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rfc:rfc6112

Internet Engineering Task Force (IETF) L. Zhu Request for Comments: 6112 P. Leach Updates: 4120, 4121, 4556 Microsoft Corporation Category: Standards Track S. Hartman ISSN: 2070-1721 Painless Security

                                                            April 2011
                   Anonymity Support for Kerberos

Abstract

 This document defines extensions to the Kerberos protocol to allow a
 Kerberos client to securely communicate with a Kerberos application
 service without revealing its identity, or without revealing more
 than its Kerberos realm.  It also defines extensions that allow a
 Kerberos client to obtain anonymous credentials without revealing its
 identity to the Kerberos Key Distribution Center (KDC).  This
 document updates RFCs 4120, 4121, and 4556.

Status of This Memo

 This is an Internet Standards Track document.
 This document is a product of the Internet Engineering Task Force
 (IETF).  It represents the consensus of the IETF community.  It has
 received public review and has been approved for publication by the
 Internet Engineering Steering Group (IESG).  Further information on
 Internet Standards is available in Section 2 of RFC 5741.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 http://www.rfc-editor.org/info/rfc6112.

Copyright Notice

 Copyright (c) 2011 IETF Trust and the persons identified as the
 document authors.  All rights reserved.
 This document is subject to BCP 78 and the IETF Trust's Legal
 Provisions Relating to IETF Documents
 (http://trustee.ietf.org/license-info) in effect on the date of
 publication of this document.  Please review these documents
 carefully, as they describe your rights and restrictions with respect
 to this document.  Code Components extracted from this document must
 include Simplified BSD License text as described in Section 4.e of
 the Trust Legal Provisions and are provided without warranty as
 described in the Simplified BSD License.

Zhu, et al. Standards Track [Page 1] RFC 6112 Kerberos Anonymity Support April 2011

 This document may contain material from IETF Documents or IETF
 Contributions published or made publicly available before November
 10, 2008.  The person(s) controlling the copyright in some of this
 material may not have granted the IETF Trust the right to allow
 modifications of such material outside the IETF Standards Process.
 Without obtaining an adequate license from the person(s) controlling
 the copyright in such materials, this document may not be modified
 outside the IETF Standards Process, and derivative works of it may
 not be created outside the IETF Standards Process, except to format
 it for publication as an RFC or to translate it into languages other
 than English.

Table of Contents

 1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
 2.  Conventions Used in This Document  . . . . . . . . . . . . . .  3
 3.  Definitions  . . . . . . . . . . . . . . . . . . . . . . . . .  3
 4.  Protocol Description . . . . . . . . . . . . . . . . . . . . .  5
   4.1.  Anonymity Support in AS Exchange . . . . . . . . . . . . .  5
     4.1.1.  Anonymous PKINIT . . . . . . . . . . . . . . . . . . .  6
   4.2.  Anonymity Support in TGS Exchange  . . . . . . . . . . . .  7
   4.3.  Subsequent Exchanges and Protocol Actions Common to AS
         and TGS for Anonymity Support  . . . . . . . . . . . . . .  9
 5.  Interoperability Requirements  . . . . . . . . . . . . . . . . 10
 6.  GSS-API Implementation Notes . . . . . . . . . . . . . . . . . 10
 7.  PKINIT Client Contribution to the Ticket Session Key . . . . . 11
   7.1.  Combining Two Protocol Keys  . . . . . . . . . . . . . . . 12
 8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 13
 9.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 14
 10. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 15
 11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 15
   11.1. Normative References . . . . . . . . . . . . . . . . . . . 15
   11.2. Informative References . . . . . . . . . . . . . . . . . . 16

Zhu, et al. Standards Track [Page 2] RFC 6112 Kerberos Anonymity Support April 2011

1. Introduction

 In certain situations, the Kerberos [RFC4120] client may wish to
 authenticate a server and/or protect communications without revealing
 the client's own identity.  For example, consider an application that
 provides read access to a research database and that permits queries
 by arbitrary requesters.  A client of such a service might wish to
 authenticate the service, to establish trust in the information
 received from it, but might not wish to disclose the client's
 identity to the service for privacy reasons.
 Extensions to Kerberos are specified in this document by which a
 client can authenticate the Key Distribution Center (KDC) and request
 an anonymous ticket.  The client can use the anonymous ticket to
 authenticate the server and protect subsequent client-server
 communications.
 By using the extensions defined in this specification, the client can
 request an anonymous ticket where the client may reveal the client's
 identity to the client's own KDC, or the client can hide the client's
 identity completely by using anonymous Public Key Cryptography for
 Initial Authentication in Kerberos (PKINIT) as defined in
 Section 4.1.  Using the returned anonymous ticket, the client remains
 anonymous in subsequent Kerberos exchanges thereafter to KDCs on the
 cross-realm authentication path and to the server with which it
 communicates.
 In this specification, the client realm in the anonymous ticket is
 the anonymous realm name when anonymous PKINIT is used to obtain the
 ticket.  The client realm is the client's real realm name if the
 client is authenticated using the client's long-term keys.  Note that
 the membership of a realm can imply a member of the community
 represented by the realm.
 The interaction with Generic Security Service Application Program
 Interface (GSS-API) is described after the protocol description.

