authentication type is sent by the KDC in a
 KRB-ERROR indicating a requirement for additional pre-authentication.
 It is usually used to notify a client of which key to use for the
 encryption of an encrypted timestamp for the purposes of sending a
 PA-ENC-TIMESTAMP pre-authentication value.  It MAY also be sent in an
 AS-REP to provide information to the client about which key salt to
 use for the string-to-key to be used by the client to obtain the key
 for decrypting the encrypted part the AS-REP.

ETYPE-INFO2-ENTRY ::= SEQUENCE {

      etype           [0] Int32,
      salt            [1] KerberosString OPTIONAL,
      s2kparams       [2] OCTET STRING OPTIONAL

}

ETYPE-INFO2 ::= SEQUENCE SIZE (1..MAX) OF ETYPE-INFO2-ENTRY

 The type of the salt is KerberosString, but existing installations
 might have locale-specific characters stored in salt strings, and
 implementors MAY choose to handle them.
 The interpretation of s2kparams is specified in the cryptosystem
 description associated with the etype.  Each cryptosystem has a
 default interpretation of s2kparams that will hold if that element is
 omitted from the encoding of ETYPE-INFO2-ENTRY.
 If ETYPE-INFO2 is sent in an AS-REP, there shall be exactly one
 ETYPE-INFO2-ENTRY, and its etype shall match that of the enc-part in
 the AS-REP.
 The preferred ordering of the "hint" pre-authentication data that
 affect client key selection is: ETYPE-INFO2, followed by ETYPE-INFO,
 followed by PW-SALT.  As noted in Section 3.1.3, a KDC MUST NOT send
 ETYPE-INFO or PW-SALT when the client's AS-REQ includes at least one
 "newer" etype.

Neuman, et al. Standards Track [Page 63] RFC 4120 Kerberos V5 July 2005

 The ETYPE-INFO2 pre-authentication type was not present in RFC 1510.

5.2.8. KerberosFlags

 For several message types, a specific constrained bit string type,
 KerberosFlags, is used.
 KerberosFlags   ::= BIT STRING (SIZE (32..MAX))
                     -- minimum number of bits shall be sent,
                     -- but no fewer than 32
 Compatibility note: The following paragraphs describe a change from
 the RFC 1510 description of bit strings that would result in
 incompatility in the case of an implementation that strictly
 conformed to ASN.1 DER and RFC 1510.
 ASN.1 bit strings have multiple uses.  The simplest use of a bit
 string is to contain a vector of bits, with no particular meaning
 attached to individual bits.  This vector of bits is not necessarily
 a multiple of eight bits long.  The use in Kerberos of a bit string
 as a compact boolean vector wherein each element has a distinct
 meaning poses some problems.  The natural notation for a compact
 boolean vector is the ASN.1 "NamedBit" notation, and the DER require
 that encodings of a bit string using "NamedBit" notation exclude any
 trailing zero bits.  This truncation is easy to neglect, especially
 given C language implementations that naturally choose to store
 boolean vectors as 32-bit integers.
 For example, if the notation for KDCOptions were to include the
 "NamedBit" notation, as in RFC 1510, and a KDCOptions value to be
 encoded had only the "forwardable" (bit number one) bit set, the DER
 encoding MUST include only two bits: the first reserved bit
 ("reserved", bit number zero, value zero) and the one-valued bit (bit
 number one) for "forwardable".
 Most existing implementations of Kerberos unconditionally send 32
 bits on the wire when encoding bit strings used as boolean vectors.
 This behavior violates the ASN.1 syntax used for flag values in RFC
 1510, but it occurs on such a widely installed base that the protocol
 description is being modified to accommodate it.
 Consequently, this document removes the "NamedBit" notations for
 individual bits, relegating them to comments.  The size constraint on
 the KerberosFlags type requires that at least 32 bits be encoded at
 all times, though a lenient implementation MAY choose to accept fewer
 than 32 bits and to treat the missing bits as set to zero.

Neuman, et al. Standards Track [Page 64] RFC 4120 Kerberos V5 July 2005

 Currently, no uses of KerberosFlags specify more than 32 bits' worth
 of flags, although future revisions of this document may do so.  When
 more than 32 bits are to be transmitted in a KerberosFlags value,
 future revisions to this document will likely specify that the
 smallest number of bits needed to encode the highest-numbered one-
 valued bit should be sent.  This is somewhat similar to the DER
 encoding of a bit string that is declared with the "NamedBit"
 notation.

5.2.9. Cryptosystem-Related Types

 Many Kerberos protocol messages contain an EncryptedData as a
 container for arbitrary encrypted data, which is often the encrypted
 encoding of another data type.  Fields within EncryptedData assist
 the recipient in selecting a key with which to decrypt the enclosed
 data.
 EncryptedData   ::= SEQUENCE {
         etype   [0] Int32 -- EncryptionType --,
         kvno    [1] UInt32 OPTIONAL,
         cipher  [2] OCTET STRING -- ciphertext
 }
 etype
    This field identifies which encryption algorithm was used to
    encipher the cipher.
 kvno
    This field contains the version number of the key under which data
    is encrypted.  It is only present in messages encrypted under long
    lasting keys, such as principals' secret keys.
 cipher
    This field contains the enciphered text, encoded as an OCTET
    STRING.  (Note that the encryption mechanisms defined in [RFC3961]
    MUST incorporate integrity protection as well, so no additional
    checksum is required.)
 The EncryptionKey type is the means by which cryptographic keys used
 for encryption are transferred.
 EncryptionKey   ::= SEQUENCE {
         keytype         [0] Int32 -- actually encryption type --,
         keyvalue        [1] OCTET STRING
 }

Neuman, et al. Standards Track [Page 65] RFC 4120 Kerberos V5 July 2005

 keytype
    This field specifies the encryption type of the encryption key
    that follows in the keyvalue field.  Although its name is
    "keytype", it actually specifies an encryption type.  Previously,
    multiple cryptosystems that performed encryption differently but
    were capable of using keys with the same characteristics were
    permitted to share an assigned number to designate the type of
    key; this usage is now deprecated.
 keyvalue
    This field contains the key itself, encoded as an octet string.
 Messages containing cleartext data to be authenticated will usually
 do so by using a member of type Checksum.  Most instances of Checksum
 use a keyed hash, though exceptions will be noted.
 Checksum        ::= SEQUENCE {
         cksumtype       [0] Int32,
         checksum        [1] OCTET STRING
 }
 cksumtype
    This field indicates the algorithm used to generate the
    accompanying checksum.
 checksum
    This field contains the checksum itself, encoded as an octet
    string.
 See Section 4 for a brief description of the use of encryption and
 checksums in Kerberos.

5.3. Tickets

 This section describes the format and encryption parameters for
 tickets and authenticators.  When a ticket or authenticator is
 included in a protocol message, it is treated as an opaque object.  A
 ticket is a record that helps a client authenticate to a service.  A
 Ticket contains the following information:
 Ticket          ::= [APPLICATION 1] SEQUENCE {
         tkt-vno         [0] INTEGER (5),
         realm           [1] Realm,
         sname           [2] PrincipalName,
         enc-part        [3] EncryptedData -- EncTicketPart
 }
  1. - Encrypted part of ticket

Neuman, et al. Standards Track [Page 66] RFC 4120 Kerberos V5 July 2005

 EncTicketPart   ::= [APPLICATION 3] SEQUENCE {
         flags                   [0] TicketFlags,
         key                     [1] EncryptionKey,
         crealm                  [2] Realm,
         cname                   [3] PrincipalName,
         transited               [4] TransitedEncoding,
         authtime                [5] KerberosTime,
         starttime               [6] KerberosTime OPTIONAL,
         endtime                 [7] KerberosTime,
         renew-till              [8] KerberosTime OPTIONAL,
         caddr                   [9] HostAddresses OPTIONAL,
         authorization-data      [10] AuthorizationData OPTIONAL
 }
  1. - encoded Transited field

TransitedEncoding ::= SEQUENCE {

         tr-type         [0] Int32 -- must be registered --,
         contents        [1] OCTET STRING
 }
 TicketFlags     ::= KerberosFlags
         -- reserved(0),
         -- forwardable(1),
         -- forwarded(2),
         -- proxiable(3),
         -- proxy(4),
         -- may-postdate(5),
         -- postdated(6),
         -- invalid(7),
         -- renewable(8),
         -- initial(9),
         -- pre-authent(10),
         -- hw-authent(11),
 -- the following are new since 1510
         -- transited-policy-checked(12),
         -- ok-as-delegate(13)
 tkt-vno
    This field specifies the version number for the ticket format.
    This document describes version number 5.
 realm
    This field specifies the realm that issued a ticket.  It also
    serves to identify the realm part of the server's principal
    identifier.  Since a Kerberos server can only issue tickets for
    servers within its realm, the two will always be identical.

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 sname
    This field specifies all components of the name part of the
    server's identity, including those parts that identify a specific
    instance of a service.
 enc-part
    This field holds the encrypted encoding of the EncTicketPart
    sequence.  It is encrypted in the key shared by Kerberos and the
    end server (the server's secret key), using a key usage value of
    2.
 flags
    This field indicates which of various options were used or
    requested when the ticket was issued.  The meanings of the flags
    are as follows:
 Bit(s)  Name             Description
 0       reserved         Reserved for future expansion of this field.
 1       forwardable      The FORWARDABLE flag is normally only
                          interpreted by the TGS, and can be ignored
                          by end servers.  When set, this flag tells
                          the ticket-granting server that it is OK to
                          issue a new TGT with a different network
                          address based on the presented ticket.
 2       forwarded        When set, this flag indicates that the
                          ticket has either been forwarded or was
                          issued based on authentication involving a
                          forwarded TGT.
 3       proxiable        The PROXIABLE flag is normally only
                          interpreted by the TGS, and can be ignored
                          by end servers.  The PROXIABLE flag has an
                          interpretation identical to that of the
                          FORWARDABLE flag, except that the PROXIABLE
                          flag tells the ticket-granting server that
                          only non-TGTs may be issued with different
                          network addresses.
 4       proxy            When set, this flag indicates that a ticket
                          is a proxy.
 5       may-postdate     The MAY-POSTDATE flag is normally only
                          interpreted by the TGS, and can be ignored
                          by end servers.  This flag tells the

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                          ticket-granting server that a post-dated
                          ticket MAY be issued based on this TGT.
 6       postdated        This flag indicates that this ticket has
                          been postdated.  The end-service can check
                          the authtime field to see when the original
                          authentication occurred.
 7       invalid          This flag indicates that a ticket is
                          invalid, and it must be validated by the KDC
                          before use.  Application servers must reject
                          tickets which have this flag set.
 8       renewable        The RENEWABLE flag is normally only
                          interpreted by the TGS, and can usually be
                          ignored by end servers (some particularly
                          careful servers MAY disallow renewable
                          tickets).  A renewable ticket can be used to
                          obtain a replacement ticket that expires at
                          a later date.
 9       initial          This flag indicates that this ticket was
                          issued using the AS protocol, and not issued
                          based on a TGT.
 10      pre-authent      This flag indicates that during initial
                          authentication, the client was authenticated
                          by the KDC before a ticket was issued.  The
                          strength of the pre-authentication method is
                          not indicated, but is acceptable to the KDC.
 11      hw-authent       This flag indicates that the protocol
                          employed for initial authentication required
                          the use of hardware expected to be possessed
                          solely by the named client.  The hardware
                          authentication method is selected by the KDC
                          and the strength of the method is not
                          indicated.
 12      transited-       This flag indicates that the KDC for
         policy-checked   the realm has checked the transited field
                          against a realm-defined policy for trusted
                          certifiers.  If this flag is reset (0), then
                          the application server must check the
                          transited field itself, and if unable to do
                          so, it must reject the authentication.  If
                          the flag is set (1), then the application
                          server MAY skip its own validation of the

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                          transited field, relying on the validation
                          performed by the KDC.  At its option the
                          application server MAY still apply its own
                          validation based on a separate policy for
                          acceptance.
                          This flag is new since RFC 1510.
 13      ok-as-delegate   This flag indicates that the server (not the
                          client) specified in the ticket has been
                          determined by policy of the realm to be a
                          suitable recipient of delegation.  A client
                          can use the presence of this flag to help it
                          decide whether to delegate credentials
                          (either grant a proxy or a forwarded TGT) to
                          this server.  The client is free to ignore
                          the value of this flag.  When setting this
                          flag, an administrator should consider the
                          security and placement of the server on
                          which the service will run, as well as
                          whether the service requires the use of
                          delegated credentials.
                          This flag is new since RFC 1510.
 14-31   reserved         Reserved for future use.
 key
    This field exists in the ticket and the KDC response and is used
    to pass the session key from Kerberos to the application server
    and the client.
 crealm
    This field contains the name of the realm in which the client is
    registered and in which initial authentication took place.
 cname
    This field contains the name part of the client's principal
    identifier.
 transited
    This field lists the names of the Kerberos realms that took part
    in authenticating the user to whom this ticket was issued.  It
    does not specify the order in which the realms were transited.
    See Section 3.3.3.2 for details on how this field encodes the
    traversed realms.  When the names of CAs are to be embedded in the
    transited field (as specified for some extensions to the

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    protocol), the X.500 names of the CAs SHOULD be mapped into items
    in the transited field using the mapping defined by RFC 2253.
 authtime
    This field indicates the time of initial authentication for the
    named principal.  It is the time of issue for the original ticket
    on which this ticket is based.  It is included in the ticket to
    provide additional information to the end service, and to provide
    the necessary information for implementation of a "hot list"
    service at the KDC.  An end service that is particularly paranoid
    could refuse to accept tickets for which the initial
    authentication occurred "too far" in the past.  This field is also
    returned as part of the response from the KDC.  When it is
    returned as part of the response to initial authentication
    (KRB_AS_REP), this is the current time on the Kerberos server.  It
    is NOT recommended that this time value be used to adjust the
    workstation's clock, as the workstation cannot reliably determine
    that such a KRB_AS_REP actually came from the proper KDC in a
    timely manner.
 starttime
    This field in the ticket specifies the time after which the ticket
    is valid.  Together with endtime, this field specifies the life of
    the ticket.  If the starttime field is absent from the ticket,
    then the authtime field SHOULD be used in its place to determine
    the life of the ticket.
 endtime
    This field contains the time after which the ticket will not be
    honored (its expiration time).  Note that individual services MAY
    place their own limits on the life of a ticket and MAY reject
    tickets which have not yet expired.  As such, this is really an
    upper bound on the expiration time for the ticket.
 renew-till
    This field is only present in tickets that have the RENEWABLE flag
    set in the flags field.  It indicates the maximum endtime that may
    be included in a renewal.  It can be thought of as the absolute
    expiration time for the ticket, including all renewals.
 caddr
    This field in a ticket contains zero (if omitted) or more (if
    present) host addresses.  These are the addresses from which the
    ticket can be used.  If there are no addresses, the ticket can be
    used from any location.  The decision by the KDC to issue or by
    the end server to accept addressless tickets is a policy decision
    and is left to the Kerberos and end-service administrators; they
    MAY refuse to issue or accept such tickets.  Because of the wide

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    deployment of network address translation, it is recommended that
    policy allow the issue and acceptance of such tickets.
    Network addresses are included in the ticket to make it harder for
    an attacker to use stolen credentials.  Because the session key is
    not sent over the network in cleartext, credentials can't be
    stolen simply by listening to the network; an attacker has to gain
    access to the session key (perhaps through operating system
    security breaches or a careless user's unattended session) to make
    use of stolen tickets.
    Note that the network address from which a connection is received
    cannot be reliably determined.  Even if it could be, an attacker
    who has compromised the client's workstation could use the
    credentials from there.  Including the network addresses only
    makes it more difficult, not impossible, for an attacker to walk
    off with stolen credentials and then to use them from a "safe"
    location.
 authorization-data
    The authorization-data field is used to pass authorization data
    from the principal on whose behalf a ticket was issued to the
    application service.  If no authorization data is included, this
    field will be left out.  Experience has shown that the name of
    this field is confusing, and that a better name would be
    "restrictions".  Unfortunately, it is not possible to change the
    name at this time.
    This field contains restrictions on any authority obtained on the
    basis of authentication using the ticket.  It is possible for any
    principal in possession of credentials to add entries to the
    authorization data field since these entries further restrict what
    can be done with the ticket.  Such additions can be made by
    specifying the additional entries when a new ticket is obtained
    during the TGS exchange, or they MAY be added during chained
    delegation using the authorization data field of the
    authenticator.
    Because entries may be added to this field by the holder of
    credentials, except when an entry is separately authenticated by
    encapsulation in the KDC-issued element, it is not allowable for
    the presence of an entry in the authorization data field of a
    ticket to amplify the privileges one would obtain from using a
    ticket.
    The data in this field may be specific to the end service; the
    field will contain the names of service specific objects, and the
    rights to those objects.  The format for this field is described

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    in Section 5.2.6.  Although Kerberos is not concerned with the
    format of the contents of the subfields, it does carry type
    information (ad-type).
    By using the authorization_data field, a principal is able to
    issue a proxy that is valid for a specific purpose.  For example,
    a client wishing to print a file can obtain a file server proxy to
    be passed to the print server.  By specifying the name of the file
    in the authorization_data field, the file server knows that the
    print server can only use the client's rights when accessing the
    particular file to be printed.
    A separate service providing authorization or certifying group
    membership may be built using the authorization-data field.  In
    this case, the entity granting authorization (not the authorized
    entity) may obtain a ticket in its own name (e.g., the ticket is
    issued in the name of a privilege server), and this entity adds
    restrictions on its own authority and delegates the restricted
    authority through a proxy to the client.  The client would then
    present this authorization credential to the application server
    separately from the authentication exchange.  Alternatively, such
    authorization credentials MAY be embedded in the ticket
    authenticating the authorized entity, when the authorization is
    separately authenticated using the KDC-issued authorization data
    element (see 5.2.6.2).
    Similarly, if one specifies the authorization-data field of a
    proxy and leaves the host addresses blank, the resulting ticket
    and session key can be treated as a capability.  See [Neu93] for
    some suggested uses of this field.
    The authorization-data field is optional and does not have to be
    included in a ticket.

