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rfc:rfc1510

Network Working Group J. Kohl Request for Comments: 1510 Digital Equipment Corporation

                                                             C. Neuman
                                                                   ISI
                                                        September 1993
          The Kerberos Network Authentication Service (V5)

Status of this Memo

 This RFC 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" for the standardization state and status
 of this protocol.  Distribution of this memo is unlimited.

Abstract

 This document gives an overview and specification of Version 5 of the
 protocol for the Kerberos network authentication system. Version 4,
 described elsewhere [1,2], is presently in production use at MIT's
 Project Athena, and at other Internet sites.

Overview

 Project Athena, Athena, Athena MUSE, Discuss, Hesiod, Kerberos,
 Moira, and Zephyr are trademarks of the Massachusetts Institute of
 Technology (MIT).  No commercial use of these trademarks may be made
 without prior written permission of MIT.
 This RFC 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; for Version 4 they are fully
 described in the Kerberos portion of the Athena Technical Plan [1].
 The protocols are under review, and are not being submitted for
 consideration as an Internet standard at this time.  Comments are
 encouraged.  Requests for addition to an electronic mailing list for
 discussion of Kerberos, kerberos@MIT.EDU, may be addressed to
 kerberos-request@MIT.EDU.  This mailing list is gatewayed onto the
 Usenet as the group comp.protocols.kerberos.  Requests for further
 information, including documents and code availability, may be sent
 to info-kerberos@MIT.EDU.

Kohl & Neuman [Page 1] RFC 1510 Kerberos September 1993

Background

 The Kerberos model is based in part on Needham and Schroeder's
 trusted third-party authentication protocol [3] and on modifications
 suggested by Denning and Sacco [4].  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 4 is publicly available, and has seen wide use across the
 Internet.
 Version 5 (described in this document) has evolved from Version 4
 based on new requirements and desires for features not available in
 Version 4.  Details on the differences between Kerberos Versions 4
 and 5 can be found in [5].

Table of Contents

 1. Introduction .......................................    5
 1.1. Cross-Realm Operation ............................    7
 1.2. Environmental assumptions ........................    8
 1.3. Glossary of terms ................................    9
 2. Ticket flag uses and requests ......................   12
 2.1. Initial and pre-authenticated tickets ............   12
 2.2. Invalid tickets ..................................   12
 2.3. Renewable tickets ................................   12
 2.4. Postdated tickets ................................   13
 2.5. Proxiable and proxy tickets ......................   14
 2.6. Forwardable tickets ..............................   15
 2.7. Other KDC options ................................   15
 3. Message Exchanges ..................................   16
 3.1. The Authentication Service Exchange ..............   16
 3.1.1. Generation of KRB_AS_REQ message ...............   17
 3.1.2. Receipt of KRB_AS_REQ message ..................   17
 3.1.3. Generation of KRB_AS_REP message ...............   17
 3.1.4. Generation of KRB_ERROR message ................   19
 3.1.5. Receipt of KRB_AS_REP message ..................   19
 3.1.6. Receipt of KRB_ERROR message ...................   20
 3.2. The Client/Server Authentication Exchange ........   20
 3.2.1. The KRB_AP_REQ message .........................   20
 3.2.2. Generation of a KRB_AP_REQ message .............   20
 3.2.3. Receipt of KRB_AP_REQ message ..................   21
 3.2.4. Generation of a KRB_AP_REP message .............   23
 3.2.5. Receipt of KRB_AP_REP message ..................   23

Kohl & Neuman [Page 2] RFC 1510 Kerberos September 1993

 3.2.6. Using the encryption key .......................   24
 3.3. The Ticket-Granting Service (TGS) Exchange .......   24
 3.3.1. Generation of KRB_TGS_REQ message ..............   25
 3.3.2. Receipt of KRB_TGS_REQ message .................   26
 3.3.3. Generation of KRB_TGS_REP message ..............   27
 3.3.3.1. Encoding the transited field .................   29
 3.3.4. Receipt of KRB_TGS_REP message .................   31
 3.4. The KRB_SAFE Exchange ............................   31
 3.4.1. Generation of a KRB_SAFE message ...............   31
 3.4.2. Receipt of KRB_SAFE message ....................   32
 3.5. The KRB_PRIV Exchange ............................   33
 3.5.1. Generation of a KRB_PRIV message ...............   33
 3.5.2. Receipt of KRB_PRIV message ....................   33
 3.6. The KRB_CRED Exchange ............................   34
 3.6.1. Generation of a KRB_CRED message ...............   34
 3.6.2. Receipt of KRB_CRED message ....................   34
 4. The Kerberos Database ..............................   35
 4.1. Database contents ................................   35
 4.2. Additional fields ................................   36
 4.3. Frequently Changing Fields .......................   37
 4.4. Site Constants ...................................   37
 5. Message Specifications .............................   38
 5.1. ASN.1 Distinguished Encoding Representation ......   38
 5.2. ASN.1 Base Definitions ...........................   38
 5.3. Tickets and Authenticators .......................   42
 5.3.1. Tickets ........................................   42
 5.3.2. Authenticators .................................   47
 5.4. Specifications for the AS and TGS exchanges ......   49
 5.4.1. KRB_KDC_REQ definition .........................   49
 5.4.2. KRB_KDC_REP definition .........................   56
 5.5. Client/Server (CS) message specifications ........   58
 5.5.1. KRB_AP_REQ definition ..........................   58
 5.5.2. KRB_AP_REP definition ..........................   60
 5.5.3. Error message reply ............................   61
 5.6. KRB_SAFE message specification ...................   61
 5.6.1. KRB_SAFE definition ............................   61
 5.7. KRB_PRIV message specification ...................   62
 5.7.1. KRB_PRIV definition ............................   62
 5.8. KRB_CRED message specification ...................   63
 5.8.1. KRB_CRED definition ............................   63
 5.9. Error message specification ......................   65
 5.9.1. KRB_ERROR definition ...........................   66
 6. Encryption and Checksum Specifications .............   67
 6.1. Encryption Specifications ........................   68
 6.2. Encryption Keys ..................................   71
 6.3. Encryption Systems ...............................   71
 6.3.1. The NULL Encryption System (null) ..............   71
 6.3.2. DES in CBC mode with a CRC-32 checksum (descbc-crc)71

Kohl & Neuman [Page 3] RFC 1510 Kerberos September 1993

 6.3.3. DES in CBC mode with an MD4 checksum (descbc-md4)  72
 6.3.4. DES in CBC mode with an MD5 checksum (descbc-md5)  72
 6.4. Checksums ........................................   74
 6.4.1. The CRC-32 Checksum (crc32) ....................   74
 6.4.2. The RSA MD4 Checksum (rsa-md4) .................   75
 6.4.3. RSA MD4 Cryptographic Checksum Using DES
 (rsa-md4-des) .........................................   75
 6.4.4. The RSA MD5 Checksum (rsa-md5) .................   76
 6.4.5. RSA MD5 Cryptographic Checksum Using DES
 (rsa-md5-des) .........................................   76
 6.4.6. DES cipher-block chained checksum (des-mac)
 6.4.7. RSA MD4 Cryptographic Checksum Using DES
 alternative (rsa-md4-des-k) ...........................   77
 6.4.8. DES cipher-block chained checksum alternative
 (des-mac-k) ...........................................   77
 7. Naming Constraints .................................   78
 7.1. Realm Names ......................................   77
 7.2. Principal Names ..................................   79
 7.2.1. Name of server principals ......................   80
 8. Constants and other defined values .................   80
 8.1. Host address types ...............................   80
 8.2. KDC messages .....................................   81
 8.2.1. IP transport ...................................   81
 8.2.2. OSI transport ..................................   82
 8.2.3. Name of the TGS ................................   82
 8.3. Protocol constants and associated values .........   82
 9. Interoperability requirements ......................   86
 9.1. Specification 1 ..................................   86
 9.2. Recommended KDC values ...........................   88
 10. Acknowledgments ...................................   88
 11. References ........................................   89
 12. Security Considerations ...........................   90
 13. Authors' Addresses ................................   90
 A. Pseudo-code for protocol processing ................   91
 A.1. KRB_AS_REQ generation ............................   91
 A.2. KRB_AS_REQ verification and KRB_AS_REP generation    92
 A.3. KRB_AS_REP verification ..........................   95
 A.4. KRB_AS_REP and KRB_TGS_REP common checks .........   96
 A.5. KRB_TGS_REQ generation ...........................   97
 A.6. KRB_TGS_REQ verification and KRB_TGS_REP generation  98
 A.7. KRB_TGS_REP verification .........................  104
 A.8. Authenticator generation .........................  104
 A.9. KRB_AP_REQ generation ............................  105
 A.10. KRB_AP_REQ verification .........................  105
 A.11. KRB_AP_REP generation ...........................  106
 A.12. KRB_AP_REP verification .........................  107
 A.13. KRB_SAFE generation .............................  107
 A.14. KRB_SAFE verification ...........................  108

Kohl & Neuman [Page 4] RFC 1510 Kerberos September 1993

 A.15. KRB_SAFE and KRB_PRIV common checks .............  108
 A.16. KRB_PRIV generation .............................  109
 A.17. KRB_PRIV verification ...........................  110
 A.18. KRB_CRED generation .............................  110
 A.19. KRB_CRED verification ...........................  111
 A.20. KRB_ERROR generation ............................  112

1. Introduction

 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
 authentication 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. (Note,
 however, that many applications use Kerberos' functions only upon the
 initiation of a stream-based network connection, and assume the
 absence of any "hijackers" who might subvert such a connection.  Such
 use implicitly trusts the host addresses involved.)  Kerberos
 performs authentication under these conditions as a trusted third-
 party authentication service by using conventional cryptography,
 i.e., shared secret key.  (shared secret key - Secret and private are
 often used interchangeably in the literature.  In our usage, it takes
 two (or more) to share a secret, thus a shared DES key is a secret
 key.  Something is only private when no one but its owner knows it.
 Thus, in public key cryptosystems, one has a public and a private
 key.)
 The authentication process proceeds as follows: A client sends a
 request to the authentication server (AS) requesting "credentials"
 for a given server.  The AS responds with these credentials,
 encrypted in the client's key.  The credentials consist of 1) a
 "ticket" for the server and 2) 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.
 The implementation 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

Kohl & Neuman [Page 5] RFC 1510 Kerberos September 1993

 transactions, a typical network application adds one or two calls to
 the Kerberos library, which results in the transmission of the
 necessary messages to achieve authentication.
 The Kerberos protocol consists of several sub-protocols (or
 exchanges).  There are two 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 sends the TGT to
 the TGS in the same manner as if it were contacting any other
 application server which requires Kerberos credentials.  The reply is
 encrypted in the session key from the TGT.
 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 server.  Since 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
 was originated by 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 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 using the session key passed in the ticket, and
 contained in the credentials.
 The authentication exchanges mentioned above require read-only access
 to the Kerberos database.  Sometimes, however, the entries in the

Kohl & Neuman [Page 6] RFC 1510 Kerberos September 1993

 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).  The administration protocol is not described in this
 document. There is also a protocol for maintaining multiple copies of
 the Kerberos database, but this can be considered an implementation
 detail and may vary to support different database technologies.

1.1. 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 use its
 authentication remotely (Of course, with appropriate permission the
 client could 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).  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 ticket-granting ticket for the remote realm's ticket-
 granting service from its local realm. When that ticket-granting
 ticket is used, the remote ticket-granting service uses the inter-
 realm key (which usually differs from its own normal TGS key) to
 decrypt the ticket-granting ticket, and is thus certain that it 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.
 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 are typically 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.  If a hierarchical organization is not used, it may be
 necessary to consult some database in order to construct an

Kohl & Neuman [Page 7] RFC 1510 Kerberos September 1993

 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.

1.2. Environmental assumptions

 Kerberos imposes a few assumptions on the environment in which it can
 properly function:
 +    "Denial of service" attacks are not solved with Kerberos.  There
      are places in these protocols where an intruder 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) is 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 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.  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

Kohl & Neuman [Page 8] RFC 1510 Kerberos September 1993

      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.

1.3. 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.
 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.

Kohl & Neuman [Page 9] RFC 1510 Kerberos September 1993

 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 successfully use that
                     ticket in an authentication exchange.
 KDC                 Key Distribution Center, a network service
                     that supplies tickets and temporary
                     session keys; or an instance of that
                     service or the 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            Aside from the 3-headed dog guarding
                     Hades, the name given to Project
                     Athena's authentication service, the
                     protocol used by that service, or the
                     code used to implement the authentication
                     service.
 Plaintext           The input to an encryption function  or
                     the output of a decryption function.
                     Decryption transforms ciphertext into
                     plaintext.
 Principal           A uniquely named client or server
                     instance that participates in a network
                     communication.

Kohl & Neuman [Page 10] RFC 1510 Kerberos September 1993

 Principal identifier The name used to uniquely identify each
                      different principal.
 Seal                To encipher a record containing several
                     fields in such a way that the fields
                     cannot be individually replaced without
                     either 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 is derived
                     from a password.
 Server              A particular Principal which provides a
                     resource to network clients.
 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".
 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.
 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.

Kohl & Neuman [Page 11] RFC 1510 Kerberos September 1993

2. Ticket flag uses and requests

 Each Kerberos ticket contains a set of flags which are used to
 indicate various 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 gives
 examples of reasons to use such a flag.

2.1. Initial and pre-authenticated tickets

 The INITIAL flag indicates that a ticket was issued using the AS
 protocol and not issued based on a ticket-granting ticket.
 Application servers that want to require the knowledge of a client's
 secret key (e.g., a passwordchanging program) can insist that this
 flag be set in any tickets they accept, and thus be assured that the
 client's key was recently presented to the application client.
 The PRE-AUTHENT and HW-AUTHENT flags provide addition 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 ticket-granting ticket (in which case the
 INITIAL flag is clear, but the PRE-AUTHENT and HW-AUTHENT flags are
 carried forward from the ticket-granting ticket).

2.2. Invalid tickets

 The INVALID flag indicates that a ticket is invalid.  Application
 servers must reject tickets which have this flag set.  A postdated
 ticket will usually be issued in this form. Invalid tickets must be
 validated by the KDC before use, by presenting them 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 which have been stolen before
 their starttime can be rendered permanently invalid (through a hot-
 list mechanism).

2.3. Renewable tickets

 Applications may desire to hold tickets which 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 shortlived 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

Kohl & Neuman [Page 12] RFC 1510 Kerberos September 1993

 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 if the ticket had been reported stolen since its last
 renewal; it will refuse to renew such 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 wish to 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 ticket-granting ticket in order to issue a
 postdated ticket based on the presented ticket. It is reset by
 default; it may be requested by a client by setting the ALLOW-
 POSTDATE option in the KRB_AS_REQ message.  This flag does not allow
 a client to obtain a postdated ticket-granting ticket; postdated
 ticket-granting tickets can only by 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 ticket-

Kohl & Neuman [Page 13] RFC 1510 Kerberos September 1993

 granting ticket 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 ticket-granting ticket.  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 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 take on the principal's identity for
 a particular purpose by granting it a proxy.
 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 ticket-granting ticket) with a
 different network address based on this ticket.  This flag is set by
 default.
 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 usually valid from only those network addresses
 specifically included in the ticket (It is permissible to request or
 issue tickets with no network addresses specified, but we do not
 recommend it).  For this reason, a client wishing to grant a proxy
 must request a new ticket valid for the network address of the
 service to be granted the proxy.
 The PROXY flag is set in a ticket by the  TGS  when  it issues a
 proxy ticket.  Application servers may check this flag and require
 additional authentication  from  the  agent presenting the proxy in
 order to provide an audit trail.

Kohl & Neuman [Page 14] RFC 1510 Kerberos September 1993

2.6. Forwardable tickets

 Authentication forwarding is an instance of the proxy case where the
 service is granted complete use of the client's identity.  An example
 where it might be used is when a user logs in to a remote system 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 ticket-granting tickets 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 ticket-granting
 ticket.
 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 end result
 can still be achieved if the user engages in the AS exchange with 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 it be set 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 wish to process
 FORWARDED tickets differently than non-FORWARDED tickets.

2.7. Other KDC options

 There are two additional options which may be set in a client's
 request of the KDC.  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 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.
 The ENC-TKT-IN-SKEY option is honored only by the ticket-granting
 service.  It indicates that the to-be-issued ticket for the end
 server is to be encrypted in the session key from the additional
 ticket-granting ticket provided with the request.  See section 3.3.3
 for specific details.