2. Conventions Used in This Document

 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
 document are to be interpreted as described in [RFC2119].

3. Definitions

 The anonymous Kerberos realm name is defined as a well-known realm
 name based on [RFC6111], and the value of this well-known realm name
 is the literal "WELLKNOWN:ANONYMOUS".

Zhu, et al. Standards Track [Page 3] RFC 6112 Kerberos Anonymity Support April 2011

 The anonymous Kerberos principal name is defined as a well-known
 Kerberos principal name based on [RFC6111].  The value of the name-
 type field is KRB_NT_WELLKNOWN [RFC6111], and the value of the name-
 string field is a sequence of two KerberosString components:
 "WELLKNOWN", "ANONYMOUS".
 The anonymous ticket flag is defined as bit 16 (with the first bit
 being bit 0) in the TicketFlags:
         TicketFlags     ::= KerberosFlags
           -- anonymous(16)
           -- TicketFlags and KerberosFlags are defined in [RFC4120]
 This is a new ticket flag that is used to indicate that a ticket is
 an anonymous one.
 An anonymous ticket is a ticket that has all of the following
 properties:
 o  The cname field contains the anonymous Kerberos principal name.
 o  The crealm field contains the client's realm name or the anonymous
    realm name.
 o  The anonymous ticket contains no information that can reveal the
    client's identity.  However, the ticket may contain the client
    realm, intermediate realms on the client's authentication path,
    and authorization data that may provide information related to the
    client's identity.  For example, an anonymous principal that is
    identifiable only within a particular group of users can be
    implemented using authorization data and such authorization data,
    if included in the anonymous ticket, would disclose the client's
    membership of that group.
 o  The anonymous ticket flag is set.
 The anonymous KDC option is defined as bit 16 (with the first bit
 being bit 0) in the KDCOptions:
         KDCOptions      ::= KerberosFlags
           -- anonymous(16)
           -- KDCOptions and KerberosFlags are defined in [RFC4120]

Zhu, et al. Standards Track [Page 4] RFC 6112 Kerberos Anonymity Support April 2011

 As described in Section 4, the anonymous KDC option is set to request
 an anonymous ticket in an Authentication Service (AS) request or a
 Ticket Granting Service (TGS) request.

4. Protocol Description

 In order to request an anonymous ticket, the client sets the
 anonymous KDC option in an AS request or a TGS request.
 The rest of this section is organized as follows: it first describes
 protocol actions specific to AS exchanges, then it describes those of
 TGS exchanges.  These are then followed by the description of
 protocol actions common to both AS and TGS and those in subsequent
 exchanges.

4.1. Anonymity Support in AS Exchange

 The client requests an anonymous ticket by setting the anonymous KDC
 option in an AS exchange.
 The Kerberos client can use the client's long-term keys, the client's
 X.509 certificates [RFC4556], or any other pre-authentication data,
 to authenticate to the KDC and requests an anonymous ticket in an AS
 exchange where the client's identity is known to the KDC.
 If the client in the AS request is anonymous, the anonymous KDC
 option MUST be set in the request.  Otherwise, the KDC MUST return a
 KRB-ERROR message with the code KDC_ERR_BADOPTION.
 If the client is anonymous and the KDC does not have a key to encrypt
 the reply (this can happen when, for example, the KDC does not
 support PKINIT [RFC4556]), the KDC MUST return an error message with
 the code KDC_ERR_NULL_KEY [RFC4120].
 When policy allows, the KDC issues an anonymous ticket.  If the
 client name in the request is the anonymous principal, the client
 realm (crealm) in the reply is the anonymous realm, otherwise, the
 client realm is the realm of the AS.  According to [RFC4120], the
 client name and the client realm in the EncTicketPart of the reply
 MUST match with the corresponding client name and the client realm of
 the KDC reply; the client MUST use the client name and the client
 realm returned in the KDC-REP in subsequent message exchanges when
 using the obtained anonymous ticket.
 Care MUST be taken by the KDC not to reveal the client's identity in
 the authorization data of the returned ticket when populating the
 authorization data in a returned anonymous ticket.