5.4. Specifications for the AS and TGS Exchanges

 This section specifies the format of the messages used in the
 exchange between the client and the Kerberos server.  The format of
 possible error messages appears in Section 5.9.1.

5.4.1. KRB_KDC_REQ Definition

 The KRB_KDC_REQ message has no application tag number of its own.
 Instead, it is incorporated into either KRB_AS_REQ or KRB_TGS_REQ,
 each of which has an application tag, depending on whether the
 request is for an initial ticket or an additional ticket.  In either
 case, the message is sent from the client to the KDC to request
 credentials for a service.

Neuman, et al. Standards Track [Page 73] RFC 4120 Kerberos V5 July 2005

 The message fields are as follows:

AS-REQ ::= [APPLICATION 10] KDC-REQ

TGS-REQ ::= [APPLICATION 12] KDC-REQ

KDC-REQ ::= SEQUENCE {

  1. - NOTE: first tag is [1], not [0]

pvno [1] INTEGER (5) ,

      msg-type        [2] INTEGER (10 -- AS -- | 12 -- TGS --),
      padata          [3] SEQUENCE OF PA-DATA OPTIONAL
                          -- NOTE: not empty --,
      req-body        [4] KDC-REQ-BODY

}

KDC-REQ-BODY ::= SEQUENCE {

      kdc-options             [0] KDCOptions,
      cname                   [1] PrincipalName OPTIONAL
                                  -- Used only in AS-REQ --,
      realm                   [2] Realm
                                  -- Server's realm
                                  -- Also client's in AS-REQ --,
      sname                   [3] PrincipalName OPTIONAL,
      from                    [4] KerberosTime OPTIONAL,
      till                    [5] KerberosTime,
      rtime                   [6] KerberosTime OPTIONAL,
      nonce                   [7] UInt32,
      etype                   [8] SEQUENCE OF Int32 -- EncryptionType
                                  -- in preference order --,
      addresses               [9] HostAddresses OPTIONAL,
      enc-authorization-data  [10] EncryptedData OPTIONAL
                                  -- AuthorizationData --,
      additional-tickets      [11] SEQUENCE OF Ticket OPTIONAL
                                     -- NOTE: not empty

}

KDCOptions ::= KerberosFlags

  1. - reserved(0),
  2. - forwardable(1),
  3. - forwarded(2),
  4. - proxiable(3),
  5. - proxy(4),
  6. - allow-postdate(5),
  7. - postdated(6),
  8. - unused7(7),
  9. - renewable(8),
  10. - unused9(9),
  11. - unused10(10),

Neuman, et al. Standards Track [Page 74] RFC 4120 Kerberos V5 July 2005

  1. - opt-hardware-auth(11),
  2. - unused12(12),
  3. - unused13(13),

– 15 is reserved for canonicalize

  1. - unused15(15),

– 26 was unused in 1510

  1. - disable-transited-check(26),

  1. - renewable-ok(27),
  2. - enc-tkt-in-skey(28),
  3. - renew(30),
  4. - validate(31)
 The fields in this message are as follows:
 pvno
    This field is included in each message, and specifies the protocol
    version number.  This document specifies protocol version 5.
 msg-type
    This field indicates the type of a protocol message.  It will
    almost always be the same as the application identifier associated
    with a message.  It is included to make the identifier more
    readily accessible to the application.  For the KDC-REQ message,
    this type will be KRB_AS_REQ or KRB_TGS_REQ.
 padata
    Contains pre-authentication data.  Requests for additional tickets
    (KRB_TGS_REQ) MUST contain a padata of PA-TGS-REQ.
    The padata (pre-authentication data) field contains a sequence of
    authentication information that may be needed before credentials
    can be issued or decrypted.
 req-body
    This field is a placeholder delimiting the extent of the remaining
    fields.  If a checksum is to be calculated over the request, it is
    calculated over an encoding of the KDC-REQ-BODY sequence which is
    enclosed within the req-body field.
 kdc-options
    This field appears in the KRB_AS_REQ and KRB_TGS_REQ requests to
    the KDC and indicates the flags that the client wants set on the
    tickets as well as other information that is to modify the
    behavior of the KDC.  Where appropriate, the name of an option may
    be the same as the flag that is set by that option.  Although in
    most cases, the bit in the options field will be the same as that
    in the flags field, this is not guaranteed, so it is not

Neuman, et al. Standards Track [Page 75] RFC 4120 Kerberos V5 July 2005

    acceptable simply to copy the options field to the flags field.
    There are various checks that must be made before an option is
    honored anyway.
    The kdc_options field is a bit-field, where the selected options
    are indicated by the bit being set (1), and the unselected options
    and reserved fields being reset (0).  The encoding of the bits is
    specified in Section 5.2.  The options are described in more
    detail above in Section 2.  The meanings of the options are as
    follows:
 Bits    Name                     Description
 0       RESERVED                 Reserved for future expansion of
                                  this field.
 1       FORWARDABLE              The FORWARDABLE option indicates
                                  that the ticket to be issued is to
                                  have its forwardable flag set.  It
                                  may only be set on the initial
                                  request, or in a subsequent request
                                  if the TGT on which it is based is
                                  also forwardable.
 2       FORWARDED                The FORWARDED option is only
                                  specified in a request to the
                                  ticket-granting server and will only
                                  be honored if the TGT in the request
                                  has its FORWARDABLE bit set.  This
                                  option indicates that this is a
                                  request for forwarding.  The
                                  address(es) of the host from which
                                  the resulting ticket is to be valid
                                  are included in the addresses field
                                  of the request.
 3       PROXIABLE                The PROXIABLE option indicates that
                                  the ticket to be issued is to have
                                  its proxiable flag set.  It may only
                                  be set on the initial request, or a
                                  subsequent request if the TGT on
                                  which it is based is also proxiable.
 4       PROXY                    The PROXY option indicates that this
                                  is a request for a proxy.  This
                                  option will only be honored if the
                                  TGT in the request has its PROXIABLE
                                  bit set.  The address(es) of the

Neuman, et al. Standards Track [Page 76] RFC 4120 Kerberos V5 July 2005

                                  host from which the resulting ticket
                                  is to be valid are included in the
                                  addresses field of the request.
 5       ALLOW-POSTDATE           The ALLOW-POSTDATE option indicates
                                  that the ticket to be issued is to
                                  have its MAY-POSTDATE flag set.  It
                                  may only be set on the initial
                                  request, or in a subsequent request
                                  if the TGT on which it is based also
                                  has its MAY-POSTDATE flag set.
 6       POSTDATED                The POSTDATED option indicates that
                                  this is a request for a postdated
                                  ticket.  This option will only be
                                  honored if the TGT on which it is
                                  based has its MAY-POSTDATE flag set.
                                  The resulting ticket will also have
                                  its INVALID flag set, and that flag
                                  may be reset by a subsequent request
                                  to the KDC after the starttime in
                                  the ticket has been reached.
 7       RESERVED                 This option is presently unused.
 8       RENEWABLE                The RENEWABLE option indicates that
                                  the ticket to be issued is to have
                                  its RENEWABLE flag set.  It may only
                                  be set on the initial request, or
                                  when the TGT on which the request is
                                  based is also renewable.  If this
                                  option is requested, then the rtime
                                  field in the request contains the
                                  desired absolute expiration time for
                                  the ticket.
 9       RESERVED                 Reserved for PK-Cross.
 10      RESERVED                 Reserved for future use.
 11      RESERVED                 Reserved for opt-hardware-auth.
 12-25   RESERVED                 Reserved for future use.
 26      DISABLE-TRANSITED-CHECK  By default the KDC will check the
                                  transited field of a TGT against the
                                  policy of the local realm before it
                                  will issue derivative tickets based

Neuman, et al. Standards Track [Page 77] RFC 4120 Kerberos V5 July 2005

                                  on the TGT.  If this flag is set in
                                  the request, checking of the
                                  transited field is disabled.
                                  Tickets issued without the
                                  performance of this check will be
                                  noted by the reset (0) value of the
                                  TRANSITED-POLICY-CHECKED flag,
                                  indicating to the application server
                                  that the transited field must be
                                  checked locally.  KDCs are
                                  encouraged but not required to honor
                                  the DISABLE-TRANSITED-CHECK option.
                                  This flag is new since RFC 1510.
 27      RENEWABLE-OK             The RENEWABLE-OK option indicates
                                  that a renewable ticket will be
                                  acceptable if a ticket with the
                                  requested life cannot otherwise be
                                  provided, in which case a renewable
                                  ticket may be issued with a renew-
                                  till equal to the requested endtime.
                                  The value of the renew-till field
                                  may still be limited by local
                                  limits, or limits selected by the
                                  individual principal or server.
 28      ENC-TKT-IN-SKEY          This option is used only by the
                                  ticket-granting service.  The ENC-
                                  TKT-IN-SKEY option indicates that
                                  the ticket for the end server is to
                                  be encrypted in the session key from
                                  the additional TGT provided.
 29      RESERVED                 Reserved for future use.
 30      RENEW                    This option is used only by the
                                  ticket-granting service.  The RENEW
                                  option indicates that the present
                                  request is for a renewal.  The
                                  ticket provided is encrypted in the
                                  secret key for the server on which
                                  it is valid.  This option will only
                                  be honored if the ticket to be
                                  renewed has its RENEWABLE flag set
                                  and if the time in its renew-till
                                  field has not passed.  The ticket to
                                  be renewed is passed in the padata

Neuman, et al. Standards Track [Page 78] RFC 4120 Kerberos V5 July 2005

                                  field as part of the authentication
                                  header.
 31      VALIDATE                 This option is used only by the
                                  ticket-granting service.  The
                                  VALIDATE option indicates that the
                                  request is to validate a postdated
                                  ticket.  It will only be honored if
                                  the ticket presented is postdated,
                                  presently has its INVALID flag set,
                                  and would otherwise be usable at
                                  this time.  A ticket cannot be
                                  validated before its starttime.  The
                                  ticket presented for validation is
                                  encrypted in the key of the server
                                  for which it is valid and is passed
                                  in the padata field as part of the
                                  authentication header.
 cname and sname
    These fields are the same as those described for the ticket in
    section 5.3.  The sname may only be absent when the ENC-TKT-IN-
    SKEY option is specified.  If the sname is absent, the name of the
    server is taken from the name of the client in the ticket passed
    as additional-tickets.
 enc-authorization-data
    The enc-authorization-data, if present (and it can only be present
    in the TGS_REQ form), is an encoding of the desired
    authorization-data encrypted under the sub-session key if present
    in the Authenticator, or alternatively from the session key in the
    TGT (both the Authenticator and TGT come from the padata field in
    the KRB_TGS_REQ).  The key usage value used when encrypting is 5
    if a sub-session key is used, or 4 if the session key is used.
 realm
    This field specifies the realm part of the server's principal
    identifier.  In the AS exchange, this is also the realm part of
    the client's principal identifier.
 from
    This field is included in the KRB_AS_REQ and KRB_TGS_REQ ticket
    requests when the requested ticket is to be postdated.  It
    specifies the desired starttime for the requested ticket.  If this
    field is omitted, then the KDC SHOULD use the current time
    instead.

Neuman, et al. Standards Track [Page 79] RFC 4120 Kerberos V5 July 2005

 till
    This field contains the expiration date requested by the client in
    a ticket request.  It is not optional, but if the requested
    endtime is "19700101000000Z", the requested ticket is to have the
    maximum endtime permitted according to KDC policy.  Implementation
    note: This special timestamp corresponds to a UNIX time_t value of
    zero on most systems.
 rtime
    This field is the requested renew-till time sent from a client to
    the KDC in a ticket request.  It is optional.
 nonce
    This field is part of the KDC request and response.  It is
    intended to hold a random number generated by the client.  If the
    same number is included in the encrypted response from the KDC, it
    provides evidence that the response is fresh and has not been
    replayed by an attacker.  Nonces MUST NEVER be reused.
 etype
    This field specifies the desired encryption algorithm to be used
    in the response.
 addresses
    This field is included in the initial request for tickets, and it
    is optionally included in requests for additional tickets from the
    ticket-granting server.  It specifies the addresses from which the
    requested ticket is to be valid.  Normally it includes the
    addresses for the client's host.  If a proxy is requested, this
    field will contain other addresses.  The contents of this field
    are usually copied by the KDC into the caddr field of the
    resulting ticket.
 additional-tickets
    Additional tickets MAY be optionally included in a request to the
    ticket-granting server.  If the ENC-TKT-IN-SKEY option has been
    specified, then the session key from the additional ticket will be
    used in place of the server's key to encrypt the new ticket.  When
    the ENC-TKT-IN-SKEY option is used for user-to-user
    authentication, this additional ticket MAY be a TGT issued by the
    local realm or an inter-realm TGT issued for the current KDC's
    realm by a remote KDC.  If more than one option that requires
    additional tickets has been specified, then the additional tickets
    are used in the order specified by the ordering of the options
    bits (see kdc-options, above).

Neuman, et al. Standards Track [Page 80] RFC 4120 Kerberos V5 July 2005

 The application tag number will be either ten (10) or twelve (12)
 depending on whether the request is for an initial ticket (AS-REQ) or
 for an additional ticket (TGS-REQ).
 The optional fields (addresses, authorization-data, and additional-
 tickets) are only included if necessary to perform the operation
 specified in the kdc-options field.
 Note that in KRB_TGS_REQ, the protocol version number appears twice
 and two different message types appear: the KRB_TGS_REQ message
 contains these fields as does the authentication header (KRB_AP_REQ)
 that is passed in the padata field.

5.4.2. KRB_KDC_REP Definition

 The KRB_KDC_REP message format is used for the reply from the KDC for
 either an initial (AS) request or a subsequent (TGS) request.  There
 is no message type for KRB_KDC_REP.  Instead, the type will be either
 KRB_AS_REP or KRB_TGS_REP.  The key used to encrypt the ciphertext
 part of the reply depends on the message type.  For KRB_AS_REP, the
 ciphertext is encrypted in the client's secret key, and the client's
 key version number is included in the key version number for the
 encrypted data.  For KRB_TGS_REP, the ciphertext is encrypted in the
 sub-session key from the Authenticator; if it is absent, the
 ciphertext is encrypted in the session key from the TGT used in the
 request.  In that case, no version number will be present in the
 EncryptedData sequence.
 The KRB_KDC_REP message contains the following fields:
 AS-REP          ::= [APPLICATION 11] KDC-REP
 TGS-REP         ::= [APPLICATION 13] KDC-REP
 KDC-REP         ::= SEQUENCE {
         pvno            [0] INTEGER (5),
         msg-type        [1] INTEGER (11 -- AS -- | 13 -- TGS --),
         padata          [2] SEQUENCE OF PA-DATA OPTIONAL
                                 -- NOTE: not empty --,
         crealm          [3] Realm,
         cname           [4] PrincipalName,
         ticket          [5] Ticket,
         enc-part        [6] EncryptedData
                                 -- EncASRepPart or EncTGSRepPart,
                                 -- as appropriate
 }
 EncASRepPart    ::= [APPLICATION 25] EncKDCRepPart

Neuman, et al. Standards Track [Page 81] RFC 4120 Kerberos V5 July 2005

 EncTGSRepPart   ::= [APPLICATION 26] EncKDCRepPart
 EncKDCRepPart   ::= SEQUENCE {
         key             [0] EncryptionKey,
         last-req        [1] LastReq,
         nonce           [2] UInt32,
         key-expiration  [3] KerberosTime OPTIONAL,
         flags           [4] TicketFlags,
         authtime        [5] KerberosTime,
         starttime       [6] KerberosTime OPTIONAL,
         endtime         [7] KerberosTime,
         renew-till      [8] KerberosTime OPTIONAL,
         srealm          [9] Realm,
         sname           [10] PrincipalName,
         caddr           [11] HostAddresses OPTIONAL
 }
 LastReq         ::=     SEQUENCE OF SEQUENCE {
         lr-type         [0] Int32,
         lr-value        [1] KerberosTime
 }
 pvno and msg-type
    These fields are described above in Section 5.4.1.  msg-type is
    either KRB_AS_REP or KRB_TGS_REP.
 padata
    This field is described in detail in Section 5.4.1.  One possible
    use for it is to encode an alternate "salt" string to be used with
    a string-to-key algorithm.  This ability is useful for easing
    transitions if a realm name needs to change (e.g., when a company
    is acquired); in such a case all existing password-derived entries
    in the KDC database would be flagged as needing a special salt
    string until the next password change.
 crealm, cname, srealm, and sname
    These fields are the same as those described for the ticket in
    section 5.3.
 ticket
    The newly-issued ticket, from Section 5.3.
 enc-part
    This field is a place holder for the ciphertext and related
    information that forms the encrypted part of a message.  The
    description of the encrypted part of the message follows each
    appearance of this field.