Kohl & Neuman [Page 15] RFC 1510 Kerberos September 1993

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 usually initiated by a client when
 it wishes to obtain authentication credentials for a given server but
 currently holds no credentials.  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) without requiring
 further use of the client's secret key.  This exchange is also used
 to request credentials for services which must not be mediated
 through the Ticket-Granting Service, but rather require a principal's
 secret key, such as the password-changing service.  (The password-
 changing request must not be honored unless the requester can provide
 the old password (the user's current secret key).  Otherwise, it
 would be possible for someone to walk up to an unattended session and
 change another user's password.)  This exchange does not by itself
 provide any assurance of the 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 the local server through successful completion of the
 Client/Server exchange.)
 The 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.
 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 KRB_AS_REP message
 contains information which can be used to detect replays, and to

Kohl & Neuman [Page 16] RFC 1510 Kerberos September 1993

 associate it with the message to which it replies.  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 also contains information which can
 be used to associate it with the message to which it replies.  The
 lack of encryption in the KRB_ERROR message precludes the ability to
 detect replays or fabrications of such messages.
 In the normal case 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.  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 may be used for preauthentication if desired, but the
 mechanism is not currently specified.

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 preauthentication 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 nonrenewable ticket (due to
 configuration constraints; see section 4).  See section A.1 for
 pseudocode.
 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 5.3.1.  The
 contents of the ticket are determined as follows.

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 required, 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 the server cannot accommodate
 the requested encryption type, an error message with code

Kohl & Neuman [Page 17] RFC 1510 Kerberos September 1993

 KDC_ERR_ETYPE_NOSUPP is returned. Otherwise it generates a "random"
 session key ("Random" means that, among other things, it should be
 impossible to guess the next session key based on knowledge of past
 session keys.  This can only be achieved in a pseudo-random number
 generator if it is based on cryptographic principles.  It would be
 more desirable to use a truly random number generator, such as one
 based on measurements of random physical phenomena.).
 If the requested start time is absent or indicates a time in the
 past, then the start time of the ticket is set to the authentication
 server's current time. If it indicates a time in the future, but the
 POSTDATED option has not been specified, then the error
 KDC_ERR_CANNOT_POSTDATE is returned.  Otherwise the requested start
 time is checked against the policy of the local realm (the
 administrator might decide to prohibit certain types or ranges of
 postdated tickets), and if acceptable, the ticket's start time 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 start time has been reached.
 The expiration time of the ticket will be set to the minimum of the
 following:
 +The expiration time (endtime) requested in the KRB_AS_REQ
  message.
 +The ticket's start time plus the maximum allowable lifetime
  associated with the client principal (the authentication
  server's database includes a maximum ticket lifetime field
  in each principal's record; see section 4).
 +The ticket's start time plus the maximum allowable lifetime
  associated with the server principal.
 +The ticket's start time plus the maximum lifetime set by
  the policy of the local realm.
 If the requested expiration time minus the start time (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"
 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 is set to the
 minimum of:

Kohl & Neuman [Page 18] RFC 1510 Kerberos September 1993

 +Its requested value.
 +The start time of the ticket plus the minimum of the two
  maximum renewable lifetimes associated with the principals'
  database entries.
 +The start time 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 start time is in the future), its
 INVALID flag will also be set.
 If all of the above succeed, the server 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 the requested encryption method, and sends it
 to the client.  See section A.2 for pseudocode.

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
 message.  The client decrypts the encrypted part of the response
 using its secret key, 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, 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 key-expiration field from the
 encrypted part of the response may be checked to notify the user of
 impending key expiration (the client program could then suggest
 remedial action, such as a password change).  See section A.3 for
 pseudocode.
 Proper decryption of the KRB_AS_REP message is not sufficient to

Kohl & Neuman [Page 19] RFC 1510 Kerberos September 1993

 verify the identity of the user; the user and an attacker could
 cooperate to generate a KRB_AS_REP format message which 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 which can be verified using a securely-stored
 secret key.  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 to recover.

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 which 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.), so 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 it.  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

Kohl & Neuman [Page 20] RFC 1510 Kerberos September 1993

 TGS exchange) a ticket and session key for the desired service.  The
 client may re-use any tickets it holds until they expire.  The client
 then constructs a new Authenticator from the the system time, its
 name, and optionally 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 may not be re-used and will
 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.  In
 such cases, a new authenticator must be generated for each retry.).
 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 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.  See section A.9 for pseudocode.

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 the ticket is encrypted in the session key from the
 server's ticket-granting ticket rather than its secret key (This is
 used for user-to-user authentication as described in [6]).  Since 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

Kohl & Neuman [Page 21] RFC 1510 Kerberos September 1993

 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; see section 6), the
 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 it to have 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 (they might not match, for example, if the wrong
 session key was used to encrypt the authenticator).  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.  If the server name, along with
 the client name, time and microsecond fields from the Authenticator
 match any recently-seen such tuples, 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 be have their authenticators rejected if the time and microsecond
 fields happen to match some other client's authenticator.).  The
 server must remember any authenticator presented within the allowable
 clock skew, so that a replay attempt is guaranteed to fail. If a
 server loses track of any authenticator presented within the
 allowable clock skew, it must reject all requests until the clock
 skew interval has passed.  This assures that any lost or re-played
 authenticators will fall outside the allowable clock skew and can no
 longer be successfully replayed (If this is not done, an attacker
 could conceivably record the ticket and authenticator sent over the
 network to a server, then disable the client's host, pose as the
 disabled host, and replay the ticket and authenticator to subvert the
 authentication.).  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.

Kohl & Neuman [Page 22] RFC 1510 Kerberos September 1993

 The server computes the age of the ticket: local (server) time minus
 the start time inside the Ticket.  If the start time 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, the client has been authenticated to the server.
 See section A.10 for pseudocode.

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 (not only authenticating 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). [Note: 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.]  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.  See section A.11 for
 pseudocode.

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 (Note that for
 encrypting the KRB_AP_REP message, the sub-session key is not used,
 even if present in the Authenticator.) 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.  See section A.12 for

Kohl & Neuman [Page 23] RFC 1510 Kerberos September 1993

 pseudocode.

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 which can be used by the application.
 The "true session key" to be used for KRB_PRIV, KRB_SAFE, or other
 application-specific uses may be chosen by the application based on
 the subkeys in the KRB_AP_REP message and the authenticator
 (Implementations of the protocol may wish to provide routines to
 choose subkeys based on session keys and random numbers and to
 orchestrate a negotiated key to be returned in the KRB_AP_REP
 message.).  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 a several alternatives.  We leave the protocol negotiations of
 how to use the key (e.g., selecting an encryption or checksum type)
 to the application programmer; the Kerberos protocol does not
 constrain the implementation options.
 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 or KRB_ERROR responses
 from the server to 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 assure 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 Ticket-Granting
 Server is initiated by a client when it wishes to obtain
 authentication credentials for a given server (which might be
 registered in a remote realm), when it wishes to renew or validate an
 existing ticket, or when it wishes 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 ticket-granting
 ticket 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

Kohl & Neuman [Page 24] RFC 1510 Kerberos September 1993

 exchange does not take place under the client's key.  Instead, the
 session key from the ticket-granting ticket 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 ticket-granting ticket 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 ticket-granting
 ticket and proxy cases, the request may include one or more of: 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 ticket-granting ticket or
 renewable ticket, or if present, in the subsession 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 which can be used to detect replays, and to
 associate it with the message to which it replies.  The KRB_ERROR
 message also contains information which can be used to associate it
 with the message to which it replies, but the lack of encryption in
 the KRB_ERROR message precludes the ability to detect replays or
 fabrications of such messages.

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 registered
 [Note: This can be accomplished in several ways.  It might be known
 beforehand (since the realm is part of the principal identifier), or
 it might be stored in a nameserver.  Presently, however, this
 information is 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 which has
 been compromised, and would result in an attacker's ability to
 compromise the authentication of the application server to the
 client.].  If the client does not already possess a ticket-granting
 ticket for the appropriate realm, then one must be obtained.  This is
 first attempted by requesting a ticket-granting ticket for the
 destination realm from the local Kerberos server (using the

Kohl & Neuman [Page 25] RFC 1510 Kerberos September 1993

 KRB_TGS_REQ message recursively).  The 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 which
 is "closer" to the desired realm (further along the standard
 hierarchical path), in which case this step must be repeated with a
 Kerberos server in the realm specified in the returned TGT.  If
 neither are returned, then the request must be retried with a
 Kerberos server for a realm higher in the hierarchy.  This request
 will itself require a ticket-granting ticket for the higher realm
 which must be obtained by recursively applying these directions.
 Once the client obtains a ticket-granting ticket for the appropriate
 realm, it determines which Kerberos servers serve that realm, and
 contacts one. The list might be obtained through a configuration file
 or network service; as long as the secret keys exchanged by realms
 are kept secret, only denial of service results from a false Kerberos
 server.
 As in the AS exchange, the client may specify a number of options in
 the KRB_TGS_REQ message.  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-authorization-
 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 subsession key.  One
 approach would be to generate a random number and XOR it with the
 session key from the ticket-granting ticket.). If the sub-session key
 is not specified, the session key from the ticket-granting ticket
 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 in the
 session key from the ticket-granting ticket.
 Once prepared, the message is sent to a Kerberos server for the
 destination realm.  See section A.5 for pseudocode.

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

Kohl & Neuman [Page 26] RFC 1510 Kerberos September 1993

 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 the accompanying ticket is not a ticket granting
 ticket for the current realm, but is for an application server in the
 current realm, the RENEW, VALIDATE, or PROXY options are specified in
 the request, and 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 rejected if the checksums do not
 match (with an error code of KRB_AP_ERR_MODIFIED) or if the checksum
 is not keyed or 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.
 If any of the decryptions indicate failed integrity checks, the
 KRB_AP_ERR_BAD_INTEGRITY error is returned.

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.  The
 Kerberos database is queried to retrieve the record for the requested
 server (including the key with which the ticket will be encrypted).
 If the request is for a ticket granting ticket 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 from 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 ticket-granting ticket (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.

Kohl & Neuman [Page 27] RFC 1510 Kerberos September 1993

 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 (endtimestarttime) 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
 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 ticket-granting tickets.
 If the requested start time is absent or indicates a time in the
 past, then the start time of the ticket is set to the authentication
 server's current time.  If it indicates a time in the future, 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 ticket-granting ticket has the
 MAYPOSTDATE 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 start time 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 ticket-granting ticket.
 If the ENC-TKT-IN-SKEY option has been specified and an additional
 ticket has been included in the request, the KDC will decrypt the
 additional ticket using the key for the server to which the
 additional ticket was issued and verify that it is a ticket-granting
 ticket.  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 and if 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 (This allows easy implementation of user-to-
 user authentication [6], which uses ticket-granting ticket session
 keys in lieu of secret server keys in situations where such secret

Kohl & Neuman [Page 28] RFC 1510 Kerberos September 1993

 keys could be easily compromised.).
 If 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 ticket-
 granting server itself, and the server is registered in the realm of
 the KDC, If the RENEW option is requested, then the KDC will verify
 that the RENEWABLE flag is set in the ticket and that the renew_till
 time is still in the future.  If the VALIDATE option is rqeuested,
 the KDC will check that the starttime has passed and the INVALID flag
 is set.  If the PROXY option is requested, then the KDC will check
 that the PROXIABLE flag is set in the ticket.  If the tests succeed,
 the KDC will issue the appropriate new ticket.
 Whenever a request is made to the ticket-granting server, the
 presented ticket(s) is(are) checked against a hot-list of tickets
 which have been canceled.  This hot-list might be implemented by
 storing a range of issue dates for "suspect tickets"; if a presented
 ticket had an authtime in that range, it would be rejected.  In this
 way, a stolen ticket-granting ticket or renewable ticket cannot be
 used to gain additional tickets (renewals or otherwise) once the
 theft has been reported.  Any normal ticket obtained before it was
 reported stolen will still be valid (because they require no
 interaction with the KDC), but only until their normal expiration
 time.
 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 the session key key from the ticket-granting ticket.  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
 ticket-granting ticket.  See section A.6 for pseudocode.

3.3.3.1. 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 ticket-granting ticket.  The name of the realm
 that issued the ticket-granting ticket will be added to the transited
 field of the ticket to be issued.  This is accomplished by reading
 the transited field from the ticket-granting ticket (which is treated
 as an unordered set of realm names), adding the new realm to the set,

Kohl & Neuman [Page 29] RFC 1510 Kerberos September 1993

 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
 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.  Since the endpoints are not included, both local and
 single-hop inter-realm authentication result in a transited field
 that is empty.
 Because the name of each realm transited  is  added  to this field,
 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 ATHENA.MIT.EDU, or CS.WASHINGTON.EDU were endpoints,
 that 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 to be the null realm ("")).  If
 it 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".

Kohl & Neuman [Page 30] RFC 1510 Kerberos September 1993

 Like the example above, if /COM/HP/APOLLO and /COM/DEC are endpoints,
 they they would not be included in this field, and we would have:
            "/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 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 session key from the ticket-granting ticket
 rather than the client's secret key.  See section A.7 for pseudocode.

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 collisionproof 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 occured).

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 some sort of
 keyed one-way hash function (such as the RSA-MD5-DES checksum
 algorithm specified in section 6.4.5, or the DES MAC), generated
 using the sub-session key if present, or the session key.  Different
 algorithms may be selected by changing the checksum type in the
 message.  Unkeyed or non-collision-proof checksums are not suitable
 for this use.
 The control information for the KRB_SAFE message includes both a

Kohl & Neuman [Page 31] RFC 1510 Kerberos September 1993

 timestamp and a sequence number.  The designer of an application
 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 them being resent, the use of the timestamp is the
 appropriate replay detection mechanism.  Using timestamps is also the
 appropriate mechanism for multi-cast protocols where 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
 collisionproof keyed checksum, and if it is not, a
 KRB_AP_ERR_INAPP_CKSUM error is generated.  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.  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 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 recently-seen such tuples, the KRB_AP_ERR_REPEAT error is
 generated.  If an incorrect sequence number is included, or a
 sequence number is expected but not present, the KRB_AP_ERR_BADORDER
 error is generated.  If neither a timestamp and usec or a sequence
 number is present, a KRB_AP_ERR_MODIFIED error is generated.

Kohl & Neuman [Page 32] RFC 1510 Kerberos September 1993

 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.

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 occured).  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
 the resultant plaintext. If decryption shows the data to have been
 modified, a KRB_AP_ERR_BAD_INTEGRITY error is generated.  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.  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 they are present but not current, the
 KRB_AP_ERR_SKEW error is generated.  If the server name, along with

Kohl & Neuman [Page 33] RFC 1510 Kerberos September 1993

 the client name, time and microsecond fields from the Authenticator
 match any recently-seen such tuples, the KRB_AP_ERR_REPEAT error is
 generated.  If an incorrect sequence number is included, or a
 sequence number is expected but not present, the KRB_AP_ERR_BADORDER
 error is generated.  If neither a timestamp and usec or 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 able to see 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
 ticket in the key field of the corresponding KrbCredInfo sequence of
 the encrypted part of the 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 specifically required by the application the nonce, s-
 address, and raddress fields, are placed in the encrypted part of the
 KRB_CRED message which is then encrypted under an encryption key
 previosuly exchanged in the KRB_AP exchange (usually the last key
 negotiated via subkeys, or the session key if no negotiation has
 occured).

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.

Kohl & Neuman [Page 34] RFC 1510 Kerberos September 1993

 If present or required, the recipient verifies 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.  A failed match for either
 case generates a KRB_AP_ERR_BADADDR error.  The timestamp and usec
 fields (and the nonce field if required) are checked next.  If the
 timestamp and usec are not present, or 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 ticket cache together with the session key and other
 information in the corresponding KrbCredInfo sequence from the
 encrypted part of the KRB_CRED message.

4. The Kerberos Database

 The Kerberos server must have access to a database containing the
 principal identifiers and secret keys of principals to be
 authenticated (The implementation of the Kerberos server need not
 combine the database and the server on the same machine; it is
 feasible to store the principal database in, say, a network name
 service, as long as the entries stored therein are protected from
 disclosure to and modification by unauthorized parties.  However, we
 recommend against such strategies, as they can make system management
 and threat analysis quite complex.).