Zhu, et al. Standards Track [Page 5] RFC 6112 Kerberos Anonymity Support April 2011

 The AD-INITIAL-VERIFIED-CAS authorization data, as defined in
 [RFC4556], contains the issuer name of the client certificate.  This
 authorization is not applicable and MUST NOT be present in the
 returned anonymous ticket when anonymous PKINIT is used.  When the
 client is authenticated (i.e., anonymous PKINIT is not used), if it
 is undesirable to disclose such information about the client's
 identity, the AD-INITIAL-VERIFIED-CAS authorization data SHOULD be
 removed from the returned anonymous ticket.
 The client can use the client keys to mutually authenticate with the
 KDC and request an anonymous Ticket Granting Ticket (TGT) in the AS
 request.  In that case, the reply key is selected as normal,
 according to Section 3.1.3 of [RFC4120].

4.1.1. Anonymous PKINIT

 This sub-section defines anonymous PKINIT.
 As described earlier in this section, the client can request an
 anonymous ticket by authenticating to the KDC using the client's
 identity; alternatively, without revealing the client's identity to
 the KDC, the Kerberos client can request an anonymous ticket as
 follows: the client sets the client name as the anonymous principal
 in the AS exchange and provides PA_PK_AS_REQ pre-authentication data
 [RFC4556] where the signerInfos field of the SignedData [RFC5652] of
 the PA_PK_AS_REQ is empty, and the certificates field is absent.
 Because the anonymous client does not have an associated asymmetric
 key pair, the client MUST choose the Diffie-Hellman key agreement
 method by filling in the Diffie-Hellman domain parameters in the
 clientPublicValue [RFC4556].  This use of the anonymous client name
 in conjunction with PKINIT is referred to as anonymous PKINIT.  If
 anonymous PKINIT is used, the realm name in the returned anonymous
 ticket MUST be the anonymous realm.
 Upon receiving the anonymous PKINIT request from the client, the KDC
 processes the request, according to Section 3.1.2 of [RFC4120].  The
 KDC skips the checks for the client's signature and the client's
 public key (such as the verification of the binding between the
 client's public key and the client name), but performs otherwise
 applicable checks, and proceeds as normal, according to [RFC4556].
 For example, the AS MUST check if the client's Diffie-Hellman domain
 parameters are acceptable.  The Diffie-Hellman key agreement method
 MUST be used and the reply key is derived according to Section
 3.2.3.1 of [RFC4556].  If the clientPublicValue is not present in the
 request, the KDC MUST return a KRB-ERROR with the code
 KDC_ERR_PUBLIC_KEY_ENCRYPTION_NOT_SUPPORTED [RFC4556].  If all goes
 well, an anonymous ticket is generated, according to Section 3.1.3 of
 [RFC4120], and PA_PK_AS_REP [RFC4556] pre-authentication data is

Zhu, et al. Standards Track [Page 6] RFC 6112 Kerberos Anonymity Support April 2011

 included in the KDC reply, according to [RFC4556].  If the KDC does
 not have an asymmetric key pair, it MAY reply anonymously or reject
 the authentication attempt.  If the KDC replies anonymously, the
 signerInfos field of the SignedData [RFC5652] of PA_PK_AS_REP in the
 reply is empty, and the certificates field is absent.  The server
 name in the anonymous KDC reply contains the name of the TGS.
 Upon receipt of the KDC reply that contains an anonymous ticket and
 PA_PK_AS_REP [RFC4556] pre-authentication data, the client can then
 authenticate the KDC based on the KDC's signature in the
 PA_PK_AS_REP.  If the KDC's signature is missing in the KDC reply
 (the reply is anonymous), the client MUST reject the returned ticket
 if it cannot authenticate the KDC otherwise.
 A KDC that supports anonymous PKINIT MUST indicate the support of
 PKINIT, according to Section 3.4 of [RFC4556].  In addition, such a
 KDC MUST indicate support for anonymous PKINIT by including a padata
 element of padata-type PA_PKINIT_KX and empty padata-value when
 including PA-PK-AS-REQ in an error reply.
 When included in a KDC error, PA_PKINIT_KX indicates support for
 anonymous PKINIT.  As discussed in Section 7, when included in an AS-
 REP, PA_PKINIT_KX proves that the KDC and client both contributed to
 the session key for any use of Diffie-Hellman key agreement with
 PKINIT.
 Note that in order to obtain an anonymous ticket with the anonymous
 realm name, the client MUST set the client name as the anonymous
 principal in the request when requesting an anonymous ticket in an AS
 exchange.  Anonymity PKINIT is the only way via which an anonymous
 ticket with the anonymous realm as the client realm can be generated
 in this specification.