Neuman, et al. Standards Track [Page 82] RFC 4120 Kerberos V5 July 2005

    The key usage value for encrypting this field is 3 in an AS-REP
    message, using the client's long-term key or another key selected
    via pre-authentication mechanisms.  In a TGS-REP message, the key
    usage value is 8 if the TGS session key is used, or 9 if a TGS
    authenticator subkey is used.
    Compatibility note: Some implementations unconditionally send an
    encrypted EncTGSRepPart (application tag number 26) in this field
    regardless of whether the reply is a AS-REP or a TGS-REP.  In the
    interest of compatibility, implementors MAY relax the check on the
    tag number of the decrypted ENC-PART.
 key
    This field is the same as described for the ticket in Section 5.3.
 last-req
    This field is returned by the KDC and specifies the time(s) of the
    last request by a principal.  Depending on what information is
    available, this might be the last time that a request for a TGT
    was made, or the last time that a request based on a TGT was
    successful.  It also might cover all servers for a realm, or just
    the particular server.  Some implementations MAY display this
    information to the user to aid in discovering unauthorized use of
    one's identity.  It is similar in spirit to the last login time
    displayed when logging in to timesharing systems.
 lr-type
    This field indicates how the following lr-value field is to be
    interpreted.  Negative values indicate that the information
    pertains only to the responding server.  Non-negative values
    pertain to all servers for the realm.
    If the lr-type field is zero (0), then no information is conveyed
    by the lr-value subfield.  If the absolute value of the lr-type
    field is one (1), then the lr-value subfield is the time of last
    initial request for a TGT.  If it is two (2), then the lr-value
    subfield is the time of last initial request.  If it is three (3),
    then the lr-value subfield is the time of issue for the newest TGT
    used.  If it is four (4), then the lr-value subfield is the time
    of the last renewal.  If it is five (5), then the lr-value
    subfield is the time of last request (of any type).  If it is (6),
    then the lr-value subfield is the time when the password will
    expire.  If it is (7), then the lr-value subfield is the time when
    the account will expire.

Neuman, et al. Standards Track [Page 83] RFC 4120 Kerberos V5 July 2005

 lr-value
    This field contains the time of the last request.  The time MUST
    be interpreted according to the contents of the accompanying lr-
    type subfield.
 nonce
    This field is described above in Section 5.4.1.
 key-expiration
    The key-expiration field is part of the response from the KDC and
    specifies the time that the client's secret key is due to expire.
    The expiration might be the result of password aging or an account
    expiration.  If present, it SHOULD be set to the earlier of the
    user's key expiration and account expiration.  The use of this
    field is deprecated, and the last-req field SHOULD be used to
    convey this information instead.  This field will usually be left
    out of the TGS reply since the response to the TGS request is
    encrypted in a session key and no client information has to be
    retrieved from the KDC database.  It is up to the application
    client (usually the login program) to take appropriate action
    (such as notifying the user) if the expiration time is imminent.
 flags, authtime, starttime, endtime, renew-till and caddr
    These fields are duplicates of those found in the encrypted
    portion of the attached ticket (see Section 5.3), provided so the
    client MAY verify that they match the intended request and in
    order to assist in proper ticket caching.  If the message is of
    type KRB_TGS_REP, the caddr field will only be filled in if the
    request was for a proxy or forwarded ticket, or if the user is
    substituting a subset of the addresses from the TGT.  If the
    client-requested addresses are not present or not used, then the
    addresses contained in the ticket will be the same as those
    included in the TGT.

5.5. Client/Server (CS) Message Specifications

 This section specifies the format of the messages used for the
 authentication of the client to the application server.

5.5.1. KRB_AP_REQ Definition

 The KRB_AP_REQ message contains the Kerberos protocol version number,
 the message type KRB_AP_REQ, an options field to indicate any options
 in use, and the ticket and authenticator themselves.  The KRB_AP_REQ
 message is often referred to as the "authentication header".

Neuman, et al. Standards Track [Page 84] RFC 4120 Kerberos V5 July 2005

 AP-REQ          ::= [APPLICATION 14] SEQUENCE {
         pvno            [0] INTEGER (5),
         msg-type        [1] INTEGER (14),
         ap-options      [2] APOptions,
         ticket          [3] Ticket,
         authenticator   [4] EncryptedData -- Authenticator
 }
 APOptions       ::= KerberosFlags
         -- reserved(0),
         -- use-session-key(1),
         -- mutual-required(2)
 pvno and msg-type
    These fields are described above in Section 5.4.1. msg-type is
    KRB_AP_REQ.
 ap-options
    This field appears in the application request (KRB_AP_REQ) and
    affects the way the request is processed.  It is a bit-field,
    where the selected options are indicated by the bit being set (1),
    and the unselected options and reserved fields by being reset (0).
    The encoding of the bits is specified in Section 5.2.  The
    meanings of the options are as follows:
 Bit(s)  Name             Description
 0       reserved         Reserved for future expansion of this field.
 1       use-session-key  The USE-SESSION-KEY option indicates that
                          the ticket the client is presenting to a
                          server is encrypted in the session key from
                          the server's TGT.  When this option is not
                          specified, the ticket is encrypted in the
                          server's secret key.
 2       mutual-required  The MUTUAL-REQUIRED option tells the server
                          that the client requires mutual
                          authentication, and that it must respond
                          with a KRB_AP_REP message.
 3-31    reserved         Reserved for future use.
 ticket
    This field is a ticket authenticating the client to the server.

Neuman, et al. Standards Track [Page 85] RFC 4120 Kerberos V5 July 2005

 authenticator
    This contains the encrypted authenticator, which includes the
    client's choice of a subkey.
 The encrypted authenticator is included in the AP-REQ; it certifies
 to a server that the sender has recent knowledge of the encryption
 key in the accompanying ticket, to help the server detect replays.
 It also assists in the selection of a "true session key" to use with
 the particular session.  The DER encoding of the following is
 encrypted in the ticket's session key, with a key usage value of 11
 in normal application exchanges, or 7 when used as the PA-TGS-REQ
 PA-DATA field of a TGS-REQ exchange (see Section 5.4.1):
  1. - Unencrypted authenticator

Authenticator ::= [APPLICATION 2] SEQUENCE {

         authenticator-vno       [0] INTEGER (5),
         crealm                  [1] Realm,
         cname                   [2] PrincipalName,
         cksum                   [3] Checksum OPTIONAL,
         cusec                   [4] Microseconds,
         ctime                   [5] KerberosTime,
         subkey                  [6] EncryptionKey OPTIONAL,
         seq-number              [7] UInt32 OPTIONAL,
         authorization-data      [8] AuthorizationData OPTIONAL
 }
 authenticator-vno
    This field specifies the version number for the format of the
    authenticator.  This document specifies version 5.
 crealm and cname
    These fields are the same as those described for the ticket in
    section 5.3.
 cksum
    This field contains a checksum of the application data that
    accompanies the KRB_AP_REQ, computed using a key usage value of 10
    in normal application exchanges, or 6 when used in the TGS-REQ
    PA-TGS-REQ AP-DATA field.
 cusec
    This field contains the microsecond part of the client's
    timestamp.  Its value (before encryption) ranges from 0 to 999999.
    It often appears along with ctime.  The two fields are used
    together to specify a reasonably accurate timestamp.
 ctime
    This field contains the current time on the client's host.

Neuman, et al. Standards Track [Page 86] RFC 4120 Kerberos V5 July 2005

 subkey
    This field contains the client's choice for an encryption key to
    be used to protect this specific application session.  Unless an
    application specifies otherwise, if this field is left out, the
    session key from the ticket will be used.
 seq-number
    This optional field includes the initial sequence number to be
    used by the KRB_PRIV or KRB_SAFE messages when sequence numbers
    are used to detect replays.  (It may also be used by application
    specific messages.)  When included in the authenticator, this
    field specifies the initial sequence number for messages from the
    client to the server.  When included in the AP-REP message, the
    initial sequence number is that for messages from the server to
    the client.  When used in KRB_PRIV or KRB_SAFE messages, it is
    incremented by one after each message is sent.  Sequence numbers
    fall in the range 0 through 2^32 - 1 and wrap to zero following
    the value 2^32 - 1.
    For sequence numbers to support the detection of replays
    adequately, they SHOULD be non-repeating, even across connection
    boundaries.  The initial sequence number SHOULD be random and
    uniformly distributed across the full space of possible sequence
    numbers, so that it cannot be guessed by an attacker and so that
    it and the successive sequence numbers do not repeat other
    sequences.  In the event that more than 2^32 messages are to be
    generated in a series of KRB_PRIV or KRB_SAFE messages, rekeying
    SHOULD be performed before sequence numbers are reused with the
    same encryption key.
    Implmentation note: Historically, some implementations transmit
    signed twos-complement numbers for sequence numbers.  In the
    interests of compatibility, implementations MAY accept the
    equivalent negative number where a positive number greater than
    2^31 - 1 is expected.
    Implementation note: As noted before, some implementations omit
    the optional sequence number when its value would be zero.
    Implementations MAY accept an omitted sequence number when
    expecting a value of zero, and SHOULD NOT transmit an
    Authenticator with a initial sequence number of zero.
 authorization-data
    This field is the same as described for the ticket in Section 5.3.
    It is optional and will only appear when additional restrictions
    are to be placed on the use of a ticket, beyond those carried in
    the ticket itself.

Neuman, et al. Standards Track [Page 87] RFC 4120 Kerberos V5 July 2005

5.5.2. KRB_AP_REP Definition

 The KRB_AP_REP message contains the Kerberos protocol version number,
 the message type, and an encrypted time-stamp.  The message is sent
 in response to an application request (KRB_AP_REQ) for which the
 mutual authentication option has been selected in the ap-options
 field.
 AP-REP          ::= [APPLICATION 15] SEQUENCE {
         pvno            [0] INTEGER (5),
         msg-type        [1] INTEGER (15),
         enc-part        [2] EncryptedData -- EncAPRepPart
 }
 EncAPRepPart    ::= [APPLICATION 27] SEQUENCE {
         ctime           [0] KerberosTime,
         cusec           [1] Microseconds,
         subkey          [2] EncryptionKey OPTIONAL,
         seq-number      [3] UInt32 OPTIONAL
 }
 The encoded EncAPRepPart is encrypted in the shared session key of
 the ticket.  The optional subkey field can be used in an
 application-arranged negotiation to choose a per association session
 key.
 pvno and msg-type
    These fields are described above in Section 5.4.1.  msg-type is
    KRB_AP_REP.
 enc-part
    This field is described above in Section 5.4.2.  It is computed
    with a key usage value of 12.
 ctime
    This field contains the current time on the client's host.
 cusec
    This field contains the microsecond part of the client's
    timestamp.
 subkey
    This field contains an encryption key that is to be used to
    protect this specific application session.  See Section 3.2.6 for
    specifics on how this field is used to negotiate a key.  Unless an
    application specifies otherwise, if this field is left out, the
    sub-session key from the authenticator or if the latter is also
    left out, the session key from the ticket will be used.

Neuman, et al. Standards Track [Page 88] RFC 4120 Kerberos V5 July 2005

 seq-number
    This field is described above in Section 5.3.2.

5.5.3. Error Message Reply

 If an error occurs while processing the application request, the
 KRB_ERROR message will be sent in response.  See Section 5.9.1 for
 the format of the error message.  The cname and crealm fields MAY be
 left out if the server cannot determine their appropriate values from
 the corresponding KRB_AP_REQ message.  If the authenticator was
 decipherable, the ctime and cusec fields will contain the values from
 it.

5.6. KRB_SAFE Message Specification

 This section specifies the format of a message that can be used by
 either side (client or server) of an application to send a tamper-
 proof message to its peer.  It presumes that a session key has
 previously been exchanged (for example, by using the
 KRB_AP_REQ/KRB_AP_REP messages).

5.6.1. KRB_SAFE definition

 The KRB_SAFE message contains user data along with a collision-proof
 checksum keyed with the last encryption key negotiated via subkeys,
 or with the session key if no negotiation has occurred.  The message
 fields are as follows:
 KRB-SAFE        ::= [APPLICATION 20] SEQUENCE {
         pvno            [0] INTEGER (5),
         msg-type        [1] INTEGER (20),
         safe-body       [2] KRB-SAFE-BODY,
         cksum           [3] Checksum
 }
 KRB-SAFE-BODY   ::= SEQUENCE {
         user-data       [0] OCTET STRING,
         timestamp       [1] KerberosTime OPTIONAL,
         usec            [2] Microseconds OPTIONAL,
         seq-number      [3] UInt32 OPTIONAL,
         s-address       [4] HostAddress,
         r-address       [5] HostAddress OPTIONAL
 }
 pvno and msg-type
    These fields are described above in Section 5.4.1.  msg-type is
    KRB_SAFE.

Neuman, et al. Standards Track [Page 89] RFC 4120 Kerberos V5 July 2005

 safe-body
    This field is a placeholder for the body of the KRB-SAFE message.
 cksum
    This field contains the checksum of the application data, computed
    with a key usage value of 15.
    The checksum is computed over the encoding of the KRB-SAFE
    sequence.  First, the cksum is set to a type zero, zero-length
    value, and the checksum is computed over the encoding of the KRB-
    SAFE sequence.  Then the checksum is set to the result of that
    computation.  Finally, the KRB-SAFE sequence is encoded again.
    This method, although different than the one specified in RFC
    1510, corresponds to existing practice.
 user-data
    This field is part of the KRB_SAFE and KRB_PRIV messages, and
    contains the application-specific data that is being passed from
    the sender to the recipient.
 timestamp
    This field is part of the KRB_SAFE and KRB_PRIV messages.  Its
    contents are the current time as known by the sender of the
    message.  By checking the timestamp, the recipient of the message
    is able to make sure that it was recently generated, and is not a
    replay.
 usec
    This field is part of the KRB_SAFE and KRB_PRIV headers.  It
    contains the microsecond part of the timestamp.
 seq-number
    This field is described above in Section 5.3.2.
 s-address
    Sender's address.
    This field specifies the address in use by the sender of the
    message.
 r-address
    This field specifies the address in use by the recipient of the
    message.  It MAY be omitted for some uses (such as broadcast
    protocols), but the recipient MAY arbitrarily reject such
    messages.  This field, along with s-address, can be used to help
    detect messages that have been incorrectly or maliciously
    delivered to the wrong recipient.

Neuman, et al. Standards Track [Page 90] RFC 4120 Kerberos V5 July 2005

5.7. KRB_PRIV Message Specification

 This section specifies the format of a message that can be used by
 either side (client or server) of an application to send a message to
 its peer securely and privately.  It presumes that a session key has
 previously been exchanged (for example, by using the
 KRB_AP_REQ/KRB_AP_REP messages).

5.7.1. KRB_PRIV Definition

 The KRB_PRIV message contains user data encrypted in the Session Key.
 The message fields are as follows:
 KRB-PRIV        ::= [APPLICATION 21] SEQUENCE {
         pvno            [0] INTEGER (5),
         msg-type        [1] INTEGER (21),
                         -- NOTE: there is no [2] tag
         enc-part        [3] EncryptedData -- EncKrbPrivPart
 }
 EncKrbPrivPart  ::= [APPLICATION 28] SEQUENCE {
         user-data       [0] OCTET STRING,
         timestamp       [1] KerberosTime OPTIONAL,
         usec            [2] Microseconds OPTIONAL,
         seq-number      [3] UInt32 OPTIONAL,
         s-address       [4] HostAddress -- sender's addr --,
         r-address       [5] HostAddress OPTIONAL -- recip's addr
 }
 pvno and msg-type
    These fields are described above in Section 5.4.1.  msg-type is
    KRB_PRIV.
 enc-part
    This field holds an encoding of the EncKrbPrivPart sequence
    encrypted under the session key, with a key usage value of 13.
    This encrypted encoding is used for the enc-part field of the
    KRB-PRIV message.
 user-data, timestamp, usec, s-address, and r-address
    These fields are described above in Section 5.6.1.
 seq-number
    This field is described above in Section 5.3.2.

Neuman, et al. Standards Track [Page 91] RFC 4120 Kerberos V5 July 2005

5.8. KRB_CRED Message Specification

 This section specifies the format of a message that can be used to
 send Kerberos credentials from one principal to another.  It is
 presented here to encourage a common mechanism to be used by
 applications when forwarding tickets or providing proxies to
 subordinate servers.  It presumes that a session key has already been
 exchanged, perhaps by using the KRB_AP_REQ/KRB_AP_REP messages.