4.1. Database contents

 A database entry should contain at least the following fields:
 Field                Value
 name                 Principal's identifier
 key                  Principal's secret key
 p_kvno               Principal's key version
 max_life             Maximum lifetime for Tickets
 max_renewable_life   Maximum total lifetime for renewable
                      Tickets
 The name field is an encoding of the principal's identifier.  The key
 field contains an encryption key.  This key is the principal's secret
 key.  (The key can be encrypted before storage under a Kerberos
 "master key" to protect it in case the database is compromised but
 the master key is not.  In that case, an extra field must be added to
 indicate the master key version used, see below.) The p_kvno field is
 the key version number of the principal's secret key.  The max_life
 field contains the maximum allowable lifetime (endtime - starttime)
 for any Ticket issued for this principal.  The max_renewable_life

Kohl & Neuman [Page 35] RFC 1510 Kerberos September 1993

 field contains the maximum allowable total lifetime for any renewable
 Ticket issued for this principal.  (See section 3.1 for a description
 of how these lifetimes are used in determining the lifetime of a
 given Ticket.)
 A server may provide KDC service to several realms, as long as the
 database representation provides a mechanism to distinguish between
 principal records with identifiers which differ only in the realm
 name.
 When an application server's key changes, if the change is routine
 (i.e.,  not the result of disclosure of the old key), the old key
 should be retained by the server until all tickets that had been
 issued using that key have expired.  Because of this, it is possible
 for several keys to be active for a single principal.  Ciphertext
 encrypted in a principal's key is always tagged with the version of
 the key that was used for encryption, to help the recipient find the
 proper key for decryption.
 When more than one key is active for a particular principal, the
 principal will have more than one record in the Kerberos database.
 The keys and key version numbers will differ between the records (the
 rest of the fields may or may not be the same). Whenever Kerberos
 issues a ticket, or responds to a request for initial authentication,
 the most recent key (known by the Kerberos server) will be used for
 encryption.  This is the key with the highest key version number.

4.2. Additional fields

 Project Athena's KDC implementation uses additional fields in its
 database:
 Field        Value
 K_kvno       Kerberos' key version
 expiration   Expiration date for entry
 attributes   Bit field of attributes
 mod_date     Timestamp of last modification
 mod_name     Modifying principal's identifier
 The K_kvno field indicates the key version of the Kerberos master key
 under which the principal's secret key is encrypted.
 After an entry's expiration date has passed, the KDC will return an
 error to any client attempting to gain tickets as or for the
 principal.  (A database may want to maintain two expiration dates:
 one for the principal, and one for the principal's current key.  This
 allows password aging to work independently of the principal's

Kohl & Neuman [Page 36] RFC 1510 Kerberos September 1993

 expiration date.  However, due to the limited space in the responses,
 the KDC must combine the key expiration and principal expiration date
 into a single value called "key_exp", which is used as a hint to the
 user to take administrative action.)
 The attributes field is a bitfield used to govern the operations
 involving the principal.  This field might be useful in conjunction
 with user registration procedures, for site-specific policy
 implementations (Project Athena currently uses it for their user
 registration process controlled by the system-wide database service,
 Moira [7]), or to identify the "string to key" conversion algorithm
 used for a principal's key.  (See the discussion of the padata field
 in section 5.4.2 for details on why this can be useful.)  Other bits
 are used to indicate that certain ticket options should not be
 allowed in tickets encrypted under a principal's key (one bit each):
 Disallow issuing postdated tickets, disallow issuing forwardable
 tickets, disallow issuing tickets based on TGT authentication,
 disallow issuing renewable tickets, disallow issuing proxiable
 tickets, and disallow issuing tickets for which the principal is the
 server.
 The mod_date field contains the time of last modification of the
 entry, and the mod_name field contains the name of the principal
 which last modified the entry.

4.3. Frequently Changing Fields

 Some KDC implementations may wish to maintain the last time that a
 request was made by a particular principal.  Information that might
 be maintained includes the time of the last request, the time of the
 last request for a ticket-granting ticket, the time of the last use
 of a ticket-granting ticket, or other times.  This information can
 then be returned to the user in the last-req field (see section 5.2).
 Other frequently changing information that can be maintained is the
 latest expiration time for any tickets that have been issued using
 each key.  This field would be used to indicate how long old keys
 must remain valid to allow the continued use of outstanding tickets.

4.4. Site Constants

 The KDC implementation should have the following configurable
 constants or options, to allow an administrator to make and enforce
 policy decisions:
 + The minimum supported lifetime (used to determine whether the
    KDC_ERR_NEVER_VALID error should be returned). This constant
    should reflect reasonable expectations of round-trip time to the

Kohl & Neuman [Page 37] RFC 1510 Kerberos September 1993

    KDC, encryption/decryption time, and processing time by the client
    and target server, and it should allow for a minimum "useful"
    lifetime.
 + The maximum allowable total (renewable) lifetime of a ticket
    (renew_till - starttime).
 + The maximum allowable lifetime of a ticket (endtime - starttime).
 + Whether to allow the issue of tickets with empty address fields
    (including the ability to specify that such tickets may only be
    issued if the request specifies some authorization_data).
 + Whether proxiable, forwardable, renewable or post-datable tickets
    are to be issued.

5. Message Specifications

 The following sections describe the exact contents and encoding of
 protocol messages and objects.  The ASN.1 base definitions are
 presented in the first subsection.  The remaining subsections specify
 the protocol objects (tickets and authenticators) and messages.
 Specification of encryption and checksum techniques, and the fields
 related to them, appear in section 6.

5.1. ASN.1 Distinguished Encoding Representation

 All uses of ASN.1 in Kerberos shall use the Distinguished Encoding
 Representation of the data elements as described in the X.509
 specification, section 8.7 [8].

5.2. ASN.1 Base Definitions

 The following ASN.1 base definitions are used in the rest of this
 section. Note that since the underscore character (_) is not
 permitted in ASN.1 names, the hyphen (-) is used in its place for the
 purposes of ASN.1 names.
 Realm ::=           GeneralString
 PrincipalName ::=   SEQUENCE {
                     name-type[0]     INTEGER,
                     name-string[1]   SEQUENCE OF GeneralString
 }
 Kerberos realms are encoded as GeneralStrings. Realms shall not
 contain a character with the code 0 (the 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

Kohl & Neuman [Page 38] RFC 1510 Kerberos September 1993

 the style of X.500 names.  Acceptable forms for realm names are
 specified in section 7.  A PrincipalName is a typed sequence of
 components consisting of the following sub-fields:
 name-type This field specifies the type of name that follows.
           Pre-defined values for this field are
           specified in section 7.2.  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).
           This constraint may be eliminated in the future.
 name-string This field encodes a sequence of components that
             form a name, each component encoded as a General
             String.  Taken together, a PrincipalName and a Realm
             form a principal identifier.  Most PrincipalNames
             will have only a few components (typically one or two).
         KerberosTime ::=   GeneralizedTime
                            -- Specifying UTC time zone (Z)
 The timestamps used in Kerberos are encoded as GeneralizedTimes.  An
 encoding shall specify the UTC time zone (Z) and shall not include
 any fractional portions of the seconds.  It further shall not include
 any separators.  Example: The only valid format for UTC time 6
 minutes, 27 seconds after 9 pm on 6 November 1985 is 19851106210627Z.
  HostAddress ::=     SEQUENCE  {
                      addr-type[0]             INTEGER,
                      address[1]               OCTET STRING
  }
  HostAddresses ::=   SEQUENCE OF SEQUENCE {
                      addr-type[0]             INTEGER,
                      address[1]               OCTET STRING
  }
 The host adddress encodings consists of two fields:
 addr-type  This field specifies the type of  address that
            follows. Pre-defined values for this field are
            specified in section 8.1.
 address   This field encodes a single address of type addr-type.
 The two forms differ slightly. HostAddress contains exactly one

Kohl & Neuman [Page 39] RFC 1510 Kerberos September 1993

 address; HostAddresses contains a sequence of possibly many
 addresses.
 AuthorizationData ::=   SEQUENCE OF SEQUENCE {
                         ad-type[0]               INTEGER,
                         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.
                 APOptions ::=   BIT STRING {
                                 reserved(0),
                                 use-session-key(1),
                                 mutual-required(2)
                 }
                 TicketFlags ::=   BIT STRING {
                                   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)
                 }
                KDCOptions ::=   BIT STRING {
                                 reserved(0),
                                 forwardable(1),
                                 forwarded(2),
                                 proxiable(3),
                                 proxy(4),
                                 allow-postdate(5),
                                 postdated(6),

Kohl & Neuman [Page 40] RFC 1510 Kerberos September 1993

                                 unused7(7),
                                 renewable(8),
                                 unused9(9),
                                 unused10(10),
                                 unused11(11),
                                 renewable-ok(27),
                                 enc-tkt-in-skey(28),
                                 renew(30),
                                 validate(31)
                }
          LastReq ::=   SEQUENCE OF SEQUENCE {
                        lr-type[0]               INTEGER,
                        lr-value[1]              KerberosTime
          }
 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
           ticket-granting ticket 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).
 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.
 See section 6 for the definitions of Checksum, ChecksumType,
 EncryptedData, EncryptionKey, EncryptionType, and KeyType.

Kohl & Neuman [Page 41] RFC 1510 Kerberos September 1993

5.3. Tickets and Authenticators

 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.

5.3.1. Tickets

 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,
                            realm[1]                     Realm,
                            sname[2]                     PrincipalName,
                            enc-part[3]                  EncryptedData

} – Encrypted part of ticket 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

} – encoded Transited field TransitedEncoding ::= SEQUENCE {

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

}

 The encoding of EncTicketPart is encrypted in the key shared by
 Kerberos and the end server (the server's secret key).  See section 6
 for the format of the ciphertext.
 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

Kohl & Neuman [Page 42] RFC 1510 Kerberos September 1993

           always be identical.
 sname     This field specifies the name part of the server's
           identity.
 enc-part  This field holds the encrypted encoding of the
           EncTicketPart sequence.
 flags     This field indicates which of various options were used or
           requested when the ticket was issued.  It 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).  Bit 0 is the most significant bit.  The
           encoding of the bits is specified in section 5.2.  The
           flags are described in more detail above in section 2.  The
           meanings of the flags are:
           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
                                 ticket- granting ticket 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 ticket-granting
                                 ticket.
           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- ticket-granting tickets may be
                                 issued with different network
                                 addresses.

Kohl & Neuman [Page 43] RFC 1510 Kerberos September 1993

           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 ticket-granting server that
                                 a post- dated ticket may be issued
                                 based on this ticket-granting ticket.
           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 wish to 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 ticket-granting
                                 ticket.
           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 preauthentication 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

Kohl & Neuman [Page 44] RFC 1510 Kerberos September 1993

                                 client.  The hardware authentication
                                 method is selected by the KDC and the
                                 strength of the method is not
                                 indicated.
           12-31     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.  The field's encoding is
           described in section 6.2.
 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 3.3.3.1 for details on how
           this field encodes the traversed realms.
 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 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 since 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 it is absent from
           the ticket, its value should be treated as that of the

Kohl & Neuman [Page 45] RFC 1510 Kerberos September 1993

           authtime field.
 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 zero-
           address tickets is a policy decision and is left to the
           Kerberos and end-service administrators; they may refuse to
           issue or accept such tickets.  The suggested and default
           policy, however, is that such tickets will only be issued
           or accepted when additional information that can be used to
           restrict the use of the ticket is included in the
           authorization_data field.  Such a ticket is a capability.
           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.
           It is important to 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 use them from a "safe"
           location.

Kohl & Neuman [Page 46] RFC 1510 Kerberos September 1993

 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.  The data in this field are specific to the end
           service.  It is expected that the field will contain the
           names of service specific objects, and the rights to those
           objects.  The format for this field is described in section
           5.2.  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.
           It is interesting to note that 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 [9] for some suggested
           uses of this field.
           The authorization-data field is optional and does not have
           to be included in a ticket.

5.3.2. Authenticators

 An authenticator is a record sent with a ticket to a server to
 certify the client's knowledge of the encryption key in the ticket,
 to help the server detect replays, and to help choose a "true session
 key" to use with the particular session.  The encoding is encrypted
 in the ticket's session key shared by the client and the server:

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

             authenticator-vno[0]          INTEGER,
             crealm[1]                     Realm,
             cname[2]                      PrincipalName,
             cksum[3]                      Checksum OPTIONAL,
             cusec[4]                      INTEGER,
             ctime[5]                      KerberosTime,
             subkey[6]                     EncryptionKey OPTIONAL,
             seq-number[7]                 INTEGER OPTIONAL,

Kohl & Neuman [Page 47] RFC 1510 Kerberos September 1993

             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 5.3.1.
 cksum     This field contains a checksum of the the application data
           that accompanies the KRB_AP_REQ.
 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.
 subkey    This field contains the client's choice for an encryption
           key which is 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.
           For sequence numbers to adequately support the detection of
           replays 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.

Kohl & Neuman [Page 48] RFC 1510 Kerberos September 1993

 authorization-data This field is the same as described for the ticket
           in section 5.3.1.  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.

5.4. Specifications for the AS and TGS exchanges

 This section specifies the format of the messages used in 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 type of its own.  Instead, its type is
 one of KRB_AS_REQ or KRB_TGS_REQ 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 Authentication Server to
 request credentials for a service.

The message fields are:

AS-REQ ::= [APPLICATION 10] KDC-REQ TGS-REQ ::= [APPLICATION 12] KDC-REQ

KDC-REQ ::= SEQUENCE {

         pvno[1]               INTEGER,
         msg-type[2]           INTEGER,
         padata[3]             SEQUENCE OF PA-DATA OPTIONAL,
         req-body[4]           KDC-REQ-BODY

}

PA-DATA ::= SEQUENCE {

         padata-type[1]        INTEGER,
         padata-value[2]       OCTET STRING,
                       -- might be encoded AP-REQ

}

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]             INTEGER,

Kohl & Neuman [Page 49] RFC 1510 Kerberos September 1993

          etype[8]             SEQUENCE OF INTEGER, -- EncryptionType,
                       -- in preference order
          addresses[9]         HostAddresses OPTIONAL,
          enc-authorization-data[10]   EncryptedData OPTIONAL,
                       -- Encrypted AuthorizationData encoding
          additional-tickets[11]       SEQUENCE OF Ticket OPTIONAL

}

 The fields in this message are:
 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    The padata (pre-authentication data) field contains a of
           authentication information which may be needed before
           credentials can be issued or decrypted.  In the case of
           requests for additional tickets (KRB_TGS_REQ), this field
           will include an element with padata-type of PA-TGS-REQ and
           data of an authentication header (ticket-granting ticket
           and authenticator). The checksum in the authenticator
           (which must be collisionproof) is to be computed over the
           KDC-REQ-BODY encoding.  In most requests for initial
           authentication (KRB_AS_REQ) and most replies (KDC-REP), the
           padata field will be left out.
           This field may also contain information needed by certain
           extensions to the Kerberos protocol.  For example, it might
           be used to initially verify the identity of a client before
           any response is returned.  This is accomplished with a
           padata field with padata-type equal to PA-ENC-TIMESTAMP and
           padata-value defined as follows:
 padata-type     ::= PA-ENC-TIMESTAMP
 padata-value    ::= EncryptedData -- PA-ENC-TS-ENC
 PA-ENC-TS-ENC   ::= SEQUENCE {
         patimestamp[0]               KerberosTime, -- client's time
         pausec[1]                    INTEGER OPTIONAL
 }

Kohl & Neuman [Page 50] RFC 1510 Kerberos September 1993

           with patimestamp containing the client's time and pausec
           containing 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
           sequence, encrypted using the client's secret key.
           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
           "smartcards" with Kerberos.  The details of such extensions
           are beyond the scope of this specification.  See [10] for
           additional uses of this field.
 padata-type The padata-type element of the padata field 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.
 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 case, the bit in the
           options field will be the same as that in the flags field,
           this is not guaranteed, so it is not acceptable to simply
           copy the options field to the flags field.  There are
           various checks that must be made before honoring an option
           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:

Kohl & Neuman [Page 51] RFC 1510 Kerberos September 1993

           Bit(s)  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 ticket-
                                granting ticket 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
                                ticket-granting ticket 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 in a
                                subsequent request if the ticket-
                                granting ticket 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 ticket-
                                granting ticket in the request has its
                                PROXIABLE bit set.  The address(es) of
                                the 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

Kohl & Neuman [Page 52] RFC 1510 Kerberos September 1993

                                the ticket-granting ticket 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 ticket-granting ticket
                                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       UNUSED       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 ticket-granting ticket 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.
           9-26    RESERVED     Reserved for future use.
           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.  If a
                                ticket with the requested life cannot
                                be provided, then a renewable ticket
                                may be issued with a renew-till equal
                                to the 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

Kohl & Neuman [Page 53] RFC 1510 Kerberos September 1993

                                encrypted in the session key from the
                                additional ticket-granting ticket
                                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 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 be
                                otherwise 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 5.3.1.  sname may only be absent when the
           ENC-TKT-IN-SKEY option is specified.  If 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 ticket-granting
           ticket, both from the padata field in the KRB_AP_REQ.

Kohl & Neuman [Page 54] RFC 1510 Kerberos September 1993

 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 start time for the
           requested ticket.
 till      This field contains the expiration date requested by the
           client in a ticket request.
 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 it
           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 re-used.  Ideally, it should be gen erated
           randomly, but if the correct time is known, it may suffice
           (Note, however, that if the time is used as the nonce, one
           must make sure that the workstation time is monotonically
           increasing.  If the time is ever reset backwards, there is
           a small, but finite, probability that a nonce will 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 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.  If more than one option
           which 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).

Kohl & Neuman [Page 55] RFC 1510 Kerberos September 1993

 The application code 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.
 It should be noted 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, or if absent, the session key
 from the ticket-granting ticket 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,
               msg-type[1]                INTEGER,
               padata[2]                  SEQUENCE OF PA-DATA OPTIONAL,
               crealm[3]                  Realm,
               cname[4]                   PrincipalName,
               ticket[5]                  Ticket,
               enc-part[6]                EncryptedData
 }
 EncASRepPart ::=    [APPLICATION 25[25]] EncKDCRepPart
 EncTGSRepPart ::=   [APPLICATION 26] EncKDCRepPart
 EncKDCRepPart ::=   SEQUENCE {
             key[0]                       EncryptionKey,
             last-req[1]                  LastReq,

Kohl & Neuman [Page 56] RFC 1510 Kerberos September 1993

             nonce[2]                     INTEGER,
             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
 }
 NOTE: In EncASRepPart, the application code in the encrypted
       part of a message provides an additional check that
       the message was decrypted properly.
 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 this field is to encode an alternate
           "mix-in" string to be used with a string-to-key algorithm
           (such as is described in section 6.3.2). This ability is
           useful to ease 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 mix-in string until the
           next password change.
 crealm, cname, srealm and sname These fields are the same as those
           described for the ticket in section 5.3.1.
 ticket    The newly-issued ticket, from section 5.3.1.
 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.  The encrypted part
           is encoded as described in section 6.1.
 key       This field is the same as described for the ticket in
           section 5.3.1.
 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 ticket-granting ticket was made, or the
           last time that a request based on a ticket-granting ticket

Kohl & Neuman [Page 57] RFC 1510 Kerberos September 1993

           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 into timesharing systems.
 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.  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 need 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
           expira    tion 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 5.3.1),
           provided so the client may verify they match the intended
           request and 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 ticket granting ticket.  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 ticket-granting ticket.

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".
 AP-REQ ::=      [APPLICATION 14] SEQUENCE {
                 pvno[0]                       INTEGER,
                 msg-type[1]                   INTEGER,

Kohl & Neuman [Page 58] RFC 1510 Kerberos September 1993

                 ap-options[2]                 APOptions,
                 ticket[3]                     Ticket,
                 authenticator[4]              EncryptedData
 }
 APOptions ::=   BIT STRING {
                 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 being reset (0).  The encoding of the bits is
           specified in section 5.2.  The meanings of the options are:
           Bit(s)  Name           Description
           0       RESERVED       Reserved for future expansion of
                                this field.
           1       USE-SESSION-KEYThe 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
                                ticket-granting ticket. When this
                                option is not specified, the ticket is
                                encrypted in the server's secret key.
           2       MUTUAL-REQUIREDThe 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.
 authenticator This contains the authenticator, which includes the
           client's choice of a subkey.  Its encoding is described in
           section 5.3.2.

Kohl & Neuman [Page 59] RFC 1510 Kerberos September 1993

5.5.2. KRB_AP_REP definition

 The KRB_AP_REP message contains the Kerberos protocol version number,
 the message type, and an encrypted timestamp. The message is sent in
 in response to an application request (KRB_AP_REQ) where the mutual
 authentication option has been selected in the ap-options field.
 AP-REP ::=         [APPLICATION 15] SEQUENCE {
            pvno[0]                   INTEGER,
            msg-type[1]               INTEGER,
            enc-part[2]               EncryptedData
 }
 EncAPRepPart ::=   [APPLICATION 27]     SEQUENCE {
            ctime[0]                  KerberosTime,
            cusec[1]                  INTEGER,
            subkey[2]                 EncryptionKey OPTIONAL,
            seq-number[3]             INTEGER OPTIONAL
 }
 NOTE: in EncAPRepPart, the application code in the encrypted part of
 a message provides an additional check that the message was decrypted
 properly.
 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.
 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 which 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 also left out, the session key from
           the ticket will be used.

Kohl & Neuman [Page 60] RFC 1510 Kerberos September 1993

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 session key.  The message fields are:
 KRB-SAFE ::=        [APPLICATION 20] SEQUENCE {
             pvno[0]               INTEGER,
             msg-type[1]           INTEGER,
             safe-body[2]          KRB-SAFE-BODY,
             cksum[3]              Checksum
 }
 KRB-SAFE-BODY ::=   SEQUENCE {
             user-data[0]          OCTET STRING,
             timestamp[1]          KerberosTime OPTIONAL,
             usec[2]               INTEGER OPTIONAL,
             seq-number[3]         INTEGER 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.
 safe-body This field is a placeholder for the body of the KRB-SAFE
           message.  It is to be encoded separately and then have the
           checksum computed over it, for use in the cksum field.
 cksum     This field contains the checksum of the application data.
           Checksum details are described in section 6.4.  The

Kohl & Neuman [Page 61] RFC 1510 Kerberos September 1993

           checksum is computed over the encoding of the KRB-SAFE-BODY
           sequence.
 user-data This field is part of the KRB_SAFE and KRB_PRIV messages
           and contain 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 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 which have been incorrectly
           or maliciously delivered to the wrong recipient.

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 securely and
 privately send a 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).

5.7.1. KRB_PRIV definition

 The KRB_PRIV message contains user data encrypted in the Session Key.
 The message fields are:
 KRB-PRIV ::=         [APPLICATION 21] SEQUENCE {
              pvno[0]                   INTEGER,
              msg-type[1]               INTEGER,
              enc-part[3]               EncryptedData
 }

Kohl & Neuman [Page 62] RFC 1510 Kerberos September 1993

 EncKrbPrivPart ::=   [APPLICATION 28] SEQUENCE {
              user-data[0]              OCTET STRING,
              timestamp[1]              KerberosTime OPTIONAL,
              usec[2]                   INTEGER OPTIONAL,
              seq-number[3]             INTEGER OPTIONAL,
              s-address[4]              HostAddress, -- sender's addr
              r-address[5]              HostAddress OPTIONAL
                                                    -- recip's addr
 }
 NOTE: In EncKrbPrivPart, the application code in the encrypted part
 of a message provides an additional check that the message was
 decrypted properly.
 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 (If supported by the
           encryption method in use, an initialization vector may be
           passed to the encryption procedure, in order to achieve
           proper cipher chaining.  The initialization vector might
           come from the last block of the ciphertext from the
           previous KRB_PRIV message, but it is the application's
           choice whether or not to use such an initialization vector.
           If left out, the default initialization vector for the
           encryption algorithm will be used.).  This encrypted
           encoding is used for the enc-part field of the KRB-PRIV
           message.  See section 6 for the format of the ciphertext.
 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.

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

Kohl & Neuman [Page 63] RFC 1510 Kerberos September 1993

 each.  The information needed to use the tickets is encryped under an
 encryption key previously exchanged.  The message fields are:
 KRB-CRED         ::= [APPLICATION 22]   SEQUENCE {
                  pvno[0]                INTEGER,
                  msg-type[1]            INTEGER, -- KRB_CRED
                  tickets[2]             SEQUENCE OF Ticket,
                  enc-part[3]            EncryptedData
 }
 EncKrbCredPart   ::= [APPLICATION 29]   SEQUENCE {
                  ticket-info[0]         SEQUENCE OF KrbCredInfo,
                  nonce[1]               INTEGER OPTIONAL,
                  timestamp[2]           KerberosTime OPTIONAL,
                  usec[3]                INTEGER 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
 }
 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 between the sender
           and the intended recipient.  This encrypted encoding is
           used for the enc-part field of the KRB-CRED message.  See
           section 6 for the format of the ciphertext.

Kohl & Neuman [Page 64] RFC 1510 Kerberos September 1993

 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 re-
           used; it should be generated randomly by the recipient of
           the message and provided to the sender of the mes  sage in
           an application specific manner.
 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 6.2.
 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 value.
 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 since the KRB_ERROR message is not protected by any
 encryption, it is quite possible for an intruder to synthesize or

Kohl & Neuman [Page 65] RFC 1510 Kerberos September 1993

 modify such a message.  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,
                 msg-type[1]           INTEGER,
                 ctime[2]              KerberosTime OPTIONAL,
                 cusec[3]              INTEGER OPTIONAL,
                 stime[4]              KerberosTime,
                 susec[5]              INTEGER,
                 error-code[6]         INTEGER,
                 crealm[7]             Realm OPTIONAL,
                 cname[8]              PrincipalName OPTIONAL,
                 realm[9]              Realm, -- Correct realm
                 sname[10]             PrincipalName, -- Correct name
                 e-text[11]            GeneralString OPTIONAL,
                 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     This field is described above in section 5.4.1.
 cusec     This field is described above in section 5.5.2.
 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 999. 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 8.
           Implementations are encouraged to provide for national
           language support in the display of error messages.
 crealm, cname, srealm and sname These fields are described above in

Kohl & Neuman [Page 66] RFC 1510 Kerberos September 1993

           section 5.3.1.
 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).
 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
 If the error-code is KRB_AP_ERR_METHOD, then the e-data field will
 contain an encoding of the following sequence:
    METHOD-DATA ::=    SEQUENCE {
                       method-type[0]   INTEGER,
                       method-data[1]   OCTET STRING OPTIONAL
     }
 method-type will indicate the required alternate method; method-data
 will contain any required additional information.

6. Encryption and Checksum Specifications

 The Kerberos protocols described in this document are designed to use
 stream encryption ciphers, which can be simulated using commonly
 available block encryption ciphers, such as the Data Encryption
 Standard [11], in conjunction with block chaining and checksum
 methods [12].  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
 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

Kohl & Neuman [Page 67] RFC 1510 Kerberos September 1993

 extract the session key from the Ticket and prove its knowledge
 thereof in a response verifies the identity of the service.
 The Kerberos protocols generally assume that the encryption used is
 secure from cryptanalysis; however, in some cases, the order of
 fields in the encrypted portions of messages are arranged to minimize
 the effects of poorly chosen keys.  It is still important to choose
 good keys.  If keys are derived from user-typed passwords, those
 passwords need to be well chosen to make brute force attacks more
 difficult.  Poorly chosen keys still make easy targets for intruders.
 The following sections specify the encryption and checksum mechanisms
 currently defined for Kerberos.  The encodings, chaining, and padding
 requirements for each are described.  For encryption methods, it is
 often desirable to place random information (often referred to as a
 confounder) at the start of the message.  The requirements for a
 confounder are specified with each encryption mechanism.
 Some encryption systems use a block-chaining method to improve the
 the security characteristics of the ciphertext.  However, these
 chaining methods often don't provide an integrity check upon
 decryption.  Such systems (such as DES in CBC mode) must be augmented
 with a checksum of the plaintext which can be verified at decryption
 and used to detect any tampering or damage.  Such checksums should be
 good at detecting burst errors in the input.  If any damage is
 detected, the decryption routine is expected to return an error
 indicating the failure of an integrity check. Each encryption type is
 expected to provide and verify an appropriate checksum. The
 specification of each encryption method sets out its checksum
 requirements.
 Finally, where a key is to be derived from a user's password, an
 algorithm for converting the password to a key of the appropriate
 type is included.  It is desirable for the string to key function to
 be one-way, and for the mapping to be different in different realms.
 This is important because users who are registered in more than one
 realm will often use the same password in each, and it is desirable
 that an attacker compromising the Kerberos server in one realm not
 obtain or derive the user's key in another.
 For a discussion of the integrity characteristics of the candidate
 encryption and checksum methods considered for Kerberos, the the
 reader is referred to [13].

6.1. Encryption Specifications

 The following ASN.1 definition describes all encrypted messages.  The
 enc-part field which appears in the unencrypted part of messages in

Kohl & Neuman [Page 68] RFC 1510 Kerberos September 1993

 section 5 is a sequence consisting of an encryption type, an optional
 key version number, and the ciphertext.
 EncryptedData ::=   SEQUENCE {
                     etype[0]     INTEGER, -- EncryptionType
                     kvno[1]      INTEGER OPTIONAL,
                     cipher[2]    OCTET STRING -- ciphertext
 }
 etype     This field identifies which encryption algorithm was used
           to encipher the cipher.  Detailed specifications for
           selected encryption types appear later in this section.
 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.
 The cipher field is generated by applying the specified encryption
 algorithm to data composed of the message and algorithm-specific
 inputs.  Encryption mechanisms defined for use with Kerberos must
 take sufficient measures to guarantee the integrity of the plaintext,
 and we recommend they also take measures to protect against
 precomputed dictionary attacks.  If the encryption algorithm is not
 itself capable of doing so, the protections can often be enhanced by
 adding a checksum and a confounder.
 The suggested format for the data to be encrypted includes a
 confounder, a checksum, the encoded plaintext, and any necessary
 padding.  The msg-seq field contains the part of the protocol message
 described in section 5 which is to be encrypted.  The confounder,
 checksum, and padding are all untagged and untyped, and their length
 is exactly sufficient to hold the appropriate item.  The type and
 length is implicit and specified by the particular encryption type
 being used (etype).  The format for the data to be encrypted is
 described in the following diagram:
       +-----------+----------+-------------+-----+
       |confounder |   check  |   msg-seq   | pad |
       +-----------+----------+-------------+-----+
 The format cannot be described in ASN.1, but for those who prefer an
 ASN.1-like notation:

Kohl & Neuman [Page 69] RFC 1510 Kerberos September 1993

CipherText ::= ENCRYPTED SEQUENCE {

       confounder[0]   UNTAGGED OCTET STRING(conf_length)     OPTIONAL,
       check[1]        UNTAGGED OCTET STRING(checksum_length) OPTIONAL,
       msg-seq[2]      MsgSequence,
       pad             UNTAGGED OCTET STRING(pad_length) OPTIONAL

}

 In the above specification, UNTAGGED OCTET STRING(length) is the
 notation for an octet string with its tag and length removed.  It is
 not a valid ASN.1 type.  The tag bits and length must be removed from
 the confounder since the purpose of the confounder is so that the
 message starts with random data, but the tag and its length are
 fixed.  For other fields, the length and tag would be redundant if
 they were included because they are specified by the encryption type.
 One generates a random confounder of the appropriate length, placing
 it in confounder; zeroes out check; calculates the appropriate
 checksum over confounder, check, and msg-seq, placing the result in
 check; adds the necessary padding; then encrypts using the specified
 encryption type and the appropriate key.
 Unless otherwise specified, a definition of an encryption algorithm
 that specifies a checksum, a length for the confounder field, or an
 octet boundary for padding uses this ciphertext format (The ordering
 of the fields in the CipherText is important.  Additionally, messages
 encoded in this format must include a length as part of the msg-seq
 field.  This allows the recipient to verify that the message has not
 been truncated.  Without a length, an attacker could use a chosen
 plaintext attack to generate a message which could be truncated,
 while leaving the checksum intact.  Note that if the msg-seq is an
 encoding of an ASN.1 SEQUENCE or OCTET STRING, then the length is
 part of that encoding.). Those fields which are not specified will be
 omitted.
 In the interest of allowing all implementations using a particular
 encryption type to communicate with all others using that type, the
 specification of an encryption type defines any checksum that is
 needed as part of the encryption process.  If an alternative checksum
 is to be used, a new encryption type must be defined.
 Some cryptosystems require additional information beyond the key and
 the data to be encrypted. For example, DES, when used in cipher-
 block-chaining mode, requires an initialization vector.  If required,
 the description for each encryption type must specify the source of
 such additional information.