5.8.1. KRB_CRED Definition

 The KRB_CRED message contains a sequence of tickets to be sent and
 information needed to use the tickets, including the session key from
 each.  The information needed to use the tickets is encrypted under
 an encryption key previously exchanged or transferred alongside the
 KRB_CRED message.  The message fields are as follows:
 KRB-CRED        ::= [APPLICATION 22] SEQUENCE {
         pvno            [0] INTEGER (5),
         msg-type        [1] INTEGER (22),
         tickets         [2] SEQUENCE OF Ticket,
         enc-part        [3] EncryptedData -- EncKrbCredPart
 }
 EncKrbCredPart  ::= [APPLICATION 29] SEQUENCE {
         ticket-info     [0] SEQUENCE OF KrbCredInfo,
         nonce           [1] UInt32 OPTIONAL,
         timestamp       [2] KerberosTime OPTIONAL,
         usec            [3] Microseconds OPTIONAL,
         s-address       [4] HostAddress OPTIONAL,
         r-address       [5] HostAddress OPTIONAL
 }
 KrbCredInfo     ::= SEQUENCE {
         key             [0] EncryptionKey,
         prealm          [1] Realm OPTIONAL,
         pname           [2] PrincipalName OPTIONAL,
         flags           [3] TicketFlags OPTIONAL,
         authtime        [4] KerberosTime OPTIONAL,
         starttime       [5] KerberosTime OPTIONAL,
         endtime         [6] KerberosTime OPTIONAL,
         renew-till      [7] KerberosTime OPTIONAL,
         srealm          [8] Realm OPTIONAL,
         sname           [9] PrincipalName OPTIONAL,
         caddr           [10] HostAddresses OPTIONAL
 }

Neuman, et al. Standards Track [Page 92] RFC 4120 Kerberos V5 July 2005

 pvno and msg-type
    These fields are described above in Section 5.4.1.  msg-type is
    KRB_CRED.
 tickets
    These are the tickets obtained from the KDC specifically for use
    by the intended recipient.  Successive tickets are paired with the
    corresponding KrbCredInfo sequence from the enc-part of the KRB-
    CRED message.
 enc-part
    This field holds an encoding of the EncKrbCredPart sequence
    encrypted under the session key shared by the sender and the
    intended recipient, with a key usage value of 14.  This encrypted
    encoding is used for the enc-part field of the KRB-CRED message.
    Implementation note: Implementations of certain applications, most
    notably certain implementations of the Kerberos GSS-API mechanism,
    do not separately encrypt the contents of the EncKrbCredPart of
    the KRB-CRED message when sending it.  In the case of those GSS-
    API mechanisms, this is not a security vulnerability, as the
    entire KRB-CRED message is itself embedded in an encrypted
    message.
 nonce
    If practical, an application MAY require the inclusion of a nonce
    generated by the recipient of the message.  If the same value is
    included as the nonce in the message, it provides evidence that
    the message is fresh and has not been replayed by an attacker.  A
    nonce MUST NEVER be reused.
 timestamp and usec
    These fields specify the time that the KRB-CRED message was
    generated.  The time is used to provide assurance that the message
    is fresh.
 s-address and r-address
    These fields are described above in Section 5.6.1.  They are used
    optionally to provide additional assurance of the integrity of the
    KRB-CRED message.
 key
    This field exists in the corresponding ticket passed by the KRB-
    CRED message and is used to pass the session key from the sender
    to the intended recipient.  The field's encoding is described in
    Section 5.2.9.

Neuman, et al. Standards Track [Page 93] RFC 4120 Kerberos V5 July 2005

 The following fields are optional.  If present, they can be
 associated with the credentials in the remote ticket file.  If left
 out, then it is assumed that the recipient of the credentials already
 knows their values.
 prealm and pname
    The name and realm of the delegated principal identity.
 flags, authtime, starttime, endtime, renew-till, srealm, sname,
 and caddr
    These fields contain the values of the corresponding fields from
    the ticket found in the ticket field.  Descriptions of the fields
    are identical to the descriptions in the KDC-REP message.

5.9. Error Message Specification

 This section specifies the format for the KRB_ERROR message.  The
 fields included in the message are intended to return as much
 information as possible about an error.  It is not expected that all
 the information required by the fields will be available for all
 types of errors.  If the appropriate information is not available
 when the message is composed, the corresponding field will be left
 out of the message.
 Note that because the KRB_ERROR message is not integrity protected,
 it is quite possible for an intruder to synthesize or modify it.  In
 particular, this means that the client SHOULD NOT use any fields in
 this message for security-critical purposes, such as setting a system
 clock or generating a fresh authenticator.  The message can be
 useful, however, for advising a user on the reason for some failure.

5.9.1. KRB_ERROR Definition

 The KRB_ERROR message consists of the following fields:
 KRB-ERROR       ::= [APPLICATION 30] SEQUENCE {
         pvno            [0] INTEGER (5),
         msg-type        [1] INTEGER (30),
         ctime           [2] KerberosTime OPTIONAL,
         cusec           [3] Microseconds OPTIONAL,
         stime           [4] KerberosTime,
         susec           [5] Microseconds,
         error-code      [6] Int32,
         crealm          [7] Realm OPTIONAL,
         cname           [8] PrincipalName OPTIONAL,
         realm           [9] Realm -- service realm --,
         sname           [10] PrincipalName -- service name --,
         e-text          [11] KerberosString OPTIONAL,

Neuman, et al. Standards Track [Page 94] RFC 4120 Kerberos V5 July 2005

         e-data          [12] OCTET STRING OPTIONAL
 }
 pvno and msg-type
    These fields are described above in Section 5.4.1.  msg-type is
    KRB_ERROR.
 ctime and cusec
    These fields are described above in Section 5.5.2.  If the values
    for these fields are known to the entity generating the error (as
    they would be if the KRB-ERROR is generated in reply to, e.g., a
    failed authentication service request), they should be populated
    in the KRB-ERROR.  If the values are not available, these fields
    can be omitted.
 stime
    This field contains the current time on the server.  It is of type
    KerberosTime.
 susec
    This field contains the microsecond part of the server's
    timestamp.  Its value ranges from 0 to 999999.  It appears along
    with stime.  The two fields are used in conjunction to specify a
    reasonably accurate timestamp.
 error-code
    This field contains the error code returned by Kerberos or the
    server when a request fails.  To interpret the value of this field
    see the list of error codes in Section 7.5.9.  Implementations are
    encouraged to provide for national language support in the display
    of error messages.
 crealm, and cname
    These fields are described above in Section 5.3.  When the entity
    generating the error knows these values, they should be populated
    in the KRB-ERROR.  If the values are not known, the crealm and
    cname fields SHOULD be omitted.
 realm and sname
    These fields are described above in Section 5.3.
 e-text
    This field contains additional text to help explain the error code
    associated with the failed request (for example, it might include
    a principal name which was unknown).

Neuman, et al. Standards Track [Page 95] RFC 4120 Kerberos V5 July 2005

 e-data
    This field contains additional data about the error for use by the
    application to help it recover from or handle the error.  If the
    errorcode is KDC_ERR_PREAUTH_REQUIRED, then the e-data field will
    contain an encoding of a sequence of padata fields, each
    corresponding to an acceptable pre-authentication method and
    optionally containing data for the method:
    METHOD-DATA     ::= SEQUENCE OF PA-DATA
 For error codes defined in this document other than
 KDC_ERR_PREAUTH_REQUIRED, the format and contents of the e-data field
 are implementation-defined.  Similarly, for future error codes, the
 format and contents of the e-data field are implementation-defined
 unless specified otherwise.  Whether defined by the implementation or
 in a future document, the e-data field MAY take the form of TYPED-
 DATA:
 TYPED-DATA      ::= SEQUENCE SIZE (1..MAX) OF SEQUENCE {
         data-type       [0] Int32,
         data-value      [1] OCTET STRING OPTIONAL
 }

5.10. Application Tag Numbers

 The following table lists the application class tag numbers used by
 various data types defined in this section.
 Tag Number(s)  Type Name      Comments
 0                             unused
 1              Ticket         PDU
 2              Authenticator  non-PDU
 3              EncTicketPart  non-PDU
 4-9                           unused
 10             AS-REQ         PDU
 11             AS-REP         PDU
 12             TGS-REQ        PDU
 13             TGS-REP        PDU

Neuman, et al. Standards Track [Page 96] RFC 4120 Kerberos V5 July 2005

 14             AP-REQ         PDU
 15             AP-REP         PDU
 16             RESERVED16     TGT-REQ (for user-to-user)
 17             RESERVED17     TGT-REP (for user-to-user)
 18-19                         unused
 20             KRB-SAFE       PDU
 21             KRB-PRIV       PDU
 22             KRB-CRED       PDU
 23-24                         unused
 25             EncASRepPart   non-PDU
 26             EncTGSRepPart  non-PDU
 27             EncApRepPart   non-PDU
 28             EncKrbPrivPart non-PDU
 29             EncKrbCredPart non-PDU
 30             KRB-ERROR      PDU
 The ASN.1 types marked above as "PDU" (Protocol Data Unit) are the
 only ASN.1 types intended as top-level types of the Kerberos
 protocol, and are the only types that may be used as elements in
 another protocol that makes use of Kerberos.

6. Naming Constraints

6.1. Realm Names

 Although realm names are encoded as GeneralStrings and technically a
 realm can select any name it chooses, interoperability across realm
 boundaries requires agreement on how realm names are to be assigned,
 and what information they imply.
 To enforce these conventions, each realm MUST conform to the
 conventions itself, and it MUST require that any realms with which
 inter-realm keys are shared also conform to the conventions and
 require the same from its neighbors.

Neuman, et al. Standards Track [Page 97] RFC 4120 Kerberos V5 July 2005

 Kerberos realm names are case sensitive.  Realm names that differ
 only in the case of the characters are not equivalent.  There are
 presently three styles of realm names: domain, X500, and other.
 Examples of each style follow:
      domain:   ATHENA.MIT.EDU
        X500:   C=US/O=OSF
       other:   NAMETYPE:rest/of.name=without-restrictions
 Domain style realm names MUST look like domain names: they consist of
 components separated by periods (.) and they contain neither colons
 (:) nor slashes (/).  Though domain names themselves are case
 insensitive, in order for realms to match, the case must match as
 well.  When establishing a new realm name based on an internet domain
 name it is recommended by convention that the characters be converted
 to uppercase.
 X.500 names contain an equals sign (=) and cannot contain a colon (:)
 before the equals sign.  The realm names for X.500 names will be
 string representations of the names with components separated by
 slashes.  Leading and trailing slashes will not be included.  Note
 that the slash separator is consistent with Kerberos implementations
 based on RFC 1510, but it is different from the separator recommended
 in RFC 2253.
 Names that fall into the other category MUST begin with a prefix that
 contains no equals sign (=) or period (.), and the prefix MUST be
 followed by a colon (:) and the rest of the name.  All prefixes
 expect those beginning with used.  Presently none are assigned.
 The reserved category includes strings that do not fall into the
 first three categories.  All names in this category are reserved.  It
 is unlikely that names will be assigned to this category unless there
 is a very strong argument for not using the 'other' category.
 These rules guarantee that there will be no conflicts between the
 various name styles.  The following additional constraints apply to
 the assignment of realm names in the domain and X.500 categories:
 either the name of a realm for the domain or X.500 formats must be
 used by the organization owning (to whom it was assigned) an Internet
 domain name or X.500 name, or, in the case that no such names are
 registered, authority to use a realm name MAY be derived from the
 authority of the parent realm.  For example, if there is no domain
 name for E40.MIT.EDU, then the administrator of the MIT.EDU realm can
 authorize the creation of a realm with that name.
 This is acceptable because the organization to which the parent is
 assigned is presumably the organization authorized to assign names to

Neuman, et al. Standards Track [Page 98] RFC 4120 Kerberos V5 July 2005

 its children in the X.500 and domain name systems as well.  If the
 parent assigns a realm name without also registering it in the domain
 name or X.500 hierarchy, it is the parent's responsibility to make
 sure that in the future there will not exist a name identical to the
 realm name of the child unless it is assigned to the same entity as
 the realm name.

6.2. Principal Names

 As was the case for realm names, conventions are needed to ensure
 that all agree on what information is implied by a principal name.
 The name-type field that is part of the principal name indicates the
 kind of information implied by the name.  The name-type SHOULD be
 treated only as a hint to interpreting the meaning of a name.  It is
 not significant when checking for equivalence.  Principal names that
 differ only in the name-type identify the same principal.  The name
 type does not partition the name space.  Ignoring the name type, no
 two names can be the same (i.e., at least one of the components, or
 the realm, MUST be different).  The following name types are defined:
 Name Type       Value  Meaning
 NT-UNKNOWN        0    Name type not known
 NT-PRINCIPAL      1    Just the name of the principal as in DCE,
                          or for users
 NT-SRV-INST       2    Service and other unique instance (krbtgt)
 NT-SRV-HST        3    Service with host name as instance
                          (telnet, rcommands)
 NT-SRV-XHST       4    Service with host as remaining components
 NT-UID            5    Unique ID
 NT-X500-PRINCIPAL 6    Encoded X.509 Distinguished name [RFC2253]
 NT-SMTP-NAME      7    Name in form of SMTP email name
                          (e.g., user@example.com)
 NT-ENTERPRISE    10    Enterprise name - may be mapped to principal
                          name
 When a name implies no information other than its uniqueness at a
 particular time, the name type PRINCIPAL SHOULD be used.  The
 principal name type SHOULD be used for users, and it might also be
 used for a unique server.  If the name is a unique machine-generated
 ID that is guaranteed never to be reassigned, then the name type of
 UID SHOULD be used.  (Note that it is generally a bad idea to
 reassign names of any type since stale entries might remain in access
 control lists.)
 If the first component of a name identifies a service and the
 remaining components identify an instance of the service in a
 server-specified manner, then the name type of SRV-INST SHOULD be

Neuman, et al. Standards Track [Page 99] RFC 4120 Kerberos V5 July 2005

 used.  An example of this name type is the Kerberos ticket-granting
 service whose name has a first component of krbtgt and a second
 component identifying the realm for which the ticket is valid.
 If the first component of a name identifies a service and there is a
 single component following the service name identifying the instance
 as the host on which the server is running, then the name type
 SRV-HST SHOULD be used.  This type is typically used for Internet
 services such as telnet and the Berkeley R commands.  If the separate
 components of the host name appear as successive components following
 the name of the service, then the name type SRV-XHST SHOULD be used.
 This type might be used to identify servers on hosts with X.500
 names, where the slash (/) might otherwise be ambiguous.
 A name type of NT-X500-PRINCIPAL SHOULD be used when a name from an
 X.509 certificate is translated into a Kerberos name.  The encoding
 of the X.509 name as a Kerberos principal shall conform to the
 encoding rules specified in RFC 2253.
 A name type of SMTP allows a name to be of a form that resembles an
 SMTP email name.  This name, including an "@" and a domain name, is
 used as the one component of the principal name.
 A name type of UNKNOWN SHOULD be used when the form of the name is
 not known.  When comparing names, a name of type UNKNOWN will match
 principals authenticated with names of any type.  A principal
 authenticated with a name of type UNKNOWN, however, will only match
 other names of type UNKNOWN.
 Names of any type with an initial component of 'krbtgt' are reserved
 for the Kerberos ticket-granting service.  See Section 7.3 for the
 form of such names.

6.2.1. Name of Server Principals

 The principal identifier for a server on a host will generally be
 composed of two parts: (1) the realm of the KDC with which the server
 is registered, and (2) a two-component name of type NT-SRV-HST, if
 the host name is an Internet domain name, or a multi-component name
 of type NT-SRV-XHST, if the name of the host is of a form (such as
 X.500) that allows slash (/) separators.  The first component of the
 two- or multi-component name will identify the service, and the
 latter components will identify the host.  Where the name of the host
 is not case sensitive (for example, with Internet domain names) the
 name of the host MUST be lowercase.  If specified by the application
 protocol for services such as telnet and the Berkeley R commands that
 run with system privileges, the first component MAY be the string
 'host' instead of a service-specific identifier.

Neuman, et al. Standards Track [Page 100] RFC 4120 Kerberos V5 July 2005

7. Constants and Other Defined Values

7.1. Host Address Types

 All negative values for the host address type are reserved for local
 use.  All non-negative values are reserved for officially assigned
 type fields and interpretations.
 Internet (IPv4) Addresses
    Internet (IPv4) addresses are 32-bit (4-octet) quantities, encoded
    in MSB order (most significant byte first).  The IPv4 loopback
    address SHOULD NOT appear in a Kerberos PDU.  The type of IPv4
    addresses is two (2).
 Internet (IPv6) Addresses
    IPv6 addresses [RFC3513] are 128-bit (16-octet) quantities,
    encoded in MSB order (most significant byte first).  The type of
    IPv6 addresses is twenty-four (24).  The following addresses MUST
    NOT appear in any Kerberos PDU:
  • the Unspecified Address
  • the Loopback Address
  • Link-Local addresses
    This restriction applies to the inclusion in the address fields of
    Kerberos PDUs, but not to the address fields of packets that might
    carry such PDUs.  The restriction is necessary because the use of
    an address with non-global scope could allow the acceptance of a
    message sent from a node that may have the same address, but which
    is not the host intended by the entity that added the restriction.
    If the link-local address type needs to be used for communication,
    then the address restriction in tickets must not be used (i.e.,
    addressless tickets must be used).
    IPv4-mapped IPv6 addresses MUST be represented as addresses of
    type 2.
 DECnet Phase IV Addresses
    DECnet Phase IV addresses are 16-bit addresses, encoded in LSB
    order.  The type of DECnet Phase IV addresses is twelve (12).

Neuman, et al. Standards Track [Page 101] RFC 4120 Kerberos V5 July 2005

 Netbios Addresses
    Netbios addresses are 16-octet addresses typically composed of 1
    to 15 alphanumeric characters and padded with the US-ASCII SPC
    character (code 32).  The 16th octet MUST be the US-ASCII NUL
    character (code 0).  The type of Netbios addresses is twenty (20).
 Directional Addresses
    Including the sender address in KRB_SAFE and KRB_PRIV messages is
    undesirable in many environments because the addresses may be
    changed in transport by network address translators.  However, if
    these addresses are removed, the messages may be subject to a
    reflection attack in which a message is reflected back to its
    originator.  The directional address type provides a way to avoid
    transport addresses and reflection attacks.  Directional addresses
    are encoded as four-byte unsigned integers in network byte order.
    If the message is originated by the party sending the original
    KRB_AP_REQ message, then an address of 0 SHOULD be used.  If the
    message is originated by the party to whom that KRB_AP_REQ was
    sent, then the address 1 SHOULD be used.  Applications involving
    multiple parties can specify the use of other addresses.
    Directional addresses MUST only be used for the sender address
    field in the KRB_SAFE or KRB_PRIV messages.  They MUST NOT be used
    as a ticket address or in a KRB_AP_REQ message.  This address type
    SHOULD only be used in situations where the sending party knows
    that the receiving party supports the address type.  This
    generally means that directional addresses may only be used when
    the application protocol requires their support.  Directional
    addresses are type (3).