Kohl & Neuman [Page 70] RFC 1510 Kerberos September 1993

6.2. Encryption Keys

 The sequence below shows the encoding of an encryption key:
        EncryptionKey ::=   SEQUENCE {
                            keytype[0]    INTEGER,
                            keyvalue[1]   OCTET STRING
        }
 keytype   This field specifies the type of encryption key that
           follows in the keyvalue field.  It will almost always
           correspond to the encryption algorithm used to generate the
           EncryptedData, though more than one algorithm may use the
           same type of key (the mapping is many to one).  This might
           happen, for example, if the encryption algorithm uses an
           alternate checksum algorithm for an integrity check, or a
           different chaining mechanism.
 keyvalue  This field contains the key itself, encoded as an octet
           string.
 All negative values for the  encryption key type are reserved for
 local use.  All non-negative values are reserved for officially
 assigned type fields and interpretations.

6.3. Encryption Systems

6.3.1. The NULL Encryption System (null)

 If no encryption is in use, the encryption system is said to be the
 NULL encryption system.  In the NULL encryption system there is no
 checksum, confounder or padding.  The ciphertext is simply the
 plaintext.  The NULL Key is used by the null encryption system and is
 zero octets in length, with keytype zero (0).

6.3.2. DES in CBC mode with a CRC-32 checksum (des-cbc-crc)

 The des-cbc-crc encryption mode encrypts information under the Data
 Encryption Standard [11] using the cipher block chaining mode [12].
 A CRC-32 checksum (described in ISO 3309 [14]) is applied to the
 confounder and message sequence (msg-seq) and placed in the cksum
 field.  DES blocks are 8 bytes.  As a result, the data to be
 encrypted (the concatenation of confounder, checksum, and message)
 must be padded to an 8 byte boundary before encryption.  The details
 of the encryption of this data are identical to those for the des-
 cbc-md5 encryption mode.
 Note that, since the CRC-32 checksum is not collisionproof, an

Kohl & Neuman [Page 71] RFC 1510 Kerberos September 1993

 attacker could use a probabilistic chosenplaintext attack to generate
 a valid message even if a confounder is used [13]. The use of
 collision-proof checksums is recommended for environments where such
 attacks represent a significant threat.  The use of the CRC-32 as the
 checksum for ticket or authenticator is no longer mandated as an
 interoperability requirement for Kerberos Version 5 Specification 1
 (See section 9.1 for specific details).

6.3.3. DES in CBC mode with an MD4 checksum (des-cbc-md4)

 The des-cbc-md4 encryption mode encrypts information under the Data
 Encryption Standard [11] using the cipher block chaining mode [12].
 An MD4 checksum (described in [15]) is applied to the confounder and
 message sequence (msg-seq) and placed in the cksum field.  DES blocks
 are 8 bytes.  As a result, the data to be encrypted (the
 concatenation of confounder, checksum, and message) must be padded to
 an 8 byte boundary before encryption.  The details of the encryption
 of this data are identical to those for the descbc-md5 encryption
 mode.

6.3.4. DES in CBC mode with an MD5 checksum (des-cbc-md5)

 The des-cbc-md5 encryption mode encrypts information under the Data
 Encryption Standard [11] using the cipher block chaining mode [12].
 An MD5 checksum (described in [16]) is applied to the confounder and
 message sequence (msg-seq) and placed in the cksum field.  DES blocks
 are 8 bytes.  As a result, the data to be encrypted (the
 concatenation of confounder, checksum, and message) must be padded to
 an 8 byte boundary before encryption.
 Plaintext and DES ciphtertext are encoded as 8-octet blocks which are
 concatenated to make the 64-bit inputs for the DES algorithms.  The
 first octet supplies the 8 most significant bits (with the octet's
 MSbit used as the DES input block's MSbit, etc.), the second octet
 the next 8 bits, ..., and the eighth octet supplies the 8 least
 significant bits.
 Encryption under DES using cipher block chaining requires an
 additional input in the form of an initialization vector.  Unless
 otherwise specified, zero should be used as the initialization
 vector.  Kerberos' use of DES requires an 8-octet confounder.
 The DES specifications identify some "weak" and "semiweak" keys;
 those keys shall not be used for encrypting messages for use in
 Kerberos.  Additionally, because of the way that keys are derived for
 the encryption of checksums, keys shall not be used that yield "weak"
 or "semi-weak" keys when eXclusive-ORed with the constant
 F0F0F0F0F0F0F0F0.

Kohl & Neuman [Page 72] RFC 1510 Kerberos September 1993

 A DES key is 8 octets of data, with keytype one (1).  This consists
 of 56 bits of key, and 8 parity bits (one per octet).  The key is
 encoded as a series of 8 octets written in MSB-first order. The bits
 within the key are also encoded in MSB order.  For example, if the
 encryption key is:
 (B1,B2,...,B7,P1,B8,...,B14,P2,B15,...,B49,P7,B50,...,B56,P8) where
 B1,B2,...,B56 are the key bits in MSB order, and P1,P2,...,P8 are the
 parity bits, the first octet of the key would be B1,B2,...,B7,P1
 (with B1 as the MSbit).  [See the FIPS 81 introduction for
 reference.]
 To generate a DES key from a text string (password), the text string
 normally must have the realm and each component of the principal's
 name appended(In some cases, it may be necessary to use a different
 "mix-in" string for compatibility reasons; see the discussion of
 padata in section 5.4.2.), then padded with ASCII nulls to an 8 byte
 boundary.  This string is then fan-folded and eXclusive-ORed with
 itself to form an 8 byte DES key.  The parity is corrected on the
 key, and it is used to generate a DES CBC checksum on the initial
 string (with the realm and name appended).  Next, parity is corrected
 on the CBC checksum.  If the result matches a "weak" or "semiweak"
 key as described in the DES specification, it is eXclusive-ORed with
 the constant 00000000000000F0.  Finally, the result is returned as
 the key.  Pseudocode follows:
      string_to_key(string,realm,name) {
           odd = 1;
           s = string + realm;
           for(each component in name) {
                s = s + component;
           }
           tempkey = NULL;
           pad(s); /* with nulls to 8 byte boundary */
           for(8byteblock in s) {
                if(odd == 0)  {
                    odd = 1;
                    reverse(8byteblock)
                }
                else odd = 0;
                tempkey = tempkey XOR 8byteblock;
           }
           fixparity(tempkey);
           key = DES-CBC-check(s,tempkey);
           fixparity(key);
           if(is_weak_key_key(key))
                key = key XOR 0xF0;
           return(key);
      }

Kohl & Neuman [Page 73] RFC 1510 Kerberos September 1993

6.4. Checksums

 The following is the ASN.1 definition used for a checksum:
          Checksum ::=   SEQUENCE {
                         cksumtype[0]   INTEGER,
                         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.
 Detailed specification of selected checksum types appear later in
 this section.  Negative values for the checksum type are reserved for
 local use.  All non-negative values are reserved for officially
 assigned type fields and interpretations.
 Checksums used by Kerberos can be classified by two properties:
 whether they are collision-proof, and whether they are keyed.  It is
 infeasible to find two plaintexts which generate the same checksum
 value for a collision-proof checksum.  A key is required to perturb
 or initialize the algorithm in a keyed checksum.  To prevent
 message-stream modification by an active attacker, unkeyed checksums
 should only be used when the checksum and message will be
 subsequently encrypted (e.g., the checksums defined as part of the
 encryption algorithms covered earlier in this section).  Collision-
 proof checksums can be made tamper-proof as well if the checksum
 value is encrypted before inclusion in a message.  In such cases, the
 composition of the checksum and the encryption algorithm must be
 considered a separate checksum algorithm (e.g., RSA-MD5 encrypted
 using DES is a new checksum algorithm of type RSA-MD5-DES).  For most
 keyed checksums, as well as for the encrypted forms of collisionproof
 checksums, Kerberos prepends a confounder before the checksum is
 calculated.

6.4.1. The CRC-32 Checksum (crc32)

 The CRC-32 checksum calculates a checksum based on a cyclic
 redundancy check as described in ISO 3309 [14].  The resulting
 checksum is four (4) octets in length.  The CRC-32 is neither keyed
 nor collision-proof.  The use of this checksum is not recommended.
 An attacker using a probabilistic chosen-plaintext attack as
 described in [13] might be able to generate an alternative message
 that satisfies the checksum.  The use of collision-proof checksums is
 recommended for environments where such attacks represent a

Kohl & Neuman [Page 74] RFC 1510 Kerberos September 1993

 significant threat.

6.4.2. The RSA MD4 Checksum (rsa-md4)

 The RSA-MD4 checksum calculates a checksum using the RSA MD4
 algorithm [15].  The algorithm takes as input an input message of
 arbitrary length and produces as output a 128-bit (16 octet)
 checksum.  RSA-MD4 is believed to be collision-proof.

6.4.3. RSA MD4 Cryptographic Checksum Using DES (rsa-md4des)

 The RSA-MD4-DES checksum calculates a keyed collisionproof checksum
 by prepending an 8 octet confounder before the text, applying the RSA
 MD4 checksum algorithm, and encrypting the confounder and the
 checksum using DES in cipher-block-chaining (CBC) mode using a
 variant of the key, where the variant is computed by eXclusive-ORing
 the key with the constant F0F0F0F0F0F0F0F0 (A variant of the key is
 used to limit the use of a key to a particular function, separating
 the functions of generating a checksum from other encryption
 performed using the session key.  The constant F0F0F0F0F0F0F0F0 was
 chosen because it maintains key parity.  The properties of DES
 precluded the use of the complement.  The same constant is used for
 similar purpose in the Message Integrity Check in the Privacy
 Enhanced Mail standard.).  The initialization vector should be zero.
 The resulting checksum is 24 octets long (8 octets of which are
 redundant).  This checksum is tamper-proof and believed to be
 collision-proof.
 The DES specifications identify some "weak keys"; those keys shall
 not be used for generating RSA-MD4 checksums for use in Kerberos.
 The format for the checksum is described in the following diagram:
    +--+--+--+--+--+--+--+--
    |  des-cbc(confounder
    +--+--+--+--+--+--+--+--
                  +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
                      rsa-md4(confounder+msg),key=var(key),iv=0)  |
                  +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
 The format cannot be described in ASN.1, but for those who prefer an
 ASN.1-like notation:
 rsa-md4-des-checksum ::=   ENCRYPTED       UNTAGGED SEQUENCE {
                            confounder[0]   UNTAGGED OCTET STRING(8),
                            check[1]        UNTAGGED OCTET STRING(16)
 }

Kohl & Neuman [Page 75] RFC 1510 Kerberos September 1993

6.4.4. The RSA MD5 Checksum (rsa-md5)

 The RSA-MD5 checksum calculates a checksum using the RSA MD5
 algorithm [16].  The algorithm takes as input an input message of
 arbitrary length and produces as output a 128-bit (16 octet)
 checksum.  RSA-MD5 is believed to be collision-proof.

6.4.5. RSA MD5 Cryptographic Checksum Using DES (rsa-md5des)

 The RSA-MD5-DES checksum calculates a keyed collisionproof checksum
 by prepending an 8 octet confounder before the text, applying the RSA
 MD5 checksum algorithm, and encrypting the confounder and the
 checksum using DES in cipher-block-chaining (CBC) mode using a
 variant of the key, where the variant is computed by eXclusive-ORing
 the key with the constant F0F0F0F0F0F0F0F0.  The initialization
 vector should be zero.  The resulting checksum is 24 octets long (8
 octets of which are redundant).  This checksum is tamper-proof and
 believed to be collision-proof.
 The DES specifications identify some "weak keys"; those keys shall
 not be used for encrypting RSA-MD5 checksums for use in Kerberos.
 The format for the checksum is described in the following diagram:
    +--+--+--+--+--+--+--+--
    |  des-cbc(confounder
    +--+--+--+--+--+--+--+--
                   +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
                       rsa-md5(confounder+msg),key=var(key),iv=0)  |
                   +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
 The format cannot be described in ASN.1, but for those who prefer an
 ASN.1-like notation:
 rsa-md5-des-checksum ::=   ENCRYPTED       UNTAGGED SEQUENCE {
                            confounder[0]   UNTAGGED OCTET STRING(8),
                            check[1]        UNTAGGED OCTET STRING(16)
 }

6.4.6. DES cipher-block chained checksum (des-mac)

 The DES-MAC checksum is computed by prepending an 8 octet confounder
 to the plaintext, performing a DES CBC-mode encryption on the result
 using the key and an initialization vector of zero, taking the last
 block of the ciphertext, prepending the same confounder and
 encrypting the pair using DES in cipher-block-chaining (CBC) mode
 using a a variant of the key, where the variant is computed by

Kohl & Neuman [Page 76] RFC 1510 Kerberos September 1993

 eXclusive-ORing the key with the constant F0F0F0F0F0F0F0F0.  The
 initialization vector should be zero.  The resulting checksum is 128
 bits (16 octets) long, 64 bits of which are redundant. This checksum
 is tamper-proof and collision-proof.
 The format for the checksum is described in the following diagram:
    +--+--+--+--+--+--+--+--
    |   des-cbc(confounder
    +--+--+--+--+--+--+--+--
                   +-----+-----+-----+-----+-----+-----+-----+-----+
                     des-mac(conf+msg,iv=0,key),key=var(key),iv=0) |
                   +-----+-----+-----+-----+-----+-----+-----+-----+
 The format cannot be described in ASN.1, but for those who prefer an
 ASN.1-like notation:
 des-mac-checksum ::=    ENCRYPTED       UNTAGGED SEQUENCE {
                         confounder[0]   UNTAGGED OCTET STRING(8),
                         check[1]        UNTAGGED OCTET STRING(8)
 }
 The DES specifications identify some "weak" and "semiweak" keys;
 those keys shall not be used for generating DES-MAC checksums for use
 in Kerberos, nor shall a key be used whose veriant is "weak" or
 "semi-weak".

6.4.7. RSA MD4 Cryptographic Checksum Using DES alternative

     (rsa-md4-des-k)
 The RSA-MD4-DES-K checksum calculates a keyed collision-proof
 checksum by applying the RSA MD4 checksum algorithm and encrypting
 the results using DES in cipherblock-chaining (CBC) mode using a DES
 key as both key and initialization vector. The resulting checksum is
 16 octets long. This checksum is tamper-proof and believed to be
 collision-proof.  Note that this checksum type is the old method for
 encoding the RSA-MD4-DES checksum and it is no longer recommended.

6.4.8. DES cipher-block chained checksum alternative (desmac-k)

 The DES-MAC-K checksum is computed by performing a DES CBC-mode
 encryption of the plaintext, and using the last block of the
 ciphertext as the checksum value. It is keyed with an encryption key
 and an initialization vector; any uses which do not specify an
 additional initialization vector will use the key as both key and
 initialization vector.  The resulting checksum is 64 bits (8 octets)
 long. This checksum is tamper-proof and collision-proof.  Note that

Kohl & Neuman [Page 77] RFC 1510 Kerberos September 1993

 this checksum type is the old method for encoding the DESMAC checksum
 and it is no longer recommended.
 The DES specifications identify some "weak keys"; those keys shall
 not be used for generating DES-MAC checksums for use in Kerberos.

7. Naming Constraints

7.1. Realm Names

 Although realm names are encoded as GeneralStrings and although a
 realm can technically 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.
 There are presently four styles of realm names: domain, X500, other,
 and reserved.  Examples of each style follow:
      domain:   host.subdomain.domain (example)
        X500:   C=US/O=OSF (example)
       other:   NAMETYPE:rest/of.name=without-restrictions (example)
    reserved:   reserved, but will not conflict with above
 Domain names must look like domain names: they consist of components
 separated by periods (.) and they contain neither colons (:) nor
 slashes (/).
 X.500 names contain an equal (=) and cannot contain a colon (:)
 before the equal.  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.
 Names that fall into the other category must begin with a prefix that
 contains no equal (=) or period (.) and the prefix must be followed
 by a colon (:) and the rest of the name. All prefixes must be
 assigned before they may be used.  Presently none are assigned.
 The reserved category includes strings which 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

Kohl & Neuman [Page 78] RFC 1510 Kerberos September 1993

 various name styles.  The following additional constraints apply to
 the assignment of realm names in the domain and X.500 categories: the
 name of a realm for the domain or X.500 formats must either 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
 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 there will not in the future exists a name identical to the
 realm name of the child unless it is assigned to the same entity as
 the realm name.

7.2. 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 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).  This constraint may be eliminated in the future.  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
 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).

Kohl & Neuman [Page 79] RFC 1510 Kerberos September 1993

 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 used.  An
 example of this name type is the Kerberos ticket-granting ticket
 which has a first component of krbtgt and a second component
 identifying the realm for which the ticket is valid.
 If instance is a single component following the service name and the
 instance identifies 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 SRVXHST 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 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 8.2.3 for the
 form of such names.

7.2.1. 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 lower case.  For services such as telnet and the
 Berkeley R commands which run with system privileges, the first
 component will be the string "host" instead of a service specific
 identifier.

8. Constants and other defined values

8.1. 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

Kohl & Neuman [Page 80] RFC 1510 Kerberos September 1993

 type fields and interpretations.
 The values of the types for the following addresses are chosen to
 match the defined address family constants in the Berkeley Standard
 Distributions of Unix.  They can be found in <sys/socket.h> with
 symbolic names AF_xxx (where xxx is an abbreviation of the address
 family name).
 Internet addresses
    Internet addresses are 32-bit (4-octet) quantities, encoded in MSB
    order.  The type of internet addresses is two (2).
 CHAOSnet addresses
    CHAOSnet addresses are 16-bit (2-octet) quantities, encoded in MSB
    order.  The type of CHAOSnet addresses is five (5).
 ISO addresses
    ISO addresses are variable-length.  The type of ISO addresses is
    seven (7).
 Xerox Network Services (XNS) addresses
    XNS addresses are 48-bit (6-octet) quantities, encoded in MSB
    order.  The type of XNS addresses is six (6).
 AppleTalk Datagram Delivery Protocol (DDP) addresses
    AppleTalk DDP addresses consist of an 8-bit node number and a 16-
    bit network number.  The first octet of the address is the node
    number; the remaining two octets encode the network number in MSB
    order. The type of AppleTalk DDP addresses is sixteen (16).
 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).

8.2. KDC messages

8.2.1. IP transport

 When contacting a Kerberos server (KDC) for a KRB_KDC_REQ request
 using IP transport, the client shall send a UDP datagram containing
 only an encoding of the request to port 88 (decimal) at the KDC's IP

Kohl & Neuman [Page 81] RFC 1510 Kerberos September 1993

 address; 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.

8.2.2. OSI transport

 During authentication of an OSI client to and OSI server, the mutual
 authentication of an OSI server to an OSI client, the transfer of
 credentials from an OSI client to an OSI server, or during exchange
 of private or integrity checked messages, Kerberos protocol messages
 may be treated as opaque objects and the type of the authentication
 mechanism will be:
 OBJECT IDENTIFIER ::= {iso (1), org(3), dod(5),internet(1),
                        security(5), kerberosv5(2)}
 Depending on the situation, the opaque object will be an
 authentication header (KRB_AP_REQ), an authentication reply
 (KRB_AP_REP), a safe message (KRB_SAFE), a private message
 (KRB_PRIV), or a credentials message (KRB_CRED).  The opaque data
 contains an application code as specified in the ASN.1 description
 for each message.  The application code may be used by Kerberos to
 determine the message type.

8.2.3. Name of the TGS

 The principal identifier of the ticket-granting service shall be
 composed of three parts: (1) the realm of the KDC issuing the TGS
 ticket (2) a two-part name of type NT-SRVINST, with the first part
 "krbtgt" and the second part the name of the realm which will accept
 the ticket-granting ticket.  For example, a ticket-granting ticket
 issued by the ATHENA.MIT.EDU realm to be used to get tickets from the
 ATHENA.MIT.EDU KDC has a principal identifier of "ATHENA.MIT.EDU"
 (realm), ("krbtgt", "ATHENA.MIT.EDU") (name).  A ticket-granting
 ticket 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).

8.3. Protocol constants and associated values

 The following tables list constants used in the protocol and defines
 their meanings.

Kohl & Neuman [Page 82] RFC 1510 Kerberos September 1993

—————+———–+———-+—————-+————— Encryption type|etype value|block size|minimum pad size|confounder size —————+———–+———-+—————-+————— NULL 0 1 0 0 des-cbc-crc 1 8 4 8 des-cbc-md4 2 8 0 8 des-cbc-md5 3 8 0 8

——————————-+——————-+————- Checksum type |sumtype value |checksum size ——————————-+——————-+————- CRC32 1 4 rsa-md4 2 16 rsa-md4-des 3 24 des-mac 4 16 des-mac-k 5 8 rsa-md4-des-k 6 16 rsa-md5 7 16 rsa-md5-des 8 24

——————————-+—————– padata type |padata-type value ——————————-+—————– PA-TGS-REQ 1 PA-ENC-TIMESTAMP 2 PA-PW-SALT 3

——————————-+————- authorization data type |ad-type value ——————————-+————- reserved values 0-63 OSF-DCE 64 SESAME 65

——————————-+—————– alternate authentication type |method-type value ——————————-+—————– reserved values 0-63 ATT-CHALLENGE-RESPONSE 64

——————————-+————- transited encoding type |tr-type value ——————————-+————- DOMAIN-X500-COMPRESS 1 reserved values all others

Kohl & Neuman [Page 83] RFC 1510 Kerberos September 1993

————–+——-+—————————————– Label |Value |Meaning or MIT code ————–+——-+—————————————–

pvno 5 current Kerberos protocol version number

message types

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_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

name types

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

error codes

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

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

Kohl & Neuman [Page 84] RFC 1510 Kerberos September 1993

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 start time 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*

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

Kohl & Neuman [Page 85] RFC 1510 Kerberos September 1993

                                 message

KRB_ERR_GENERIC 60 Generic error (description in e-text) KRB_ERR_FIELD_TOOLONG 61 Field is too long for this

                                 implementation
  • This error carries additional information in the e-data field. The

contents of the e-data field for this message is described in section

 5.9.1.

9. 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, forwarding, postdating, and renewing tickets,
 the format of realm names, and the handling of authorization data.
 In order to ensure the interoperability of realms, it is necessary to
 define a minimal configuration which 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.

9.1. Specification 1

 This section defines the first specification of these options.
 Implementations which are configured in this way can be said to
 support Kerberos Version 5 Specification 1 (5.1).
 Encryption and checksum methods
 The following encryption and checksum mechanisms must be supported.
 Implementations may support other mechanisms as well, but the
 additional mechanisms may only be used when communicating with
 principals known to also support them: Encryption: DES-CBC-MD5
 Checksums: CRC-32, DES-MAC, DES-MAC-K, and DES-MD5
 Realm Names
 All implementations must understand hierarchical realms in both the
 Internet Domain and the X.500 style.  When a ticket granting ticket
 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.

Kohl & Neuman [Page 86] RFC 1510 Kerberos September 1993

 Transited field encoding
 DOMAIN-X500-COMPRESS (described in section 3.3.3.1) must be
 supported.  Alternative encodings may be supported, but they may be
 used only when that encoding is supported by ALL intermediate realms.
 Pre-authentication methods
 The TGS-REQ method must be supported.  The TGS-REQ method 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 not used in the initial request and the
 error KDC_ERR_PREAUTH_REQUIRED is returned specifying PA-ENCTIMESTAMP
 as an acceptable method, the client should retry the initial request
 using the PA-ENC-TIMESTAMP preauthentication method. Servers need not
 support the PAENC-TIMESTAMP method, but if not supported the server
 should ignore the presence of PA-ENC-TIMESTAMP pre-authentication in
 a request.
 Mutual authentication
 Mutual authentication (via the KRB_AP_REP message) must be supported.
 Ticket addresses and flags
 All KDC's must pass on tickets that carry no addresses (i.e.,  if a
 TGT contains no addresses, the KDC will return derivative tickets),
 but each realm may set its own policy for issuing such tickets, and
 each application server will set its own policy with respect to
 accepting them. By default, servers should not accept them.
 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 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-TKTIN-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.

Kohl & Neuman [Page 87] RFC 1510 Kerberos September 1993

 Authorization data
 Implementations must pass all authorization data subfields from
 ticket-granting tickets to any derivative tickets unless 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).
 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.

9.2. Recommended KDC values

 Following is a list of recommended values for a KDC implementation,
 based on the list of suggested configuration constants (see section
 4.4).
 minimum lifetime                5 minutes
 maximum renewable lifetime      1 week
 maximum ticket lifetime         1 day
 empty addresses                 only when suitable restrictions appear
                                 in authorization data
 proxiable, etc.                 Allowed.

10. Acknowledgments

 Early versions of this document, describing version 4 of the
 protocol, were written by Jennifer Steiner (formerly at Project
 Athena); these drafts provided an excellent starting point for this
 current version 5 specification.  Many people in the Internet
 community have contributed ideas and suggested protocol changes for
 version 5. Notable contributions came from Ted Anderson, Steve
 Bellovin and Michael Merritt [17], Daniel Bernstein, Mike Burrows,
 Donald Davis, Ravi Ganesan, Morrie Gasser, Virgil Gligor, Bill
 Griffeth, Mark Lillibridge, Mark Lomas, Steve Lunt, Piers McMahon,
 Joe Pato, William Sommerfeld, Stuart Stubblebine, Ralph Swick, Ted
 T'so, and Stanley Zanarotti.  Many others commented and helped shape
 this specification into its current form.

Kohl & Neuman [Page 88] RFC 1510 Kerberos September 1993

11. References

 [1]  Miller, S., Neuman, C., Schiller, J., and  J. Saltzer, "Section
      E.2.1: Kerberos  Authentication and Authorization System",
      M.I.T. Project Athena, Cambridge, Massachusetts, December 21,
      1987.
 [2]  Steiner, J., Neuman, C., and J. Schiller, "Kerberos: An
      Authentication Service for Open Network Systems", pp. 191-202 in
      Usenix Conference Proceedings, Dallas, Texas, February, 1988.
 [3]  Needham, R., and M. Schroeder, "Using Encryption for
      Authentication in Large Networks of Computers", Communications
      of the ACM, Vol. 21 (12), pp. 993-999, December 1978.
 [4]  Denning, D., and G. Sacco, "Time stamps in Key Distribution
      Protocols", Communications of the ACM, Vol. 24 (8), pp. 533-536,
      August 1981.
 [5]  Kohl, J., Neuman, C., and T. Ts'o, "The Evolution of the
      Kerberos Authentication Service", in an IEEE Computer Society
      Text soon to be published, June 1992.
 [6]  Davis, D., and R. Swick, "Workstation Services and Kerberos
      Authentication at Project Athena", Technical Memorandum TM-424,
      MIT Laboratory for Computer Science, February 1990.
 [7]  Levine, P., Gretzinger, M, Diaz, J., Sommerfeld, W., and K.
      Raeburn, "Section E.1: Service Management System, M.I.T.
      Project Athena, Cambridge, Mas sachusetts (1987).
 [8]  CCITT, Recommendation X.509: The Directory Authentication
      Framework, December 1988.
 [9]  Neuman, C., "Proxy-Based Authorization and Accounting for
      Distributed Systems," in Proceedings of the 13th International
      Conference on Distributed Computing Systems", Pittsburgh, PA,
      May 1993.
 [10] Pato, J., "Using Pre-Authentication to Avoid Password Guessing
      Attacks", Open Software Foundation DCE Request for Comments 26,
      December 1992.
 [11] National Bureau of Standards, U.S. Department of Commerce, "Data
      Encryption Standard", Federal Information Processing Standards
      Publication 46, Washington, DC (1977).

Kohl & Neuman [Page 89] RFC 1510 Kerberos September 1993

 [12] National Bureau of Standards, U.S. Department of Commerce, "DES
      Modes of Operation", Federal Information Processing Standards
      Publication 81, Springfield, VA, December 1980.
 [13] Stubblebine S., and V. Gligor, "On Message Integrity in
      Cryptographic Protocols", in Proceedings of the IEEE Symposium
      on Research in Security and Privacy, Oakland, California, May
      1992.
 [14] International Organization for Standardization, "ISO Information
      Processing Systems - Data Communication High-Level Data Link
      Control Procedure - Frame Structure", IS 3309, October 1984, 3rd
      Edition.
 [15] Rivest, R., "The MD4 Message Digest Algorithm", RFC 1320, MIT
      Laboratory for Computer Science, April 1992.
 [16] Rivest, R., "The MD5 Message Digest Algorithm", RFC 1321, MIT
      Laboratory for Computer Science, April 1992.
 [17] Bellovin S., and M. Merritt, "Limitations of the Kerberos
      Authentication System", Computer Communications Review, Vol.
      20(5), pp. 119-132, October 1990.

12. Security Considerations

 Security issues are discussed throughout this memo.

13. Authors' Addresses

 John Kohl
 Digital Equipment Corporation
 110 Spit Brook Road, M/S ZKO3-3/U14
 Nashua, NH  03062
 Phone: 603-881-2481
 EMail: jtkohl@zk3.dec.com
 B. Clifford Neuman
 USC/Information Sciences Institute
 4676 Admiralty Way #1001
 Marina del Rey, CA 90292-6695
 Phone: 310-822-1511
 EMail: bcn@isi.edu

Kohl & Neuman [Page 90] RFC 1510 Kerberos September 1993

A. Pseudo-code for protocol processing

 This appendix provides pseudo-code describing how the messages are to
 be constructed and interpreted by clients and servers.

A.1. KRB_AS_REQ generation

      request.pvno := protocol version; /* pvno = 5 */
      request.msg-type := message type; /* type = KRB_AS_REQ */
      if(pa_enc_timestamp_required) then
              request.padata.padata-type = PA-ENC-TIMESTAMP;
              get system_time;
              padata-body.patimestamp,pausec = system_time;
              encrypt padata-body into request.padata.padata-value
                      using client.key; /* derived from password */
      endif
      body.kdc-options := users's preferences;
      body.cname := user's name;
      body.realm := user's realm;
      body.sname := service's name; /* usually "krbtgt",
                                       "localrealm" */
      if (body.kdc-options.POSTDATED is set) then
              body.from := requested starting time;
      else
              omit body.from;
      endif
      body.till := requested end time;
      if (body.kdc-options.RENEWABLE is set) then
              body.rtime := requested final renewal time;
      endif
      body.nonce := random_nonce();
      body.etype := requested etypes;
      if (user supplied addresses) then
              body.addresses := user's addresses;
      else
              omit body.addresses;
      endif
      omit body.enc-authorization-data;
      request.req-body := body;
      kerberos := lookup(name of local kerberos server (or servers));
      send(packet,kerberos);
      wait(for response);
      if (timed_out) then
              retry or use alternate server;
      endif

Kohl & Neuman [Page 91] RFC 1510 Kerberos September 1993

A.2. KRB_AS_REQ verification and KRB_AS_REP generation

      decode message into req;
      client := lookup(req.cname,req.realm);
      server := lookup(req.sname,req.realm);
      get system_time;
      kdc_time := system_time.seconds;
      if (!client) then
              /* no client in Database */
              error_out(KDC_ERR_C_PRINCIPAL_UNKNOWN);
      endif
      if (!server) then
              /* no server in Database */
              error_out(KDC_ERR_S_PRINCIPAL_UNKNOWN);
      endif
      if(client.pa_enc_timestamp_required and
         pa_enc_timestamp not present) then
              error_out(KDC_ERR_PREAUTH_REQUIRED(PA_ENC_TIMESTAMP));
      endif
      if(pa_enc_timestamp present) then
              decrypt req.padata-value into decrypted_enc_timestamp
                      using client.key;
                      using auth_hdr.authenticator.subkey;
              if (decrypt_error()) then
                      error_out(KRB_AP_ERR_BAD_INTEGRITY);
              if(decrypted_enc_timestamp is not within allowable
                      skew) then error_out(KDC_ERR_PREAUTH_FAILED);
              endif
              if(decrypted_enc_timestamp and usec is replay)
                      error_out(KDC_ERR_PREAUTH_FAILED);
              endif
              add decrypted_enc_timestamp and usec to replay cache;
      endif
      use_etype := first supported etype in req.etypes;
      if (no support for req.etypes) then
              error_out(KDC_ERR_ETYPE_NOSUPP);
      endif
      new_tkt.vno := ticket version; /* = 5 */
      new_tkt.sname := req.sname;
      new_tkt.srealm := req.srealm;
      reset all flags in new_tkt.flags;