7.2. KDC Messaging: IP Transports

 Kerberos defines two IP transport mechanisms for communication
 between clients and servers: UDP/IP and TCP/IP.

7.2.1. UDP/IP transport

 Kerberos servers (KDCs) supporting IP transports MUST accept UDP
 requests and SHOULD listen for them on port 88 (decimal) unless
 specifically configured to listen on an alternative UDP port.
 Alternate ports MAY be used when running multiple KDCs for multiple
 realms on the same host.

Neuman, et al. Standards Track [Page 102] RFC 4120 Kerberos V5 July 2005

 Kerberos clients supporting IP transports SHOULD support the sending
 of UDP requests.  Clients SHOULD use KDC discovery [7.2.3] to
 identify the IP address and port to which they will send their
 request.
 When contacting a KDC for a KRB_KDC_REQ request using UDP/IP
 transport, the client shall send a UDP datagram containing only an
 encoding of the request to the KDC.  The KDC will respond with a
 reply datagram containing only an encoding of the reply message
 (either a KRB_ERROR or a KRB_KDC_REP) to the sending port at the
 sender's IP address.  The response to a request made through UDP/IP
 transport MUST also use UDP/IP transport.  If the response cannot be
 handled using UDP (for example, because it is too large), the KDC
 MUST return KRB_ERR_RESPONSE_TOO_BIG, forcing the client to retry the
 request using the TCP transport.

7.2.2. TCP/IP Transport

 Kerberos servers (KDCs) supporting IP transports MUST accept TCP
 requests and SHOULD listen for them on port 88 (decimal) unless
 specifically configured to listen on an alternate TCP port.
 Alternate ports MAY be used when running multiple KDCs for multiple
 realms on the same host.
 Clients MUST support the sending of TCP requests, but MAY choose to
 try a request initially using the UDP transport.  Clients SHOULD use
 KDC discovery [7.2.3] to identify the IP address and port to which
 they will send their request.
 Implementation note: Some extensions to the Kerberos protocol will
 not succeed if any client or KDC not supporting the TCP transport is
 involved.  Implementations of RFC 1510 were not required to support
 TCP/IP transports.
 When the KRB_KDC_REQ message is sent to the KDC over a TCP stream,
 the response (KRB_KDC_REP or KRB_ERROR message) MUST be returned to
 the client on the same TCP stream that was established for the
 request.  The KDC MAY close the TCP stream after sending a response,
 but MAY leave the stream open for a reasonable period of time if it
 expects a follow-up.  Care must be taken in managing TCP/IP
 connections on the KDC to prevent denial of service attacks based on
 the number of open TCP/IP connections.
 The client MUST be prepared to have the stream closed by the KDC at
 any time after the receipt of a response.  A stream closure SHOULD
 NOT be treated as a fatal error.  Instead, if multiple exchanges are
 required (e.g., certain forms of pre-authentication), the client may
 need to establish a new connection when it is ready to send

Neuman, et al. Standards Track [Page 103] RFC 4120 Kerberos V5 July 2005

 subsequent messages.  A client MAY close the stream after receiving a
 response, and SHOULD close the stream if it does not expect to send
 follow-up messages.
 A client MAY send multiple requests before receiving responses,
 though it must be prepared to handle the connection being closed
 after the first response.
 Each request (KRB_KDC_REQ) and response (KRB_KDC_REP or KRB_ERROR)
 sent over the TCP stream is preceded by the length of the request as
 4 octets in network byte order.  The high bit of the length is
 reserved for future expansion and MUST currently be set to zero.  If
 a KDC that does not understand how to interpret a set high bit of the
 length encoding receives a request with the high order bit of the
 length set, it MUST return a KRB-ERROR message with the error
 KRB_ERR_FIELD_TOOLONG and MUST close the TCP stream.
 If multiple requests are sent over a single TCP connection and the
 KDC sends multiple responses, the KDC is not required to send the
 responses in the order of the corresponding requests.  This may
 permit some implementations to send each response as soon as it is
 ready, even if earlier requests are still being processed (for
 example, waiting for a response from an external device or database).

7.2.3. KDC Discovery on IP Networks

 Kerberos client implementations MUST provide a means for the client
 to determine the location of the Kerberos Key Distribution Centers
 (KDCs).  Traditionally, Kerberos implementations have stored such
 configuration information in a file on each client machine.
 Experience has shown that this method of storing configuration
 information presents problems with out-of-date information and
 scaling, especially when using cross-realm authentication.  This
 section describes a method for using the Domain Name System [RFC1035]
 for storing KDC location information.

7.2.3.1. DNS vs. Kerberos: Case Sensitivity of Realm Names

 In Kerberos, realm names are case sensitive.  Although it is strongly
 encouraged that all realm names be all uppercase, this recommendation
 has not been adopted by all sites.  Some sites use all lowercase
 names and other use mixed case.  DNS, on the other hand, is case
 insensitive for queries.  Because the realm names "MYREALM",
 "myrealm", and "MyRealm" are all different, but resolve the same in
 the domain name system, it is necessary that only one of the possible
 combinations of upper- and lowercase characters be used in realm
 names.

Neuman, et al. Standards Track [Page 104] RFC 4120 Kerberos V5 July 2005

7.2.3.2. Specifying KDC Location Information with DNS SRV records

 KDC location information is to be stored using the DNS SRV RR
 [RFC2782].  The format of this RR is as follows:
    _Service._Proto.Realm TTL Class SRV Priority Weight Port Target
 The Service name for Kerberos is always "kerberos".
 The Proto can be either "udp" or "tcp".  If these SRV records are to
 be used, both "udp" and "tcp" records MUST be specified for all KDC
 deployments.
 The Realm is the Kerberos realm that this record corresponds to.  The
 realm MUST be a domain-style realm name.
 TTL, Class, SRV, Priority, Weight, and Target have the standard
 meaning as defined in RFC 2782.
 As per RFC 2782, the Port number used for "_udp" and "_tcp" SRV
 records SHOULD be the value assigned to "kerberos" by the Internet
 Assigned Number Authority: 88 (decimal), unless the KDC is configured
 to listen on an alternate TCP port.
 Implementation note: Many existing client implementations do not
 support KDC Discovery and are configured to send requests to the IANA
 assigned port (88 decimal), so it is strongly recommended that KDCs
 be configured to listen on that port.

7.2.3.3. KDC Discovery for Domain Style Realm Names on IP Networks

 These are DNS records for a Kerberos realm EXAMPLE.COM.  It has two
 Kerberos servers, kdc1.example.com and kdc2.example.com.  Queries
 should be directed to kdc1.example.com first as per the specified
 priority.  Weights are not used in these sample records.
   _kerberos._udp.EXAMPLE.COM.     IN   SRV   0 0 88 kdc1.example.com.
   _kerberos._udp.EXAMPLE.COM.     IN   SRV   1 0 88 kdc2.example.com.
   _kerberos._tcp.EXAMPLE.COM.     IN   SRV   0 0 88 kdc1.example.com.
   _kerberos._tcp.EXAMPLE.COM.     IN   SRV   1 0 88 kdc2.example.com.

7.3. Name of the TGS

 The principal identifier of the ticket-granting service shall be
 composed of three parts: the realm of the KDC issuing the TGS ticket,
 and a two-part name of type NT-SRV-INST, with the first part "krbtgt"
 and the second part the name of the realm that will accept the TGT.
 For example, a TGT issued by the ATHENA.MIT.EDU realm to be used to

Neuman, et al. Standards Track [Page 105] RFC 4120 Kerberos V5 July 2005

 get tickets from the ATHENA.MIT.EDU KDC has a principal identifier of
 "ATHENA.MIT.EDU" (realm), ("krbtgt", "ATHENA.MIT.EDU") (name).  A TGT
 issued by the ATHENA.MIT.EDU realm to be used to get tickets from the
 MIT.EDU realm has a principal identifier of "ATHENA.MIT.EDU" (realm),
 ("krbtgt", "MIT.EDU") (name).

7.4. OID Arc for KerberosV5

 This OID MAY be used to identify Kerberos protocol messages
 encapsulated in other protocols.  It also designates the OID arc for
 KerberosV5-related OIDs assigned by future IETF action.
 Implementation note: RFC 1510 had an incorrect value (5) for "dod" in
 its OID.
 id-krb5         OBJECT IDENTIFIER ::= {
         iso(1) identified-organization(3) dod(6) internet(1)
         security(5) kerberosV5(2)
 }
 Assignment of OIDs beneath the id-krb5 arc must be obtained by
 contacting the registrar for the id-krb5 arc, or its designee.  At
 the time of the issuance of this RFC, such registrations can be
 obtained by contacting krb5-oid-registrar@mit.edu.

7.5. Protocol Constants and Associated Values

 The following tables list constants used in the protocol and define
 their meanings.  In the "specification" section, ranges are specified
 that limit the values of constants for which values are defined here.
 This allows implementations to make assumptions about the maximum
 values that will be received for these constants.  Implementations
 receiving values outside the range specified in the "specification"
 section MAY reject the request, but they MUST recover cleanly.

7.5.1. Key Usage Numbers

 The encryption and checksum specifications in [RFC3961] require as
 input a "key usage number", to alter the encryption key used in any
 specific message in order to make certain types of cryptographic
 attack more difficult.  These are the key usage values assigned in
 this document:
         1.  AS-REQ PA-ENC-TIMESTAMP padata timestamp, encrypted with
             the client key (Section 5.2.7.2)

Neuman, et al. Standards Track [Page 106] RFC 4120 Kerberos V5 July 2005

         2.  AS-REP Ticket and TGS-REP Ticket (includes TGS session
             key or application session key), encrypted with the
             service key (Section 5.3)
         3.  AS-REP encrypted part (includes TGS session key or
             application session key), encrypted with the client key
             (Section 5.4.2)
         4.  TGS-REQ KDC-REQ-BODY AuthorizationData, encrypted with
             the TGS session key (Section 5.4.1)
         5.  TGS-REQ KDC-REQ-BODY AuthorizationData, encrypted with
             the TGS authenticator subkey (Section 5.4.1)
         6.  TGS-REQ PA-TGS-REQ padata AP-REQ Authenticator cksum,
             keyed with the TGS session key (Section 5.5.1)
         7.  TGS-REQ PA-TGS-REQ padata AP-REQ Authenticator (includes
             TGS authenticator subkey), encrypted with the TGS session
             key (Section 5.5.1)
         8.  TGS-REP encrypted part (includes application session
             key), encrypted with the TGS session key (Section 5.4.2)
         9.  TGS-REP encrypted part (includes application session
             key), encrypted with the TGS authenticator subkey
             (Section 5.4.2)
        10.  AP-REQ Authenticator cksum, keyed with the application
             session key (Section 5.5.1)
        11.  AP-REQ Authenticator (includes application authenticator
             subkey), encrypted with the application session key
             (Section 5.5.1)
        12.  AP-REP encrypted part (includes application session
             subkey), encrypted with the application session key
             (Section 5.5.2)
        13.  KRB-PRIV encrypted part, encrypted with a key chosen by
             the application (Section 5.7.1)
        14.  KRB-CRED encrypted part, encrypted with a key chosen by
             the application (Section 5.8.1)
        15.  KRB-SAFE cksum, keyed with a key chosen by the
             application (Section 5.6.1)
     16-18.  Reserved for future use in Kerberos and related
             protocols.
        19.  AD-KDC-ISSUED checksum (ad-checksum in 5.2.6.4)
     20-21.  Reserved for future use in Kerberos and related
             protocols.
     22-25.  Reserved for use in the Kerberos Version 5 GSS-API
             mechanisms [RFC4121].
    26-511.  Reserved for future use in Kerberos and related
             protocols.
  512-1023.  Reserved for uses internal to a Kerberos implementation.
      1024.  Encryption for application use in protocols that do not
             specify key usage values

Neuman, et al. Standards Track [Page 107] RFC 4120 Kerberos V5 July 2005

      1025.  Checksums for application use in protocols that do not
             specify key usage values
 1026-2047.  Reserved for application use.

7.5.2. PreAuthentication Data Types

 Padata and Data Type    Padata-type   Comment
                          Value
 PA-TGS-REQ                  1
 PA-ENC-TIMESTAMP            2
 PA-PW-SALT                  3
 [reserved]                  4
 PA-ENC-UNIX-TIME            5        (deprecated)
 PA-SANDIA-SECUREID          6
 PA-SESAME                   7
 PA-OSF-DCE                  8
 PA-CYBERSAFE-SECUREID       9
 PA-AFS3-SALT                10
 PA-ETYPE-INFO               11
 PA-SAM-CHALLENGE            12       (sam/otp)
 PA-SAM-RESPONSE             13       (sam/otp)
 PA-PK-AS-REQ_OLD            14       (pkinit)
 PA-PK-AS-REP_OLD            15       (pkinit)
 PA-PK-AS-REQ                16       (pkinit)
 PA-PK-AS-REP                17       (pkinit)
 PA-ETYPE-INFO2              19       (replaces pa-etype-info)
 PA-USE-SPECIFIED-KVNO       20
 PA-SAM-REDIRECT             21       (sam/otp)
 PA-GET-FROM-TYPED-DATA      22       (embedded in typed data)
 TD-PADATA                   22       (embeds padata)
 PA-SAM-ETYPE-INFO           23       (sam/otp)
 PA-ALT-PRINC                24       (crawdad@fnal.gov)
 PA-SAM-CHALLENGE2           30       (kenh@pobox.com)
 PA-SAM-RESPONSE2            31       (kenh@pobox.com)
 PA-EXTRA-TGT                41       Reserved extra TGT
 TD-PKINIT-CMS-CERTIFICATES  101      CertificateSet from CMS
 TD-KRB-PRINCIPAL            102      PrincipalName
 TD-KRB-REALM                103      Realm
 TD-TRUSTED-CERTIFIERS       104      from PKINIT
 TD-CERTIFICATE-INDEX        105      from PKINIT
 TD-APP-DEFINED-ERROR        106      application specific
 TD-REQ-NONCE                107      INTEGER
 TD-REQ-SEQ                  108      INTEGER
 PA-PAC-REQUEST              128      (jbrezak@exchange.microsoft.com)

Neuman, et al. Standards Track [Page 108] RFC 4120 Kerberos V5 July 2005

7.5.3. Address Types

 Address Type                   Value
 IPv4                             2
 Directional                      3
 ChaosNet                         5
 XNS                              6
 ISO                              7
 DECNET Phase IV                 12
 AppleTalk DDP                   16
 NetBios                         20
 IPv6                            24

7.5.4. Authorization Data Types

 Authorization Data Type          Ad-type Value
 AD-IF-RELEVANT                     1
 AD-INTENDED-FOR-SERVER             2
 AD-INTENDED-FOR-APPLICATION-CLASS  3
 AD-KDC-ISSUED                      4
 AD-AND-OR                          5
 AD-MANDATORY-TICKET-EXTENSIONS     6
 AD-IN-TICKET-EXTENSIONS            7
 AD-MANDATORY-FOR-KDC               8
 Reserved values                 9-63
 OSF-DCE                           64
 SESAME                            65
 AD-OSF-DCE-PKI-CERTID             66 (hemsath@us.ibm.com)
 AD-WIN2K-PAC                     128 (jbrezak@exchange.microsoft.com)
 AD-ETYPE-NEGOTIATION             129  (lzhu@windows.microsoft.com)

7.5.5. Transited Encoding Types

 Transited Encoding Type         Tr-type Value
 DOMAIN-X500-COMPRESS            1
 Reserved values                 All others

7.5.6. Protocol Version Number

 Label               Value   Meaning or MIT Code
 pvno                  5     Current Kerberos protocol version number

Neuman, et al. Standards Track [Page 109] RFC 4120 Kerberos V5 July 2005

7.5.7. Kerberos Message Types

 Message Type   Value  Meaning
 KRB_AS_REQ      10    Request for initial authentication
 KRB_AS_REP      11    Response to KRB_AS_REQ request
 KRB_TGS_REQ     12    Request for authentication based on TGT
 KRB_TGS_REP     13    Response to KRB_TGS_REQ request
 KRB_AP_REQ      14    Application request to server
 KRB_AP_REP      15    Response to KRB_AP_REQ_MUTUAL
 KRB_RESERVED16  16    Reserved for user-to-user krb_tgt_request
 KRB_RESERVED17  17    Reserved for user-to-user krb_tgt_reply
 KRB_SAFE        20    Safe (checksummed) application message
 KRB_PRIV        21    Private (encrypted) application message
 KRB_CRED        22    Private (encrypted) message to forward
                         credentials
 KRB_ERROR       30    Error response