Kohl & Neuman [Page 92] RFC 1510 Kerberos September 1993

      /* It should be noted that local policy may affect the  */
      /* processing of any of these flags.  For example, some */
      /* realms may refuse to issue renewable tickets         */
      if (req.kdc-options.FORWARDABLE is set) then
              set new_tkt.flags.FORWARDABLE;
      endif
      if (req.kdc-options.PROXIABLE is set) then
              set new_tkt.flags.PROXIABLE;
      endif
      if (req.kdc-options.ALLOW-POSTDATE is set) then
              set new_tkt.flags.ALLOW-POSTDATE;
      endif
      if ((req.kdc-options.RENEW is set) or
          (req.kdc-options.VALIDATE is set) or
          (req.kdc-options.PROXY is set) or
          (req.kdc-options.FORWARDED is set) or
          (req.kdc-options.ENC-TKT-IN-SKEY is set)) then
              error_out(KDC_ERR_BADOPTION);
      endif
      new_tkt.session := random_session_key();
      new_tkt.cname := req.cname;
      new_tkt.crealm := req.crealm;
      new_tkt.transited := empty_transited_field();
      new_tkt.authtime := kdc_time;
      if (req.kdc-options.POSTDATED is set) then
         if (against_postdate_policy(req.from)) then
              error_out(KDC_ERR_POLICY);
         endif
         set new_tkt.flags.INVALID;
         new_tkt.starttime := req.from;
      else
         omit new_tkt.starttime; /* treated as authtime when
                                    omitted */
      endif
      if (req.till = 0) then
              till := infinity;
      else
              till := req.till;
      endif
      new_tkt.endtime := min(till,
                            new_tkt.starttime+client.max_life,
                            new_tkt.starttime+server.max_life,
                            new_tkt.starttime+max_life_for_realm);

Kohl & Neuman [Page 93] RFC 1510 Kerberos September 1993

      if ((req.kdc-options.RENEWABLE-OK is set) and
          (new_tkt.endtime < req.till)) then
              /* we set the RENEWABLE option for later processing */
              set req.kdc-options.RENEWABLE;
              req.rtime := req.till;
      endif
      if (req.rtime = 0) then
              rtime := infinity;
      else
              rtime := req.rtime;
      endif
      if (req.kdc-options.RENEWABLE is set) then
              set new_tkt.flags.RENEWABLE;
              new_tkt.renew-till := min(rtime,
              new_tkt.starttime+client.max_rlife,
              new_tkt.starttime+server.max_rlife,
              new_tkt.starttime+max_rlife_for_realm);
      else
              omit new_tkt.renew-till; /* only present if RENEWABLE */
      endif
      if (req.addresses) then
              new_tkt.caddr := req.addresses;
      else
              omit new_tkt.caddr;
      endif
      new_tkt.authorization_data := empty_authorization_data();
      encode to-be-encrypted part of ticket into OCTET STRING;
      new_tkt.enc-part := encrypt OCTET STRING
          using etype_for_key(server.key), server.key, server.p_kvno;
      /* Start processing the response */
      resp.pvno := 5;
      resp.msg-type := KRB_AS_REP;
      resp.cname := req.cname;
      resp.crealm := req.realm;
      resp.ticket := new_tkt;
      resp.key := new_tkt.session;
      resp.last-req := fetch_last_request_info(client);
      resp.nonce := req.nonce;
      resp.key-expiration := client.expiration;

Kohl & Neuman [Page 94] RFC 1510 Kerberos September 1993

      resp.flags := new_tkt.flags;
      resp.authtime := new_tkt.authtime;
      resp.starttime := new_tkt.starttime;
      resp.endtime := new_tkt.endtime;
      if (new_tkt.flags.RENEWABLE) then
              resp.renew-till := new_tkt.renew-till;
      endif
      resp.realm := new_tkt.realm;
      resp.sname := new_tkt.sname;
      resp.caddr := new_tkt.caddr;
      encode body of reply into OCTET STRING;
      resp.enc-part := encrypt OCTET STRING
                       using use_etype, client.key, client.p_kvno;
      send(resp);

A.3. KRB_AS_REP verification

      decode response into resp;
      if (resp.msg-type = KRB_ERROR) then
              if(error = KDC_ERR_PREAUTH_REQUIRED(PA_ENC_TIMESTAMP))
                      then set pa_enc_timestamp_required;
                      goto KRB_AS_REQ;
              endif
              process_error(resp);
              return;
      endif
      /* On error, discard the response, and zero the session key */
      /* from the response immediately */
      key = get_decryption_key(resp.enc-part.kvno, resp.enc-part.etype,
                               resp.padata);
      unencrypted part of resp := decode of decrypt of resp.enc-part
                              using resp.enc-part.etype and key;
      zero(key);
      if (common_as_rep_tgs_rep_checks fail) then
              destroy resp.key;
              return error;
      endif
      if near(resp.princ_exp) then

Kohl & Neuman [Page 95] RFC 1510 Kerberos September 1993

              print(warning message);
      endif
      save_for_later(ticket,session,client,server,times,flags);

A.4. KRB_AS_REP and KRB_TGS_REP common checks

      if (decryption_error() or
          (req.cname != resp.cname) or
          (req.realm != resp.crealm) or
          (req.sname != resp.sname) or
          (req.realm != resp.realm) or
          (req.nonce != resp.nonce) or
          (req.addresses != resp.caddr)) then
              destroy resp.key;
              return KRB_AP_ERR_MODIFIED;
      endif
      /* make sure no flags are set that shouldn't be, and that  */
      /* all that should be are set                              */
      if (!check_flags_for_compatability(req.kdc-options,resp.flags))
              then destroy resp.key;
              return KRB_AP_ERR_MODIFIED;
      endif
      if ((req.from = 0) and
          (resp.starttime is not within allowable skew)) then
              destroy resp.key;
              return KRB_AP_ERR_SKEW;
      endif
      if ((req.from != 0) and (req.from != resp.starttime)) then
              destroy resp.key;
              return KRB_AP_ERR_MODIFIED;
      endif
      if ((req.till != 0) and (resp.endtime > req.till)) then
              destroy resp.key;
              return KRB_AP_ERR_MODIFIED;
      endif
      if ((req.kdc-options.RENEWABLE is set) and
          (req.rtime != 0) and (resp.renew-till > req.rtime)) then
              destroy resp.key;
              return KRB_AP_ERR_MODIFIED;
      endif
      if ((req.kdc-options.RENEWABLE-OK is set) and
          (resp.flags.RENEWABLE) and
          (req.till != 0) and
          (resp.renew-till > req.till)) then
              destroy resp.key;
              return KRB_AP_ERR_MODIFIED;

Kohl & Neuman [Page 96] RFC 1510 Kerberos September 1993

      endif

A.5. KRB_TGS_REQ generation

      /* Note that make_application_request might have to     */
      /* recursivly call this routine to get the appropriate  */
      /* ticket-granting ticket                               */
      request.pvno := protocol version; /* pvno = 5 */
      request.msg-type := message type; /* type = KRB_TGS_REQ */
      body.kdc-options := users's preferences;
      /* If the TGT is not for the realm of the end-server  */
      /* then the sname will be for a TGT for the end-realm */
      /* and the realm of the requested ticket (body.realm) */
      /* will be that of the TGS to which the TGT we are    */
      /* sending applies                                    */
      body.sname := service's name;
      body.realm := service's realm;
      if (body.kdc-options.POSTDATED is set) then
              body.from := requested starting time;
      else
              omit body.from;
      endif
      body.till := requested end time;
      if (body.kdc-options.RENEWABLE is set) then
              body.rtime := requested final renewal time;
      endif
      body.nonce := random_nonce();
      body.etype := requested etypes;
      if (user supplied addresses) then
              body.addresses := user's addresses;
      else
              omit body.addresses;
      endif
      body.enc-authorization-data := user-supplied data;
      if (body.kdc-options.ENC-TKT-IN-SKEY) then
              body.additional-tickets_ticket := second TGT;
      endif
      request.req-body := body;
      check := generate_checksum (req.body,checksumtype);
      request.padata[0].padata-type := PA-TGS-REQ;
      request.padata[0].padata-value := create a KRB_AP_REQ using
                                    the TGT and checksum

Kohl & Neuman [Page 97] RFC 1510 Kerberos September 1993

      /* add in any other padata as required/supplied */
      kerberos := lookup(name of local kerberose server (or servers));
      send(packet,kerberos);
      wait(for response);
      if (timed_out) then
              retry or use alternate server;
      endif

A.6. KRB_TGS_REQ verification and KRB_TGS_REP generation

      /* note that reading the application request requires first
      determining the server for which a ticket was issued, and
      choosing the correct key for decryption.  The name of the
      server appears in the plaintext part of the ticket. */
      if (no KRB_AP_REQ in req.padata) then
              error_out(KDC_ERR_PADATA_TYPE_NOSUPP);
      endif
      verify KRB_AP_REQ in req.padata;
      /* Note that the realm in which the Kerberos server is
      operating is determined by the instance from the
      ticket-granting ticket.  The realm in the ticket-granting
      ticket is the realm under which the ticket granting ticket was
      issued.  It is possible for a single Kerberos server to
      support more than one realm. */
      auth_hdr := KRB_AP_REQ;
      tgt := auth_hdr.ticket;
      if (tgt.sname is not a TGT for local realm and is not
              req.sname) then error_out(KRB_AP_ERR_NOT_US);
      realm := realm_tgt_is_for(tgt);
      decode remainder of request;
      if (auth_hdr.authenticator.cksum is missing) then
              error_out(KRB_AP_ERR_INAPP_CKSUM);
      endif
      if (auth_hdr.authenticator.cksum type is not supported) then
              error_out(KDC_ERR_SUMTYPE_NOSUPP);
      endif
      if (auth_hdr.authenticator.cksum is not both collision-proof
          and keyed)  then
              error_out(KRB_AP_ERR_INAPP_CKSUM);
      endif

Kohl & Neuman [Page 98] RFC 1510 Kerberos September 1993

      set computed_checksum := checksum(req);
      if (computed_checksum != auth_hdr.authenticatory.cksum) then
              error_out(KRB_AP_ERR_MODIFIED);
      endif
      server := lookup(req.sname,realm);
      if (!server) then
              if (is_foreign_tgt_name(server)) then
                      server := best_intermediate_tgs(server);
              else
                      /* no server in Database */
                      error_out(KDC_ERR_S_PRINCIPAL_UNKNOWN);
              endif
      endif
      session := generate_random_session_key();
      use_etype := first supported etype in req.etypes;
      if (no support for req.etypes) then
              error_out(KDC_ERR_ETYPE_NOSUPP);
      endif
      new_tkt.vno := ticket version; /* = 5 */
      new_tkt.sname := req.sname;
      new_tkt.srealm := realm;
      reset all flags in new_tkt.flags;
      /* It should be noted that local policy may affect the  */
      /* processing of any of these flags.  For example, some */
      /* realms may refuse to issue renewable tickets         */
      new_tkt.caddr := tgt.caddr;
      resp.caddr := NULL; /* We only include this if they change */
      if (req.kdc-options.FORWARDABLE is set) then
              if (tgt.flags.FORWARDABLE is reset) then
                      error_out(KDC_ERR_BADOPTION);
              endif
              set new_tkt.flags.FORWARDABLE;
      endif
      if (req.kdc-options.FORWARDED is set) then
              if (tgt.flags.FORWARDABLE is reset) then
                      error_out(KDC_ERR_BADOPTION);
              endif
              set new_tkt.flags.FORWARDED;
              new_tkt.caddr := req.addresses;

Kohl & Neuman [Page 99] RFC 1510 Kerberos September 1993

              resp.caddr := req.addresses;
      endif
      if (tgt.flags.FORWARDED is set) then
              set new_tkt.flags.FORWARDED;
      endif
      if (req.kdc-options.PROXIABLE is set) then
              if (tgt.flags.PROXIABLE is reset)
                      error_out(KDC_ERR_BADOPTION);
              endif
              set new_tkt.flags.PROXIABLE;
      endif
      if (req.kdc-options.PROXY is set) then
              if (tgt.flags.PROXIABLE is reset) then
                      error_out(KDC_ERR_BADOPTION);
              endif
              set new_tkt.flags.PROXY;
              new_tkt.caddr := req.addresses;
              resp.caddr := req.addresses;
      endif
      if (req.kdc-options.POSTDATE is set) then
              if (tgt.flags.POSTDATE is reset)
                      error_out(KDC_ERR_BADOPTION);
              endif
              set new_tkt.flags.POSTDATE;
      endif
      if (req.kdc-options.POSTDATED is set) then
              if (tgt.flags.POSTDATE is reset) then
                      error_out(KDC_ERR_BADOPTION);
              endif
              set new_tkt.flags.POSTDATED;
              set new_tkt.flags.INVALID;
              if (against_postdate_policy(req.from)) then
                      error_out(KDC_ERR_POLICY);
              endif
              new_tkt.starttime := req.from;
      endif
      if (req.kdc-options.VALIDATE is set) then
              if (tgt.flags.INVALID is reset) then
                      error_out(KDC_ERR_POLICY);
              endif
              if (tgt.starttime > kdc_time) then
                      error_out(KRB_AP_ERR_NYV);
              endif
              if (check_hot_list(tgt)) then

Kohl & Neuman [Page 100] RFC 1510 Kerberos September 1993

                      error_out(KRB_AP_ERR_REPEAT);
              endif
              tkt := tgt;
              reset new_tkt.flags.INVALID;
      endif
      if (req.kdc-options.(any flag except ENC-TKT-IN-SKEY, RENEW,
                           and those already processed) is set) then
              error_out(KDC_ERR_BADOPTION);
      endif
      new_tkt.authtime := tgt.authtime;
      if (req.kdc-options.RENEW is set) then
        /* Note that if the endtime has already passed, the ticket */
        /* would have been rejected in the initial authentication  */
        /* stage, so there is no need to check again here          */
              if (tgt.flags.RENEWABLE is reset) then
                      error_out(KDC_ERR_BADOPTION);
              endif
              if (tgt.renew-till >= kdc_time) then
                      error_out(KRB_AP_ERR_TKT_EXPIRED);
              endif
              tkt := tgt;
              new_tkt.starttime := kdc_time;
              old_life := tgt.endttime - tgt.starttime;
              new_tkt.endtime := min(tgt.renew-till,
                                     new_tkt.starttime + old_life);
      else
              new_tkt.starttime := kdc_time;
              if (req.till = 0) then
                      till := infinity;
              else
                      till := req.till;
              endif
              new_tkt.endtime := min(till,
                                 new_tkt.starttime+client.max_life,
                                 new_tkt.starttime+server.max_life,
                                 new_tkt.starttime+max_life_for_realm,
                                 tgt.endtime);
              if ((req.kdc-options.RENEWABLE-OK is set) and
                  (new_tkt.endtime < req.till) and
                  (tgt.flags.RENEWABLE is set) then
                      /* we set the RENEWABLE option for later  */
                      /* processing                             */
                      set req.kdc-options.RENEWABLE;
                      req.rtime := min(req.till, tgt.renew-till);

Kohl & Neuman [Page 101] RFC 1510 Kerberos September 1993

              endif
      endif
      if (req.rtime = 0) then
              rtime := infinity;
      else
              rtime := req.rtime;
      endif
      if ((req.kdc-options.RENEWABLE is set) and
          (tgt.flags.RENEWABLE is set)) then
              set new_tkt.flags.RENEWABLE;
              new_tkt.renew-till := min(rtime,
              new_tkt.starttime+client.max_rlife,
              new_tkt.starttime+server.max_rlife,
              new_tkt.starttime+max_rlife_for_realm,
              tgt.renew-till);
      else
              new_tkt.renew-till := OMIT;
                            /* leave the renew-till field out */
      endif
      if (req.enc-authorization-data is present) then
              decrypt req.enc-authorization-data
                      into    decrypted_authorization_data
                      using auth_hdr.authenticator.subkey;
              if (decrypt_error()) then
                      error_out(KRB_AP_ERR_BAD_INTEGRITY);
              endif
      endif
      new_tkt.authorization_data :=
      req.auth_hdr.ticket.authorization_data +
                               decrypted_authorization_data;
      new_tkt.key := session;
      new_tkt.crealm := tgt.crealm;
      new_tkt.cname := req.auth_hdr.ticket.cname;
      if (realm_tgt_is_for(tgt) := tgt.realm) then
              /* tgt issued by local realm */
              new_tkt.transited := tgt.transited;
      else
              /* was issued for this realm by some other realm */
              if (tgt.transited.tr-type not supported) then
                      error_out(KDC_ERR_TRTYPE_NOSUPP);
              endif
              new_tkt.transited
                 := compress_transited(tgt.transited + tgt.realm)
      endif