7.5.8. Name Types

 Name Type           Value  Meaning
 KRB_NT_UNKNOWN        0    Name type not known
 KRB_NT_PRINCIPAL      1    Just the name of the principal as in DCE,
                              or for users
 KRB_NT_SRV_INST       2    Service and other unique instance (krbtgt)
 KRB_NT_SRV_HST        3    Service with host name as instance
                              (telnet, rcommands)
 KRB_NT_SRV_XHST       4    Service with host as remaining components
 KRB_NT_UID            5    Unique ID
 KRB_NT_X500_PRINCIPAL 6    Encoded X.509 Distinguished name [RFC2253]
 KRB_NT_SMTP_NAME      7    Name in form of SMTP email name
                              (e.g., user@example.com)
 KRB_NT_ENTERPRISE    10    Enterprise name; may be mapped to
                              principal name

7.5.9. Error Codes

 Error Code                         Value  Meaning
 KDC_ERR_NONE                           0  No error
 KDC_ERR_NAME_EXP                       1  Client's entry in database
                                             has expired
 KDC_ERR_SERVICE_EXP                    2  Server's entry in database
                                             has expired
 KDC_ERR_BAD_PVNO                       3  Requested protocol version
                                             number not supported

Neuman, et al. Standards Track [Page 110] RFC 4120 Kerberos V5 July 2005

 KDC_ERR_C_OLD_MAST_KVNO                4  Client's key encrypted in
                                             old master key
 KDC_ERR_S_OLD_MAST_KVNO                5  Server's key encrypted in
                                             old master key
 KDC_ERR_C_PRINCIPAL_UNKNOWN            6  Client not found in
                                             Kerberos database
 KDC_ERR_S_PRINCIPAL_UNKNOWN            7  Server not found in
                                             Kerberos database
 KDC_ERR_PRINCIPAL_NOT_UNIQUE           8  Multiple principal entries
                                             in database
 KDC_ERR_NULL_KEY                       9  The client or server has a
                                             null key
 KDC_ERR_CANNOT_POSTDATE               10  Ticket not eligible for
                                             postdating
 KDC_ERR_NEVER_VALID                   11  Requested starttime is
                                             later than end time
 KDC_ERR_POLICY                        12  KDC policy rejects request
 KDC_ERR_BADOPTION                     13  KDC cannot accommodate
                                             requested option
 KDC_ERR_ETYPE_NOSUPP                  14  KDC has no support for
                                             encryption type
 KDC_ERR_SUMTYPE_NOSUPP                15  KDC has no support for
                                             checksum type
 KDC_ERR_PADATA_TYPE_NOSUPP            16  KDC has no support for
                                             padata type
 KDC_ERR_TRTYPE_NOSUPP                 17  KDC has no support for
                                             transited type
 KDC_ERR_CLIENT_REVOKED                18  Clients credentials have
                                             been revoked
 KDC_ERR_SERVICE_REVOKED               19  Credentials for server have
                                             been revoked
 KDC_ERR_TGT_REVOKED                   20  TGT has been revoked
 KDC_ERR_CLIENT_NOTYET                 21  Client not yet valid; try
                                             again later
 KDC_ERR_SERVICE_NOTYET                22  Server not yet valid; try
                                             again later
 KDC_ERR_KEY_EXPIRED                   23  Password has expired;
                                             change password to reset
 KDC_ERR_PREAUTH_FAILED                24  Pre-authentication
                                             information was invalid
 KDC_ERR_PREAUTH_REQUIRED              25  Additional pre-
                                             authentication required
 KDC_ERR_SERVER_NOMATCH                26  Requested server and ticket
                                             don't match
 KDC_ERR_MUST_USE_USER2USER            27  Server principal valid for
                                             user2user only
 KDC_ERR_PATH_NOT_ACCEPTED             28  KDC Policy rejects
                                             transited path

Neuman, et al. Standards Track [Page 111] RFC 4120 Kerberos V5 July 2005

 KDC_ERR_SVC_UNAVAILABLE               29  A service is not available
 KRB_AP_ERR_BAD_INTEGRITY              31  Integrity check on
                                             decrypted field failed
 KRB_AP_ERR_TKT_EXPIRED                32  Ticket expired
 KRB_AP_ERR_TKT_NYV                    33  Ticket not yet valid
 KRB_AP_ERR_REPEAT                     34  Request is a replay
 KRB_AP_ERR_NOT_US                     35  The ticket isn't for us
 KRB_AP_ERR_BADMATCH                   36  Ticket and authenticator
                                             don't match
 KRB_AP_ERR_SKEW                       37  Clock skew too great
 KRB_AP_ERR_BADADDR                    38  Incorrect net address
 KRB_AP_ERR_BADVERSION                 39  Protocol version mismatch
 KRB_AP_ERR_MSG_TYPE                   40  Invalid msg type
 KRB_AP_ERR_MODIFIED                   41  Message stream modified
 KRB_AP_ERR_BADORDER                   42  Message out of order
 KRB_AP_ERR_BADKEYVER                  44  Specified version of key is
                                             not available
 KRB_AP_ERR_NOKEY                      45  Service key not available
 KRB_AP_ERR_MUT_FAIL                   46  Mutual authentication
                                             failed
 KRB_AP_ERR_BADDIRECTION               47  Incorrect message direction
 KRB_AP_ERR_METHOD                     48  Alternative authentication
                                             method required
 KRB_AP_ERR_BADSEQ                     49  Incorrect sequence number
                                             in message
 KRB_AP_ERR_INAPP_CKSUM                50  Inappropriate type of
                                             checksum in message
 KRB_AP_PATH_NOT_ACCEPTED              51  Policy rejects transited
                                             path
 KRB_ERR_RESPONSE_TOO_BIG              52  Response too big for UDP;
                                             retry with TCP
 KRB_ERR_GENERIC                       60  Generic error (description
                                             in e-text)
 KRB_ERR_FIELD_TOOLONG                 61  Field is too long for this
                                             implementation
 KDC_ERROR_CLIENT_NOT_TRUSTED          62  Reserved for PKINIT
 KDC_ERROR_KDC_NOT_TRUSTED             63  Reserved for PKINIT
 KDC_ERROR_INVALID_SIG                 64  Reserved for PKINIT
 KDC_ERR_KEY_TOO_WEAK                  65  Reserved for PKINIT
 KDC_ERR_CERTIFICATE_MISMATCH          66  Reserved for PKINIT
 KRB_AP_ERR_NO_TGT                     67  No TGT available to
                                             validate USER-TO-USER
 KDC_ERR_WRONG_REALM                   68  Reserved for future use
 KRB_AP_ERR_USER_TO_USER_REQUIRED      69  Ticket must be for
                                             USER-TO-USER
 KDC_ERR_CANT_VERIFY_CERTIFICATE       70  Reserved for PKINIT
 KDC_ERR_INVALID_CERTIFICATE           71  Reserved for PKINIT
 KDC_ERR_REVOKED_CERTIFICATE           72  Reserved for PKINIT

Neuman, et al. Standards Track [Page 112] RFC 4120 Kerberos V5 July 2005

 KDC_ERR_REVOCATION_STATUS_UNKNOWN     73  Reserved for PKINIT
 KDC_ERR_REVOCATION_STATUS_UNAVAILABLE 74  Reserved for PKINIT
 KDC_ERR_CLIENT_NAME_MISMATCH          75  Reserved for PKINIT
 KDC_ERR_KDC_NAME_MISMATCH             76  Reserved for PKINIT

8. Interoperability Requirements

 Version 5 of the Kerberos protocol supports a myriad of options.
 Among these are multiple encryption and checksum types; alternative
 encoding schemes for the transited field; optional mechanisms for
 pre-authentication; the handling of tickets with no addresses;
 options for mutual authentication; user-to-user authentication;
 support for proxies; the format of realm names; the handling of
 authorization data; and forwarding, postdating, and renewing tickets.
 In order to ensure the interoperability of realms, it is necessary to
 define a minimal configuration that must be supported by all
 implementations.  This minimal configuration is subject to change as
 technology does.  For example, if at some later date it is discovered
 that one of the required encryption or checksum algorithms is not
 secure, it will be replaced.

8.1. Specification 2

 This section defines the second specification of these options.
 Implementations which are configured in this way can be said to
 support Kerberos Version 5 Specification 2 (5.2).  Specification 1
 (deprecated) may be found in RFC 1510.
 Transport
    TCP/IP and UDP/IP transport MUST be supported by clients and KDCs
    claiming conformance to specification 2.
 Encryption and Checksum Methods
    The following encryption and checksum mechanisms MUST be
    supported:
    Encryption: AES256-CTS-HMAC-SHA1-96 [RFC3962]
    Checksums: HMAC-SHA1-96-AES256 [RFC3962]
    Implementations SHOULD support other mechanisms as well, but the
    additional mechanisms may only be used when communicating with
    principals known to also support them.  The following mechanisms
    from [RFC3961] and [RFC3962] SHOULD be supported:

Neuman, et al. Standards Track [Page 113] RFC 4120 Kerberos V5 July 2005

    Encryption: AES128-CTS-HMAC-SHA1-96, DES-CBC-MD5, DES3-CBC-SHA1-KD
    Checksums: DES-MD5, HMAC-SHA1-DES3-KD, HMAC-SHA1-96-AES128
    Implementations MAY support other mechanisms as well, but the
    additional mechanisms may only be used when communicating with
    principals known to support them also.
    Implementation note: Earlier implementations of Kerberos generate
    messages using the CRC-32 and RSA-MD5 checksum methods.  For
    interoperability with these earlier releases, implementors MAY
    consider supporting these checksum methods but should carefully
    analyze the security implications to limit the situations within
    which these methods are accepted.
 Realm Names
    All implementations MUST understand hierarchical realms in both
    the Internet Domain and the X.500 style.  When a TGT for an
    unknown realm is requested, the KDC MUST be able to determine the
    names of the intermediate realms between the KDCs realm and the
    requested realm.
 Transited Field Encoding
    DOMAIN-X500-COMPRESS (described in Section 3.3.3.2) MUST be
    supported.  Alternative encodings MAY be supported, but they may
    only be used when that encoding is supported by ALL intermediate
    realms.
 Pre-authentication Methods
    The TGS-REQ method MUST be supported.  It is not used on the
    initial request.  The PA-ENC-TIMESTAMP method MUST be supported by
    clients, but whether it is enabled by default MAY be determined on
    a realm-by-realm basis.  If the method is not used in the initial
    request and the error KDC_ERR_PREAUTH_REQUIRED is returned
    specifying PA-ENC-TIMESTAMP as an acceptable method, the client
    SHOULD retry the initial request using the PA-ENC-TIMESTAMP pre-
    authentication method.  Servers need not support the PA-ENC-
    TIMESTAMP method, but if it is not supported the server SHOULD
    ignore the presence of PA-ENC-TIMESTAMP pre-authentication in a
    request.
    The ETYPE-INFO2 method MUST be supported; this method is used to
    communicate the set of supported encryption types, and
    corresponding salt and string to key parameters.  The ETYPE-INFO
    method SHOULD be supported for interoperability with older
    implementation.

Neuman, et al. Standards Track [Page 114] RFC 4120 Kerberos V5 July 2005

 Mutual Authentication
    Mutual authentication (via the KRB_AP_REP message) MUST be
    supported.
 Ticket Addresses and Flags
    All KDCs MUST pass through tickets that carry no addresses (i.e.,
    if a TGT contains no addresses, the KDC will return derivative
    tickets).  Implementations SHOULD default to requesting
    addressless tickets, as this significantly increases
    interoperability with network address translation.  In some cases,
    realms or application servers MAY require that tickets have an
    address.
    Implementations SHOULD accept directional address type for the
    KRB_SAFE and KRB_PRIV message and SHOULD include directional
    addresses in these messages when other address types are not
    available.
    Proxies and forwarded tickets MUST be supported.  Individual
    realms and application servers can set their own policy on when
    such tickets will be accepted.
    All implementations MUST recognize renewable and postdated
    tickets, but they need not actually implement them.  If these
    options are not supported, the starttime and endtime in the ticket
    SHALL specify a ticket's entire useful life.  When a postdated
    ticket is decoded by a server, all implementations SHALL make the
    presence of the postdated flag visible to the calling server.
 User-to-User Authentication
    Support for user-to-user authentication (via the ENC-TKT-IN-SKEY
    KDC option) MUST be provided by implementations, but individual
    realms MAY decide as a matter of policy to reject such requests on
    a per-principal or realm-wide basis.
 Authorization Data
    Implementations MUST pass all authorization data subfields from
    TGTs to any derivative tickets unless they are directed to
    suppress a subfield as part of the definition of that registered
    subfield type.  (It is never incorrect to pass on a subfield, and
    no registered subfield types presently specify suppression at the
    KDC.)

Neuman, et al. Standards Track [Page 115] RFC 4120 Kerberos V5 July 2005

    Implementations MUST make the contents of any authorization data
    subfields available to the server when a ticket is used.
    Implementations are not required to allow clients to specify the
    contents of the authorization data fields.
 Constant Ranges
    All protocol constants are constrained to 32-bit (signed) values
    unless further constrained by the protocol definition.  This limit
    is provided to allow implementations to make assumptions about the
    maximum values that will be received for these constants.
    Implementations receiving values outside this range MAY reject the
    request, but they MUST recover cleanly.

8.2. Recommended KDC Values

 Following is a list of recommended values for a KDC configuration.
    Minimum lifetime              5 minutes
    Maximum renewable lifetime    1 week
    Maximum ticket lifetime       1 day
    Acceptable clock skew         5 minutes
    Empty addresses               Allowed
    Proxiable, etc.               Allowed

9. IANA Considerations

 Section 7 of this document specifies protocol constants and other
 defined values required for the interoperability of multiple
 implementations.  Until a subsequent RFC specifies otherwise, or the
 Kerberos working group is shut down, allocations of additional
 protocol constants and other defined values required for extensions
 to the Kerberos protocol will be administered by the Kerberos working
 group.  Following the recommendations outlined in [RFC2434], guidance
 is provided to the IANA as follows:
 "reserved" realm name types in Section 6.1 and "other" realm types
 except those beginning with "X-" or "x-" will not be registered
 without IETF standards action, at which point guidelines for further
 assignment will be specified.  Realm name types beginning with "X-"
 or "x-" are for private use.
 For host address types described in Section 7.1, negative values are
 for private use.  Assignment of additional positive numbers is
 subject to review by the Kerberos working group or other expert
 review.

Neuman, et al. Standards Track [Page 116] RFC 4120 Kerberos V5 July 2005

 Additional key usage numbers, as defined in Section 7.5.1, will be
 assigned subject to review by the Kerberos working group or other
 expert review.
 Additional preauthentication data type values, as defined in section
 7.5.2, will be assigned subject to review by the Kerberos working
 group or other expert review.
 Additional authorization data types as defined in Section 7.5.4, will
 be assigned subject to review by the Kerberos working group or other
 expert review.  Although it is anticipated that there may be
 significant demand for private use types, provision is intentionally
 not made for a private use portion of the namespace because conflicts
 between privately assigned values could have detrimental security
 implications.
 Additional transited encoding types, as defined in Section 7.5.5,
 present special concerns for interoperability with existing
 implementations.  As such, such assignments will only be made by
 standards action, except that the Kerberos working group or another
 other working group with competent jurisdiction may make preliminary
 assignments for documents that are moving through the standards
 process.
 Additional Kerberos message types, as described in Section 7.5.7,
 will be assigned subject to review by the Kerberos working group or
 other expert review.
 Additional name types, as described in Section 7.5.8, will be
 assigned subject to review by the Kerberos working group or other
 expert review.
 Additional error codes described in Section 7.5.9 will be assigned
 subject to review by the Kerberos working group or other expert
 review.

10. Security Considerations

 As an authentication service, Kerberos provides a means of verifying
 the identity of principals on a network.  By itself, Kerberos does
 not provide authorization.  Applications should not accept the
 issuance of a service ticket by the Kerberos server as granting
 authority to use the service, since such applications may become
 vulnerable to the bypass of this authorization check in an
 environment where they inter-operate with other KDCs or where other
 options for application authentication are provided.