Kohl & Neuman [Page 102] RFC 1510 Kerberos September 1993

      encode encrypted part of new_tkt into OCTET STRING;
      if (req.kdc-options.ENC-TKT-IN-SKEY is set) then
              if (server not specified) then
                      server = req.second_ticket.client;
              endif
              if ((req.second_ticket is not a TGT) or
                  (req.second_ticket.client != server)) then
                      error_out(KDC_ERR_POLICY);
              endif
              new_tkt.enc-part := encrypt OCTET STRING using
                      using etype_for_key(second-ticket.key),
                                                    second-ticket.key;
      else
              new_tkt.enc-part := encrypt OCTET STRING
                      using etype_for_key(server.key), server.key,
                                                    server.p_kvno;
      endif
      resp.pvno := 5;
      resp.msg-type := KRB_TGS_REP;
      resp.crealm := tgt.crealm;
      resp.cname := tgt.cname;
      resp.ticket := new_tkt;
      resp.key := session;
      resp.nonce := req.nonce;
      resp.last-req := fetch_last_request_info(client);
      resp.flags := new_tkt.flags;
      resp.authtime := new_tkt.authtime;
      resp.starttime := new_tkt.starttime;
      resp.endtime := new_tkt.endtime;
      omit resp.key-expiration;
      resp.sname := new_tkt.sname;
      resp.realm := new_tkt.realm;
      if (new_tkt.flags.RENEWABLE) then
              resp.renew-till := new_tkt.renew-till;
      endif
      encode body of reply into OCTET STRING;
      if (req.padata.authenticator.subkey)
              resp.enc-part := encrypt OCTET STRING using use_etype,

Kohl & Neuman [Page 103] RFC 1510 Kerberos September 1993

                      req.padata.authenticator.subkey;
      else resp.enc-part := encrypt OCTET STRING
                            using use_etype, tgt.key;
      send(resp);

A.7. KRB_TGS_REP verification

      decode response into resp;
      if (resp.msg-type = KRB_ERROR) then
              process_error(resp);
              return;
      endif
      /* On error, discard the response, and zero the session key from
      the response immediately */
      if (req.padata.authenticator.subkey)
              unencrypted part of resp :=
                      decode of decrypt of resp.enc-part
                      using resp.enc-part.etype and subkey;
      else unencrypted part of resp :=
                      decode of decrypt of resp.enc-part
                      using resp.enc-part.etype and tgt's session key;
      if (common_as_rep_tgs_rep_checks fail) then
              destroy resp.key;
              return error;
      endif
      check authorization_data as necessary;
      save_for_later(ticket,session,client,server,times,flags);

A.8. Authenticator generation

      body.authenticator-vno := authenticator vno; /* = 5 */
      body.cname, body.crealm := client name;
      if (supplying checksum) then
              body.cksum := checksum;
      endif
      get system_time;
      body.ctime, body.cusec := system_time;
      if (selecting sub-session key) then
              select sub-session key;
              body.subkey := sub-session key;
      endif
      if (using sequence numbers) then
              select initial sequence number;
              body.seq-number := initial sequence;
      endif

Kohl & Neuman [Page 104] RFC 1510 Kerberos September 1993

A.9. KRB_AP_REQ generation

      obtain ticket and session_key from cache;
      packet.pvno := protocol version; /* 5 */
      packet.msg-type := message type; /* KRB_AP_REQ */
      if (desired(MUTUAL_AUTHENTICATION)) then
              set packet.ap-options.MUTUAL-REQUIRED;
      else
              reset packet.ap-options.MUTUAL-REQUIRED;
      endif
      if (using session key for ticket) then
              set packet.ap-options.USE-SESSION-KEY;
      else
              reset packet.ap-options.USE-SESSION-KEY;
      endif
      packet.ticket := ticket; /* ticket */
      generate authenticator;
      encode authenticator into OCTET STRING;
      encrypt OCTET STRING into packet.authenticator
                           using session_key;

A.10. KRB_AP_REQ verification

      receive packet;
      if (packet.pvno != 5) then
              either process using other protocol spec
              or error_out(KRB_AP_ERR_BADVERSION);
      endif
      if (packet.msg-type != KRB_AP_REQ) then
              error_out(KRB_AP_ERR_MSG_TYPE);
      endif
      if (packet.ticket.tkt_vno != 5) then
              either process using other protocol spec
              or error_out(KRB_AP_ERR_BADVERSION);
      endif
      if (packet.ap_options.USE-SESSION-KEY is set) then
              retrieve session key from ticket-granting ticket for
               packet.ticket.{sname,srealm,enc-part.etype};
      else
         retrieve service key for
         packet.ticket.{sname,srealm,enc-part.etype,enc-part.skvno};
      endif
      if (no_key_available) then
              if (cannot_find_specified_skvno) then
                      error_out(KRB_AP_ERR_BADKEYVER);
              else
                      error_out(KRB_AP_ERR_NOKEY);
              endif

Kohl & Neuman [Page 105] RFC 1510 Kerberos September 1993

      endif
      decrypt packet.ticket.enc-part into decr_ticket
                                     using retrieved key;
      if (decryption_error()) then
              error_out(KRB_AP_ERR_BAD_INTEGRITY);
      endif
      decrypt packet.authenticator into decr_authenticator
              using decr_ticket.key;
      if (decryption_error()) then
              error_out(KRB_AP_ERR_BAD_INTEGRITY);
      endif
      if (decr_authenticator.{cname,crealm} !=
          decr_ticket.{cname,crealm}) then
              error_out(KRB_AP_ERR_BADMATCH);
      endif
      if (decr_ticket.caddr is present) then
              if (sender_address(packet) is not in decr_ticket.caddr)
                      then error_out(KRB_AP_ERR_BADADDR);
              endif
      elseif (application requires addresses) then
              error_out(KRB_AP_ERR_BADADDR);
      endif
      if (not in_clock_skew(decr_authenticator.ctime,
                            decr_authenticator.cusec)) then
              error_out(KRB_AP_ERR_SKEW);
      endif
      if (repeated(decr_authenticator.{ctime,cusec,cname,crealm}))
              then error_out(KRB_AP_ERR_REPEAT);
      endif
      save_identifier(decr_authenticator.{ctime,cusec,cname,crealm});
      get system_time;
      if ((decr_ticket.starttime-system_time > CLOCK_SKEW) or
          (decr_ticket.flags.INVALID is set)) then
              /* it hasn't yet become valid */
              error_out(KRB_AP_ERR_TKT_NYV);
      endif
      if (system_time-decr_ticket.endtime > CLOCK_SKEW) then
              error_out(KRB_AP_ERR_TKT_EXPIRED);
      endif
      /* caller must check decr_ticket.flags for any pertinent */
      /* details */
      return(OK, decr_ticket, packet.ap_options.MUTUAL-REQUIRED);

A.11. KRB_AP_REP generation

      packet.pvno := protocol version; /* 5 */
      packet.msg-type := message type; /* KRB_AP_REP */
      body.ctime := packet.ctime;
      body.cusec := packet.cusec;

Kohl & Neuman [Page 106] RFC 1510 Kerberos September 1993

      if (selecting sub-session key) then
              select sub-session key;
              body.subkey := sub-session key;
      endif
      if (using sequence numbers) then
              select initial sequence number;
              body.seq-number := initial sequence;
      endif
      encode body into OCTET STRING;
      select encryption type;
      encrypt OCTET STRING into packet.enc-part;

A.12. KRB_AP_REP verification

      receive packet;
      if (packet.pvno != 5) then
              either process using other protocol spec
              or error_out(KRB_AP_ERR_BADVERSION);
      endif
      if (packet.msg-type != KRB_AP_REP) then
              error_out(KRB_AP_ERR_MSG_TYPE);
      endif
      cleartext := decrypt(packet.enc-part)
                   using ticket's session key;
      if (decryption_error()) then
              error_out(KRB_AP_ERR_BAD_INTEGRITY);
      endif
      if (cleartext.ctime != authenticator.ctime) then
              error_out(KRB_AP_ERR_MUT_FAIL);
      endif
      if (cleartext.cusec != authenticator.cusec) then
              error_out(KRB_AP_ERR_MUT_FAIL);
      endif
      if (cleartext.subkey is present) then
              save cleartext.subkey for future use;
      endif
      if (cleartext.seq-number is present) then
              save cleartext.seq-number for future verifications;
      endif
      return(AUTHENTICATION_SUCCEEDED);

A.13. KRB_SAFE generation

      collect user data in buffer;
      /* assemble packet: */
      packet.pvno := protocol version; /* 5 */
      packet.msg-type := message type; /* KRB_SAFE */

Kohl & Neuman [Page 107] RFC 1510 Kerberos September 1993

      body.user-data := buffer; /* DATA */
      if (using timestamp) then
              get system_time;
              body.timestamp, body.usec := system_time;
      endif
      if (using sequence numbers) then
              body.seq-number := sequence number;
      endif
      body.s-address := sender host addresses;
      if (only one recipient) then
              body.r-address := recipient host address;
      endif
      checksum.cksumtype := checksum type;
      compute checksum over body;
      checksum.checksum := checksum value; /* checksum.checksum */
      packet.cksum := checksum;
      packet.safe-body := body;

A.14. KRB_SAFE verification

      receive packet;
      if (packet.pvno != 5) then
              either process using other protocol spec
              or error_out(KRB_AP_ERR_BADVERSION);
      endif
      if (packet.msg-type != KRB_SAFE) then
              error_out(KRB_AP_ERR_MSG_TYPE);
      endif
      if (packet.checksum.cksumtype is not both collision-proof
                                           and keyed) then
              error_out(KRB_AP_ERR_INAPP_CKSUM);
      endif
      if (safe_priv_common_checks_ok(packet)) then
              set computed_checksum := checksum(packet.body);
              if (computed_checksum != packet.checksum) then
                      error_out(KRB_AP_ERR_MODIFIED);
              endif
              return (packet, PACKET_IS_GENUINE);
      else
              return common_checks_error;
      endif

A.15. KRB_SAFE and KRB_PRIV common checks

      if (packet.s-address != O/S_sender(packet)) then
          /* O/S report of sender not who claims to have sent it */
          error_out(KRB_AP_ERR_BADADDR);
      endif
      if ((packet.r-address is present) and
          (packet.r-address != local_host_address)) then

Kohl & Neuman [Page 108] RFC 1510 Kerberos September 1993

              /* was not sent to proper place */
              error_out(KRB_AP_ERR_BADADDR);
      endif
      if (((packet.timestamp is present) and
           (not in_clock_skew(packet.timestamp,packet.usec))) or
          (packet.timestamp is not present and timestamp expected))
              then error_out(KRB_AP_ERR_SKEW);
      endif
      if (repeated(packet.timestamp,packet.usec,packet.s-address))
              then error_out(KRB_AP_ERR_REPEAT);
      endif
      if (((packet.seq-number is present) and
           ((not in_sequence(packet.seq-number)))) or
          (packet.seq-number is not present and sequence expected))
              then error_out(KRB_AP_ERR_BADORDER);
      endif
      if (packet.timestamp not present and
          packet.seq-number not present) then
              error_out(KRB_AP_ERR_MODIFIED);
      endif
      save_identifier(packet.{timestamp,usec,s-address},
                      sender_principal(packet));
      return PACKET_IS_OK;

A.16. KRB_PRIV generation

      collect user data in buffer;
      /* assemble packet: */
      packet.pvno := protocol version; /* 5 */
      packet.msg-type := message type; /* KRB_PRIV */
      packet.enc-part.etype := encryption type;
      body.user-data := buffer;
      if (using timestamp) then
              get system_time;
              body.timestamp, body.usec := system_time;
      endif
      if (using sequence numbers) then
              body.seq-number := sequence number;
      endif
      body.s-address := sender host addresses;
      if (only one recipient) then
              body.r-address := recipient host address;
      endif

Kohl & Neuman [Page 109] RFC 1510 Kerberos September 1993

      encode body into OCTET STRING;
      select encryption type;
      encrypt OCTET STRING into packet.enc-part.cipher;

A.17. KRB_PRIV verification

      receive packet;
      if (packet.pvno != 5) then
              either process using other protocol spec
              or error_out(KRB_AP_ERR_BADVERSION);
      endif
      if (packet.msg-type != KRB_PRIV) then
              error_out(KRB_AP_ERR_MSG_TYPE);
      endif
      cleartext := decrypt(packet.enc-part) using negotiated key;
      if (decryption_error()) then
              error_out(KRB_AP_ERR_BAD_INTEGRITY);
      endif
      if (safe_priv_common_checks_ok(cleartext)) then
          return(cleartext.DATA, PACKET_IS_GENUINE_AND_UNMODIFIED);
      else
              return common_checks_error;
      endif

A.18. KRB_CRED generation

      invoke KRB_TGS; /* obtain tickets to be provided to peer */
      /* assemble packet: */
      packet.pvno := protocol version; /* 5 */
      packet.msg-type := message type; /* KRB_CRED */
      for (tickets[n] in tickets to be forwarded) do
              packet.tickets[n] = tickets[n].ticket;
      done
      packet.enc-part.etype := encryption type;
      for (ticket[n] in tickets to be forwarded) do
              body.ticket-info[n].key = tickets[n].session;
              body.ticket-info[n].prealm = tickets[n].crealm;
              body.ticket-info[n].pname = tickets[n].cname;
              body.ticket-info[n].flags = tickets[n].flags;
              body.ticket-info[n].authtime = tickets[n].authtime;
              body.ticket-info[n].starttime = tickets[n].starttime;
              body.ticket-info[n].endtime = tickets[n].endtime;
              body.ticket-info[n].renew-till = tickets[n].renew-till;

Kohl & Neuman [Page 110] RFC 1510 Kerberos September 1993

              body.ticket-info[n].srealm = tickets[n].srealm;
              body.ticket-info[n].sname = tickets[n].sname;
              body.ticket-info[n].caddr = tickets[n].caddr;
      done
      get system_time;
      body.timestamp, body.usec := system_time;
      if (using nonce) then
              body.nonce := nonce;
      endif
      if (using s-address) then
              body.s-address := sender host addresses;
      endif
      if (limited recipients) then
              body.r-address := recipient host address;
      endif
      encode body into OCTET STRING;
      select encryption type;
      encrypt OCTET STRING into packet.enc-part.cipher
      using negotiated encryption key;

A.19. KRB_CRED verification

      receive packet;
      if (packet.pvno != 5) then
              either process using other protocol spec
              or error_out(KRB_AP_ERR_BADVERSION);
      endif
      if (packet.msg-type != KRB_CRED) then
              error_out(KRB_AP_ERR_MSG_TYPE);
      endif
      cleartext := decrypt(packet.enc-part) using negotiated key;
      if (decryption_error()) then
              error_out(KRB_AP_ERR_BAD_INTEGRITY);
      endif
      if ((packet.r-address is present or required) and
         (packet.s-address != O/S_sender(packet)) then
          /* O/S report of sender not who claims to have sent it */
          error_out(KRB_AP_ERR_BADADDR);
      endif
      if ((packet.r-address is present) and
          (packet.r-address != local_host_address)) then
              /* was not sent to proper place */
              error_out(KRB_AP_ERR_BADADDR);

Kohl & Neuman [Page 111] RFC 1510 Kerberos September 1993

      endif
      if (not in_clock_skew(packet.timestamp,packet.usec)) then
              error_out(KRB_AP_ERR_SKEW);
      endif
      if (repeated(packet.timestamp,packet.usec,packet.s-address))
              then error_out(KRB_AP_ERR_REPEAT);
      endif
      if (packet.nonce is required or present) and
         (packet.nonce != expected-nonce) then
              error_out(KRB_AP_ERR_MODIFIED);
      endif
      for (ticket[n] in tickets that were forwarded) do
              save_for_later(ticket[n],key[n],principal[n],
                             server[n],times[n],flags[n]);
      return

A.20. KRB_ERROR generation

      /* assemble packet: */
      packet.pvno := protocol version; /* 5 */
      packet.msg-type := message type; /* KRB_ERROR */
      get system_time;
      packet.stime, packet.susec := system_time;
      packet.realm, packet.sname := server name;
      if (client time available) then
              packet.ctime, packet.cusec := client_time;
      endif
      packet.error-code := error code;
      if (client name available) then
              packet.cname, packet.crealm := client name;
      endif
      if (error text available) then
              packet.e-text := error text;
      endif
      if (error data available) then
              packet.e-data := error data;
      endif

Kohl & Neuman [Page 112]

/data/webs/external/dokuwiki/data/pages/rfc/rfc1510.txt · Last modified: 1993/09/09 01:35 by 127.0.0.1

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