Neuman, et al. Standards Track [Page 117] RFC 4120 Kerberos V5 July 2005

 Denial of service attacks are not solved with Kerberos.  There are
 places in the protocols where an intruder can prevent an application
 from participating in the proper authentication steps.  Because
 authentication is a required step for the use of many services,
 successful denial of service attacks on a Kerberos server might
 result in the denial of other network services that rely on Kerberos
 for authentication.  Kerberos is vulnerable to many kinds of denial
 of service attacks: those on the network, which would prevent clients
 from contacting the KDC; those on the domain name system, which could
 prevent a client from finding the IP address of the Kerberos server;
 and those by overloading the Kerberos KDC itself with repeated
 requests.
 Interoperability conflicts caused by incompatible character-set usage
 (see 5.2.1) can result in denial of service for clients that utilize
 character-sets in Kerberos strings other than those stored in the KDC
 database.
 Authentication servers maintain a database of principals (i.e., users
 and servers) and their secret keys.  The security of the
 authentication server machines is critical.  The breach of security
 of an authentication server will compromise the security of all
 servers that rely upon the compromised KDC, and will compromise the
 authentication of any principals registered in the realm of the
 compromised KDC.
 Principals must keep their secret keys secret.  If an intruder
 somehow steals a principal's key, it will be able to masquerade as
 that principal or impersonate any server to the legitimate principal.
 Password-guessing attacks are not solved by Kerberos.  If a user
 chooses a poor password, it is possible for an attacker to
 successfully mount an off-line dictionary attack by repeatedly
 attempting to decrypt, with successive entries from a dictionary,
 messages obtained that are encrypted under a key derived from the
 user's password.
 Unless pre-authentication options are required by the policy of a
 realm, the KDC will not know whether a request for authentication
 succeeds.  An attacker can request a reply with credentials for any
 principal.  These credentials will likely not be of much use to the
 attacker unless it knows the client's secret key, but the
 availability of the response encrypted in the client's secret key
 provides the attacker with ciphertext that may be used to mount brute
 force or dictionary attacks to decrypt the credentials, by guessing
 the user's password.  For this reason it is strongly encouraged that
 Kerberos realms require the use of pre-authentication.  Even with

Neuman, et al. Standards Track [Page 118] RFC 4120 Kerberos V5 July 2005

 pre-authentication, attackers may try brute force or dictionary
 attacks against credentials that are observed by eavesdropping on the
 network.
 Because a client can request a ticket for any server principal and
 can attempt a brute force or dictionary attack against the server
 principal's key using that ticket, it is strongly encouraged that
 keys be randomly generated (rather than generated from passwords) for
 any principals that are usable as the target principal for a
 KRB_TGS_REQ or KRB_AS_REQ messages.  [RFC4086]
 Although the DES-CBC-MD5 encryption method and DES-MD5 checksum
 methods are listed as SHOULD be implemented for backward
 compatibility, the single DES encryption algorithm on which these are
 based is weak, and stronger algorithms should be used whenever
 possible.
 Each host on the network must have a clock that is loosely
 synchronized to the time of the other hosts; this synchronization is
 used to reduce the bookkeeping needs of application servers when they
 do replay detection.  The degree of "looseness" can be configured on
 a per-server basis, but it is typically on the order of 5 minutes.
 If the clocks are synchronized over the network, the clock
 synchronization protocol MUST itself be secured from network
 attackers.
 Principal identifiers must not recycled on a short-term basis.  A
 typical mode of access control will use access control lists (ACLs)
 to grant permissions to particular principals.  If a stale ACL entry
 remains for a deleted principal and the principal identifier is
 reused, the new principal will inherit rights specified in the stale
 ACL entry.  By not reusing principal identifiers, the danger of
 inadvertent access is removed.
 Proper decryption of an KRB_AS_REP message from the KDC is not
 sufficient for the host to verify the identity of the user; the user
 and an attacker could cooperate to generate a KRB_AS_REP format
 message that decrypts properly but is not from the proper KDC.  To
 authenticate a user logging on to a local system, the credentials
 obtained in the AS exchange may first be used in a TGS exchange to
 obtain credentials for a local server.  Those credentials must then
 be verified by a local server through successful completion of the
 Client/Server exchange.
 Many RFC 1510-compliant implementations ignore unknown authorization
 data elements.  Depending on these implementations to honor
 authorization data restrictions may create a security weakness.

Neuman, et al. Standards Track [Page 119] RFC 4120 Kerberos V5 July 2005

 Kerberos credentials contain clear-text information identifying the
 principals to which they apply.  If privacy of this information is
 needed, this exchange should itself be encapsulated in a protocol
 providing for confidentiality on the exchange of these credentials.
 Applications must take care to protect communications subsequent to
 authentication, either by using the KRB_PRIV or KRB_SAFE messages as
 appropriate, or by applying their own confidentiality or integrity
 mechanisms on such communications.  Completion of the KRB_AP_REQ and
 KRB_AP_REP exchange without subsequent use of confidentiality and
 integrity mechanisms provides only for authentication of the parties
 to the communication and not confidentiality and integrity of the
 subsequent communication.  Applications applying confidentiality and
 integrity protection mechanisms other than KRB_PRIV and KRB_SAFE must
 make sure that the authentication step is appropriately linked with
 the protected communication channel that is established by the
 application.
 Unless the application server provides its own suitable means to
 protect against replay (for example, a challenge-response sequence
 initiated by the server after authentication, or use of a server-
 generated encryption subkey), the server must utilize a replay cache
 to remember any authenticator presented within the allowable clock
 skew.  All services sharing a key need to use the same replay cache.
 If separate replay caches are used, then an authenticator used with
 one such service could later be replayed to a different service with
 the same service principal.
 If a server loses track of authenticators presented within the
 allowable clock skew, it must reject all requests until the clock
 skew interval has passed, providing assurance that any lost or
 replayed authenticators will fall outside the allowable clock skew
 and can no longer be successfully replayed.
 Implementations of Kerberos should not use untrusted directory
 servers to determine the realm of a host.  To allow this would allow
 the compromise of the directory server to enable an attacker to
 direct the client to accept authentication with the wrong principal
 (i.e., one with a similar name, but in a realm with which the
 legitimate host was not registered).
 Implementations of Kerberos must not use DNS to map one name to
 another (canonicalize) in order to determine the host part of the
 principal name with which one is to communicate.  To allow this
 canonicalization would allow a compromise of the DNS to result in a
 client obtaining credentials and correctly authenticating to the

Neuman, et al. Standards Track [Page 120] RFC 4120 Kerberos V5 July 2005

 wrong principal.  Though the client will know who it is communicating
 with, it will not be the principal with which it intended to
 communicate.
 If the Kerberos server returns a TGT for a realm 'closer' than the
 desired realm, the client may use local policy configuration to
 verify that the authentication path used is an acceptable one.
 Alternatively, a client may choose its own authentication path rather
 than rely on the Kerberos server to select one.  In either case, any
 policy or configuration information used to choose or validate
 authentication paths, whether by the Kerberos server or client, must
 be obtained from a trusted source.
 The Kerberos protocol in its basic form does not provide perfect
 forward secrecy for communications.  If traffic has been recorded by
 an eavesdropper, then messages encrypted using the KRB_PRIV message,
 or messages encrypted using application-specific encryption under
 keys exchanged using Kerberos can be decrypted if the user's,
 application server's, or KDC's key is subsequently discovered.  This
 is because the session key used to encrypt such messages, when
 transmitted over the network, is encrypted in the key of the
 application server.  It is also encrypted under the session key from
 the user's TGT when it is returned to the user in the KRB_TGS_REP
 message.  The session key from the TGT is sent to the user in the
 KRB_AS_REP message encrypted in the user's secret key and embedded in
 the TGT, which was encrypted in the key of the KDC.  Applications
 requiring perfect forward secrecy must exchange keys through
 mechanisms that provide such assurance, but may use Kerberos for
 authentication of the encrypted channel established through such
 other means.

11. Acknowledgements

 This document is a revision to RFC 1510 which was co-authored with
 John Kohl.  The specification of the Kerberos protocol described in
 this document is the result of many years of effort.  Over this
 period, many individuals have contributed to the definition of the
 protocol and to the writing of the specification.  Unfortunately, it
 is not possible to list all contributors as authors of this document,
 though there are many not listed who are authors in spirit, including
 those who contributed text for parts of some sections, who
 contributed to the design of parts of the protocol, and who
 contributed significantly to the discussion of the protocol in the
 IETF common authentication technology (CAT) and Kerberos working
 groups.

Neuman, et al. Standards Track [Page 121] RFC 4120 Kerberos V5 July 2005

 Among those contributing to the development and specification of
 Kerberos were Jeffrey Altman, John Brezak, Marc Colan, Johan
 Danielsson, Don Davis, Doug Engert, Dan Geer, Paul Hill, John Kohl,
 Marc Horowitz, Matt Hur, Jeffrey Hutzelman, Paul Leach, John Linn,
 Ari Medvinsky, Sasha Medvinsky, Steve Miller, Jon Rochlis, Jerome
 Saltzer, Jeffrey Schiller, Jennifer Steiner, Ralph Swick, Mike Swift,
 Jonathan Trostle, Theodore Ts'o, Brian Tung, Jacques Vidrine, Assar
 Westerlund, and Nicolas Williams.  Many other members of MIT Project
 Athena, the MIT networking group, and the Kerberos and CAT working
 groups of the IETF contributed but are not listed.

Neuman, et al. Standards Track [Page 122] RFC 4120 Kerberos V5 July 2005

A. ASN.1 module

KerberosV5Spec2 {

      iso(1) identified-organization(3) dod(6) internet(1)
      security(5) kerberosV5(2) modules(4) krb5spec2(2)

} DEFINITIONS EXPLICIT TAGS ::= BEGIN

– OID arc for KerberosV5 – – This OID may be used to identify Kerberos protocol messages – encapsulated in other protocols. – – This OID also designates the OID arc for KerberosV5-related OIDs. – – NOTE: RFC 1510 had an incorrect value (5) for "dod" in its OID. id-krb5 OBJECT IDENTIFIER ::= {

      iso(1) identified-organization(3) dod(6) internet(1)
      security(5) kerberosV5(2)

}

Int32 ::= INTEGER (-2147483648..2147483647)

  1. - signed values representable in 32 bits

UInt32 ::= INTEGER (0..4294967295)

  1. - unsigned 32 bit values

Microseconds ::= INTEGER (0..999999)

  1. - microseconds

KerberosString ::= GeneralString (IA5String)

Realm ::= KerberosString

PrincipalName ::= SEQUENCE {

      name-type       [0] Int32,
      name-string     [1] SEQUENCE OF KerberosString

}

KerberosTime ::= GeneralizedTime – with no fractional seconds

HostAddress ::= SEQUENCE {

      addr-type       [0] Int32,
      address         [1] OCTET STRING

}

– NOTE: HostAddresses is always used as an OPTIONAL field and – should not be empty. HostAddresses – NOTE: subtly different from rfc1510,

Neuman, et al. Standards Track [Page 123] RFC 4120 Kerberos V5 July 2005

  1. - but has a value mapping and encodes the same

::= SEQUENCE OF HostAddress

– NOTE: AuthorizationData is always used as an OPTIONAL field and – should not be empty. AuthorizationData ::= SEQUENCE OF SEQUENCE {

      ad-type         [0] Int32,
      ad-data         [1] OCTET STRING

}

PA-DATA ::= SEQUENCE {

  1. - NOTE: first tag is [1], not [0]

padata-type [1] Int32,

      padata-value    [2] OCTET STRING -- might be encoded AP-REQ

}

KerberosFlags ::= BIT STRING (SIZE (32..MAX))

  1. - minimum number of bits shall be sent,
  2. - but no fewer than 32

EncryptedData ::= SEQUENCE {

      etype   [0] Int32 -- EncryptionType --,
      kvno    [1] UInt32 OPTIONAL,
      cipher  [2] OCTET STRING -- ciphertext

}

EncryptionKey ::= SEQUENCE {

      keytype         [0] Int32 -- actually encryption type --,
      keyvalue        [1] OCTET STRING

}

Checksum ::= SEQUENCE {

      cksumtype       [0] Int32,
      checksum        [1] OCTET STRING

}

Ticket ::= [APPLICATION 1] SEQUENCE {

      tkt-vno         [0] INTEGER (5),
      realm           [1] Realm,
      sname           [2] PrincipalName,
      enc-part        [3] EncryptedData -- EncTicketPart

}

– Encrypted part of ticket EncTicketPart ::= [APPLICATION 3] SEQUENCE {

      flags                   [0] TicketFlags,
      key                     [1] EncryptionKey,
      crealm                  [2] Realm,

Neuman, et al. Standards Track [Page 124] RFC 4120 Kerberos V5 July 2005

      cname                   [3] PrincipalName,
      transited               [4] TransitedEncoding,
      authtime                [5] KerberosTime,
      starttime               [6] KerberosTime OPTIONAL,
      endtime                 [7] KerberosTime,
      renew-till              [8] KerberosTime OPTIONAL,
      caddr                   [9] HostAddresses OPTIONAL,
      authorization-data      [10] AuthorizationData OPTIONAL

}

– encoded Transited field TransitedEncoding ::= SEQUENCE {

      tr-type         [0] Int32 -- must be registered --,
      contents        [1] OCTET STRING

}

TicketFlags ::= KerberosFlags

  1. - reserved(0),
  2. - forwardable(1),
  3. - forwarded(2),
  4. - proxiable(3),
  5. - proxy(4),
  6. - may-postdate(5),
  7. - postdated(6),
  8. - invalid(7),
  9. - renewable(8),
  10. - initial(9),
  11. - pre-authent(10),
  12. - hw-authent(11),

– the following are new since 1510

  1. - transited-policy-checked(12),
  2. - ok-as-delegate(13)

AS-REQ ::= [APPLICATION 10] KDC-REQ

TGS-REQ ::= [APPLICATION 12] KDC-REQ

KDC-REQ ::= SEQUENCE {

  1. - NOTE: first tag is [1], not [0]

pvno [1] INTEGER (5) ,

      msg-type        [2] INTEGER (10 -- AS -- | 12 -- TGS --),
      padata          [3] SEQUENCE OF PA-DATA OPTIONAL
                          -- NOTE: not empty --,
      req-body        [4] KDC-REQ-BODY

}

KDC-REQ-BODY ::= SEQUENCE {

      kdc-options             [0] KDCOptions,

Neuman, et al. Standards Track [Page 125] RFC 4120 Kerberos V5 July 2005

      cname                   [1] PrincipalName OPTIONAL
                                  -- Used only in AS-REQ --,
      realm                   [2] Realm
                                  -- Server's realm
                                  -- Also client's in AS-REQ --,
      sname                   [3] PrincipalName OPTIONAL,
      from                    [4] KerberosTime OPTIONAL,
      till                    [5] KerberosTime,
      rtime                   [6] KerberosTime OPTIONAL,
      nonce                   [7] UInt32,
      etype                   [8] SEQUENCE OF Int32 -- EncryptionType
                                  -- in preference order --,
      addresses               [9] HostAddresses OPTIONAL,
      enc-authorization-data  [10] EncryptedData OPTIONAL
                                  -- AuthorizationData --,
      additional-tickets      [11] SEQUENCE OF Ticket OPTIONAL
                                      -- NOTE: not empty

}

KDCOptions ::= KerberosFlags

  1. - reserved(0),
  2. - forwardable(1),
  3. - forwarded(2),
  4. - proxiable(3),
  5. - proxy(4),
  6. - allow-postdate(5),
  7. - postdated(6),
  8. - unused7(7),
  9. - renewable(8),
  10. - unused9(9),
  11. - unused10(10),
  12. - opt-hardware-auth(11),
  13. - unused12(12),
  14. - unused13(13),

– 15 is reserved for canonicalize

  1. - unused15(15),

– 26 was unused in 1510

  1. - disable-transited-check(26),

  1. - renewable-ok(27),
  2. - enc-tkt-in-skey(28),
  3. - renew(30),
  4. - validate(31)

AS-REP ::= [APPLICATION 11] KDC-REP

TGS-REP ::= [APPLICATION 13] KDC-REP

Neuman, et al. Standards Track [Page 126] RFC 4120 Kerberos V5 July 2005

KDC-REP ::= SEQUENCE {

      pvno            [0] INTEGER (5),
      msg-type        [1] INTEGER (11 -- AS -- | 13 -- TGS --),
      padata          [2] SEQUENCE OF PA-DATA OPTIONAL
                              -- NOTE: not empty --,
      crealm          [3] Realm,
      cname           [4] PrincipalName,
      ticket          [5] Ticket,
      enc-part        [6] EncryptedData
                              -- EncASRepPart or EncTGSRepPart,
                              -- as appropriate

}

EncASRepPart ::= [APPLICATION 25] EncKDCRepPart

EncTGSRepPart ::= [APPLICATION 26] EncKDCRepPart

EncKDCRepPart ::= SEQUENCE {

      key             [0] EncryptionKey,
      last-req        [1] LastReq,
      nonce           [2] UInt32,
      key-expiration  [3] KerberosTime OPTIONAL,
      flags           [4] TicketFlags,
      authtime        [5] KerberosTime,
      starttime       [6] KerberosTime OPTIONAL,
      endtime         [7] KerberosTime,
      renew-till      [8] KerberosTime OPTIONAL,
      srealm          [9] Realm,
      sname           [10] PrincipalName,
      caddr           [11] HostAddresses OPTIONAL

}

LastReq ::= SEQUENCE OF SEQUENCE {

      lr-type         [0] Int32,
      lr-value        [1] KerberosTime

}

AP-REQ ::= [APPLICATION 14] SEQUENCE {

      pvno            [0] INTEGER (5),
      msg-type        [1] INTEGER (14),
      ap-options      [2] APOptions,
      ticket          [3] Ticket,
      authenticator   [4] EncryptedData -- Authenticator

}

APOptions ::= KerberosFlags

  1. - reserved(0),
  2. - use-session-key(1),

Neuman, et al. Standards Track [Page 127] RFC 4120 Kerberos V5 July 2005

  1. - mutual-required(2)

– Unencrypted authenticator Authenticator ::= [APPLICATION 2] SEQUENCE {

      authenticator-vno       [0] INTEGER (5),
      crealm                  [1] Realm,
      cname                   [2] PrincipalName,
      cksum                   [3] Checksum OPTIONAL,
      cusec                   [4] Microseconds,
      ctime                   [5] KerberosTime,
      subkey                  [6] EncryptionKey OPTIONAL,
      seq-number              [7] UInt32 OPTIONAL,
      authorization-data      [8] AuthorizationData OPTIONAL

}

AP-REP ::= [APPLICATION 15] SEQUENCE {

      pvno            [0] INTEGER (5),
      msg-type        [1] INTEGER (15),
      enc-part        [2] EncryptedData -- EncAPRepPart

}

EncAPRepPart ::= [APPLICATION 27] SEQUENCE {

      ctime           [0] KerberosTime,
      cusec           [1] Microseconds,
      subkey          [2] EncryptionKey OPTIONAL,
      seq-number      [3] UInt32 OPTIONAL

}

KRB-SAFE ::= [APPLICATION 20] SEQUENCE {

      pvno            [0] INTEGER (5),
      msg-type        [1] INTEGER (20),
      safe-body       [2] KRB-SAFE-BODY,
      cksum           [3] Checksum

}

KRB-SAFE-BODY ::= SEQUENCE {

      user-data       [0] OCTET STRING,
      timestamp       [1] KerberosTime OPTIONAL,
      usec            [2] Microseconds OPTIONAL,
      seq-number      [3] UInt32 OPTIONAL,
      s-address       [4] HostAddress,
      r-address       [5] HostAddress OPTIONAL

}

KRB-PRIV ::= [APPLICATION 21] SEQUENCE {

      pvno            [0] INTEGER (5),
      msg-type        [1] INTEGER (21),
                      -- NOTE: there is no [2] tag

Neuman, et al. Standards Track [Page 128] RFC 4120 Kerberos V5 July 2005

      enc-part        [3] EncryptedData -- EncKrbPrivPart

}

EncKrbPrivPart ::= [APPLICATION 28] SEQUENCE {

      user-data       [0] OCTET STRING,
      timestamp       [1] KerberosTime OPTIONAL,
      usec            [2] Microseconds OPTIONAL,
      seq-number      [3] UInt32 OPTIONAL,
      s-address       [4] HostAddress -- sender's addr --,
      r-address       [5] HostAddress OPTIONAL -- recip's addr

}

KRB-CRED ::= [APPLICATION 22] SEQUENCE {

      pvno            [0] INTEGER (5),
      msg-type        [1] INTEGER (22),
      tickets         [2] SEQUENCE OF Ticket,
      enc-part        [3] EncryptedData -- EncKrbCredPart

}

EncKrbCredPart ::= [APPLICATION 29] SEQUENCE {

      ticket-info     [0] SEQUENCE OF KrbCredInfo,
      nonce           [1] UInt32 OPTIONAL,
      timestamp       [2] KerberosTime OPTIONAL,
      usec            [3] Microseconds OPTIONAL,
      s-address       [4] HostAddress OPTIONAL,
      r-address       [5] HostAddress OPTIONAL

}

KrbCredInfo ::= SEQUENCE {

      key             [0] EncryptionKey,
      prealm          [1] Realm OPTIONAL,
      pname           [2] PrincipalName OPTIONAL,
      flags           [3] TicketFlags OPTIONAL,
      authtime        [4] KerberosTime OPTIONAL,
      starttime       [5] KerberosTime OPTIONAL,
      endtime         [6] KerberosTime OPTIONAL,
      renew-till      [7] KerberosTime OPTIONAL,
      srealm          [8] Realm OPTIONAL,
      sname           [9] PrincipalName OPTIONAL,
      caddr           [10] HostAddresses OPTIONAL

}

KRB-ERROR ::= [APPLICATION 30] SEQUENCE {

      pvno            [0] INTEGER (5),
      msg-type        [1] INTEGER (30),
      ctime           [2] KerberosTime OPTIONAL,
      cusec           [3] Microseconds OPTIONAL,
      stime           [4] KerberosTime,

Neuman, et al. Standards Track [Page 129] RFC 4120 Kerberos V5 July 2005

      susec           [5] Microseconds,
      error-code      [6] Int32,
      crealm          [7] Realm OPTIONAL,
      cname           [8] PrincipalName OPTIONAL,
      realm           [9] Realm -- service realm --,
      sname           [10] PrincipalName -- service name --,
      e-text          [11] KerberosString OPTIONAL,
      e-data          [12] OCTET STRING OPTIONAL

}

METHOD-DATA ::= SEQUENCE OF PA-DATA

TYPED-DATA ::= SEQUENCE SIZE (1..MAX) OF SEQUENCE {

      data-type       [0] Int32,
      data-value      [1] OCTET STRING OPTIONAL

}

– preauth stuff follows

PA-ENC-TIMESTAMP ::= EncryptedData – PA-ENC-TS-ENC

PA-ENC-TS-ENC ::= SEQUENCE {

      patimestamp     [0] KerberosTime -- client's time --,
      pausec          [1] Microseconds OPTIONAL

}

ETYPE-INFO-ENTRY ::= SEQUENCE {

      etype           [0] Int32,
      salt            [1] OCTET STRING OPTIONAL

}

ETYPE-INFO ::= SEQUENCE OF ETYPE-INFO-ENTRY

ETYPE-INFO2-ENTRY ::= SEQUENCE {

      etype           [0] Int32,
      salt            [1] KerberosString OPTIONAL,
      s2kparams       [2] OCTET STRING OPTIONAL

}

ETYPE-INFO2 ::= SEQUENCE SIZE (1..MAX) OF ETYPE-INFO2-ENTRY

AD-IF-RELEVANT ::= AuthorizationData

AD-KDCIssued ::= SEQUENCE {

      ad-checksum     [0] Checksum,
      i-realm         [1] Realm OPTIONAL,
      i-sname         [2] PrincipalName OPTIONAL,
      elements        [3] AuthorizationData

Neuman, et al. Standards Track [Page 130] RFC 4120 Kerberos V5 July 2005

}

AD-AND-OR ::= SEQUENCE {

      condition-count [0] Int32,
      elements        [1] AuthorizationData

}

AD-MANDATORY-FOR-KDC ::= AuthorizationData

END

B. Changes since RFC 1510

 This document replaces RFC 1510 and clarifies specification of items
 that were not completely specified.  Where changes to recommended
 implementation choices were made, or where new options were added,
 those changes are described within the document and listed in this
 section.  More significantly, "Specification 2" in Section 8 changes
 the required encryption and checksum methods to bring them in line
 with the best current practices and to deprecate methods that are no
 longer considered sufficiently strong.
 Discussion was added to Section 1 regarding the ability to rely on
 the KDC to check the transited field, and on the inclusion of a flag
 in a ticket indicating that this check has occurred.  This is a new
 capability not present in RFC 1510.  Pre-existing implementations may
 ignore or not set this flag without negative security implications.
 The definition of the secret key says that in the case of a user the
 key may be derived from a password.  In RFC 1510, it said that the
 key was derived from the password.  This change was made to
 accommodate situations where the user key might be stored on a
 smart-card, or otherwise obtained independently of a password.
 The introduction mentions the use of public key cryptography for
 initial authentication in Kerberos by reference.  RFC 1510 did not
 include such a reference.
 Section 1.3 was added to explain that while Kerberos provides
 authentication of a named principal, it is still the responsibility
 of the application to ensure that the authenticated name is the
 entity with which the application wishes to communicate.
 Discussion of extensibility has been added to the introduction.
 Discussion of how extensibility affects ticket flags and KDC options
 was added to the introduction of Section 2.  No changes were made to
 existing options and flags specified in RFC 1510, though some of the

Neuman, et al. Standards Track [Page 131] RFC 4120 Kerberos V5 July 2005

 sections in the specification were renumbered, and text was revised
 to make the description and intent of existing options clearer,
 especially with respect to the ENC-TKT-IN-SKEY option (now section
 2.9.2) which is used for user-to-user authentication.  The new option
 and ticket flag transited policy checking (Section 2.7) was added.
 A warning regarding generation of session keys for application use
 was added to Section 3, urging the inclusion of key entropy from the
 KDC generated session key in the ticket.  An example regarding use of
 the sub-session key was added to Section 3.2.6.  Descriptions of the
 pa-etype-info, pa-etype-info2, and pa-pw-salt pre-authentication data
 items were added.  The recommendation for use of pre-authentication
 was changed from "MAY" to "SHOULD" and a note was added regarding
 known plaintext attacks.
 In RFC 1510, Section 4 described the database in the KDC.  This
 discussion was not necessary for interoperability and unnecessarily
 constrained implementation.  The old Section 4 was removed.
 The current Section 4 was formerly Section 6 on encryption and
 checksum specifications.  The major part of this section was brought
 up to date to support new encryption methods, and moved to a separate
 document.  Those few remaining aspects of the encryption and checksum
 specification specific to Kerberos are now specified in Section 4.
 Significant changes were made to the layout of Section 5 to clarify
 the correct behavior for optional fields.  Many of these changes were
 made necessary because of improper ASN.1 description in the original
 Kerberos specification which left the correct behavior
 underspecified.  Additionally, the wording in this section was
 tightened wherever possible to ensure that implementations conforming
 to this specification will be extensible with the addition of new
 fields in future specifications.
 Text was added describing time_t=0 issues in the ASN.1.  Text was
 also added, clarifying issues with implementations treating omitted
 optional integers as zero.  Text was added clarifying behavior for
 optional SEQUENCE or SEQUENCE OF that may be empty.  Discussion was
 added regarding sequence numbers and behavior of some
 implementations, including "zero" behavior and negative numbers.  A
 compatibility note was added regarding the unconditional sending of
 EncTGSRepPart regardless of the enclosing reply type.  Minor changes
 were made to the description of the HostAddresses type.  Integer
 types were constrained.  KerberosString was defined as a
 (significantly) constrained GeneralString.  KerberosFlags was defined
 to reflect existing implementation behavior that departs from the

Neuman, et al. Standards Track [Page 132] RFC 4120 Kerberos V5 July 2005

 definition in RFC 1510.  The transited-policy-checked(12) and the
 ok-as-delegate(13) ticket flags were added.  The disable-transited-
 check(26) KDC option was added.
 Descriptions of commonly implemented PA-DATA were added to Section 5.
 The description of KRB-SAFE has been updated to note the existing
 implementation behavior of double-encoding.
 There were two definitions of METHOD-DATA in RFC 1510.  The second
 one, intended for use with KRB_AP_ERR_METHOD was removed leaving the
 SEQUENCE OF PA-DATA definition.
 Section 7, naming constraints, from RFC 1510 was moved to Section 6.
 Words were added describing the convention that domain-based realm
 names for newly-created realms should be specified as uppercase.
 This recommendation does not make lowercase realm names illegal.
 Words were added highlighting that the slash-separated components in
 the X.500 style of realm names is consistent with existing RFC 1510
 based implementations, but that it conflicts with the general
 recommendation of X.500 name representation specified in RFC 2253.
 Section 8, network transport, constants and defined values, from RFC
 1510 was moved to Section 7.  Since RFC 1510, the definition of the
 TCP transport for Kerberos messages was added, and the encryption and
 checksum number assignments have been moved into a separate document.
 "Specification 2" in Section 8 of the current document changes the
 required encryption and checksum methods to bring them in line with
 the best current practices and to deprecate methods that are no
 longer considered sufficiently strong.
 Two new sections, on IANA considerations and security considerations
 were added.
 The pseudo-code has been removed from the appendix.  The pseudo-code
 was sometimes misinterpreted to limit implementation choices and in
 RFC 1510, it was not always consistent with the words in the
 specification.  Effort was made to clear up any ambiguities in the
 specification, rather than to rely on the pseudo-code.
 An appendix was added containing the complete ASN.1 module drawn from
 the discussion in Section 5 of the current document.

END NOTES

 (*TM) Project Athena, Athena, and Kerberos are trademarks of the
 Massachusetts Institute of Technology (MIT).

Neuman, et al. Standards Track [Page 133] RFC 4120 Kerberos V5 July 2005

Normative References

 [RFC3961]          Raeburn, K., "Encryption and Checksum
                    Specifications for Kerberos 5", RFC 3961, February
                    2005.
 [RFC3962]          Raeburn, K., "Advanced Encryption Standard (AES)
                    Encryption for Kerberos 5", RFC 3962, February
                    2005.
 [ISO-646/ECMA-6]   International Organization for Standardization,
                    "7-bit Coded Character Set for Information
                    Interchange", ISO/IEC 646:1991.
 [ISO-2022/ECMA-35] International Organization for Standardization,
                    "Character code structure and extension
                    techniques", ISO/IEC 2022:1994.
 [RFC1035]          Mockapetris, P., "Domain names - implementation
                    and specification", STD 13, RFC 1035, November
                    1987.
 [RFC2119]          Bradner, S., "Key words for use in RFCs to
                    Indicate Requirement Levels", BCP 14, RFC 2119,
                    March 1997.
 [RFC2434]          Narten, T. and H. Alvestrand, "Guidelines for
                    Writing an IANA Considerations Section in RFCs",
                    BCP 26, RFC 2434, October 1998.
 [RFC2782]          Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS
                    RR for specifying the location of services (DNS
                    SRV)", RFC 2782, February 2000.
 [RFC2253]          Wahl, M., Kille, S., and T. Howes, "Lightweight
                    Directory Access Protocol (v3): UTF-8 String
                    Representation of Distinguished Names", RFC 2253,
                    December 1997.
 [RFC3513]          Hinden, R. and S. Deering, "Internet Protocol
                    Version 6 (IPv6) Addressing Architecture", RFC
                    3513, April 2003.
 [X680]             Abstract Syntax Notation One (ASN.1):
                    Specification of Basic Notation, ITU-T
                    Recommendation X.680 (1997) | ISO/IEC
                    International Standard 8824-1:1998.

Neuman, et al. Standards Track [Page 134] RFC 4120 Kerberos V5 July 2005

 [X690]             ASN.1 encoding rules: Specification of Basic
                    Encoding Rules (BER), Canonical Encoding Rules
                    (CER) and Distinguished Encoding Rules (DER),
                    ITU-T Recommendation X.690 (1997)| ISO/IEC
                    International Standard 8825-1:1998.

Informative References

 [ISO-8859]         International Organization for Standardization,
                    "8-bit Single-byte Coded Graphic Character Sets --
                    Latin Alphabet", ISO/IEC 8859.
 [RFC1964]          Linn, J., "The Kerberos Version 5 GSS-API
                    Mechanism", RFC 1964, June 1996.
 [DGT96]            Don Davis, Daniel Geer, and Theodore Ts'o,
                    "Kerberos With Clocks Adrift: History, Protocols,
                    and Implementation", USENIX Computing Systems 9:1,
                    January 1996.
 [DS81]             Dorothy E. Denning and Giovanni Maria Sacco,
                    "Time-stamps in Key Distribution Protocols,"
                    Communications of the ACM, Vol. 24 (8), p. 533-
                    536, August 1981.
 [KNT94]            John T. Kohl, B. Clifford Neuman, and Theodore Y.
                    Ts'o, "The Evolution of the Kerberos
                    Authentication System". In Distributed Open
                    Systems, pages 78-94. IEEE Computer Society Press,
                    1994.
 [MNSS87]           S. P. Miller, B. C. Neuman, J. I. Schiller, and J.
                    H. Saltzer, Section E.2.1: Kerberos Authentication
                    and Authorization System, M.I.T. Project Athena,
                    Cambridge, Massachusetts, December 21, 1987.
 [NS78]             Roger M. Needham and Michael D. Schroeder, "Using
                    Encryption for Authentication in Large Networks of
                    Computers," Communications of the ACM, Vol. 21
                    (12), pp. 993-999, December 1978.
 [Neu93]            B. Clifford Neuman, "Proxy-Based Authorization and
                    Accounting for Distributed Systems," in
                    Proceedings of the 13th International Conference
                    on Distributed Computing Systems, Pittsburgh, PA,
                    May 1993.

Neuman, et al. Standards Track [Page 135] RFC 4120 Kerberos V5 July 2005

 [NT94]             B. Clifford Neuman and Theodore Y. Ts'o, "An
                    Authentication Service for Computer Networks,"
                    IEEE Communications Magazine, Vol. 32 (9), p. 33-
                    38, September 1994.
 [Pat92]            J. Pato, Using Pre-Authentication to Avoid
                    Password Guessing Attacks, Open Software
                    Foundation DCE Request for Comments 26 (December
                    1992.
 [RFC1510]          Kohl, J. and C. Neuman, "The Kerberos Network
                    Authentication Service (V5)", RFC 1510, September
                    1993.
 [RFC4086]          Eastlake, D., 3rd, Schiller, J., and S. Crocker,
                    "Randomness Requirements for Security", BCP 106,
                    RFC 4086, June 2005.
 [SNS88]            J. G. Steiner, B. C. Neuman, and J. I. Schiller,
                    "Kerberos: An Authentication Service for Open
                    Network Systems," p. 191-202, Usenix Conference
                    Proceedings, Dallas, Texas, February 1988.
 [RFC4121]          Zhu, L., Jaganathan, K., and S. Hartman, "The
                    Kerberos Version 5 Generic Security Service
                    Application Program Interface (GSS-API) Mechanism:
                    Version 2", RFC 4121, July 2005.

Neuman, et al. Standards Track [Page 136] RFC 4120 Kerberos V5 July 2005

Authors' Addresses

 Clifford Neuman
 Information Sciences Institute
 University of Southern California
 4676 Admiralty Way
 Marina del Rey, CA 90292, USA
 EMail: bcn@isi.edu
 Tom Yu
 Massachusetts Institute of Technology
 77 Massachusetts Avenue
 Cambridge, MA 02139, USA
 EMail: tlyu@mit.edu
 Sam Hartman
 Massachusetts Institute of Technology
 77 Massachusetts Avenue
 Cambridge, MA 02139, USA
 EMail: hartmans-ietf@mit.edu
 Kenneth Raeburn
 Massachusetts Institute of Technology
 77 Massachusetts Avenue
 Cambridge, MA 02139, USA
 EMail: raeburn@mit.edu

Neuman, et al. Standards Track [Page 137] RFC 4120 Kerberos V5 July 2005

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Neuman, et al. Standards Track [Page 138]

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