4.2. Anonymity Support in TGS Exchange

 The client requests an anonymous ticket by setting the anonymous KDC
 option in a TGS exchange, and in that request the client can use a
 normal Ticket Granting Ticket (TGT) with the client's identity, or an
 anonymous TGT, or an anonymous cross-realm TGT.  If the client uses a
 normal TGT, the client's identity is known to the TGS.
 Note that the client can completely hide the client's identity in an
 AS exchange using anonymous PKINIT, as described in the previous
 section.

Zhu, et al. Standards Track [Page 7] RFC 6112 Kerberos Anonymity Support April 2011

 If the ticket in the PA-TGS-REQ of the TGS request is an anonymous
 one, the anonymous KDC option MUST be set in the request.  Otherwise,
 the KDC MUST return a KRB-ERROR message with the code
 KDC_ERR_BADOPTION.
 When policy allows, the KDC issues an anonymous ticket.  If the
 ticket in the TGS request is an anonymous one, the client name and
 the client realm are copied from that ticket; otherwise, the ticket
 in the TGS request is a normal ticket, the returned anonymous ticket
 contains the client name as the anonymous principal and the client
 realm as the true realm of the client.  In all cases, according to
 [RFC4120] the client name and the client realm in the EncTicketPart
 of the reply MUST match with the corresponding client name and the
 client realm of the anonymous ticket in the reply; the client MUST
 use the client name and the client realm returned in the KDC-REP in
 subsequent message exchanges when using the obtained anonymous
 ticket.
 Care MUST be taken by the TGS not to reveal the client's identity in
 the authorization data of the returned ticket.  When propagating
 authorization data in the ticket or in the enc-authorization-data
 field of the request, the TGS MUST ensure that the client
 confidentiality is not violated in the returned anonymous ticket.
 The TGS MUST process the authorization data recursively, according to
 Section 5.2.6 of [RFC4120], beyond the container levels such that all
 embedded authorization elements are interpreted.  The TGS SHOULD NOT
 populate identity-based authorization data into an anonymous ticket
 in that such authorization data typically reveals the client's
 identity.  The specification of a new authorization data type MUST
 specify the processing rules of the authorization data when an
 anonymous ticket is returned.  If there is no processing rule defined
 for an authorization data element or the authorization data element
 is unknown, the TGS MUST process it when an anonymous ticket is
 returned as follows:
 o  If the authorization data element may reveal the client's
    identity, it MUST be removed unless otherwise specified.
 o  If the authorization data element, that could reveal the client's
    identity, is intended to restrict the use of the ticket or limit
    the rights otherwise conveyed in the ticket, it cannot be removed
    in order to hide the client's identity.  In this case, the
    authentication attempt MUST be rejected, and the TGS MUST return
    an error message with the code KDC_ERR_POLICY.  Note this is
    applicable to both critical and optional authorization data.

Zhu, et al. Standards Track [Page 8] RFC 6112 Kerberos Anonymity Support April 2011

 o  If the authorization data element is unknown, the TGS MAY remove
    it, or transfer it into the returned anonymous ticket, or reject
    the authentication attempt, based on local policy for that
    authorization data type unless otherwise specified.  If there is
    no policy defined for a given unknown authorization data type, the
    authentication MUST be rejected.  The error code is KDC_ERR_POLICY
    when the authentication is rejected.
 The AD-INITIAL-VERIFIED-CAS authorization data, as defined in
 [RFC4556], contains the issuer name of the client certificate.  If it
 is undesirable to disclose such information about the client's
 identity, the AD-INITIAL-VERIFIED-CAS authorization data SHOULD be
 removed from an anonymous ticket.
 The TGS encodes the name of the previous realm into the transited
 field, according to Section 3.3.3.2 of [RFC4120].  Based on local
 policy, the TGS MAY omit the previous realm, if the cross realm TGT
 is an anonymous one, in order to hide the authentication path of the
 client.  The unordered set of realms in the transited field, if
 present, can reveal which realm may potentially be the realm of the
 client or the realm that issued the anonymous TGT.  The anonymous
 Kerberos realm name MUST NOT be present in the transited field of a
 ticket.  The true name of the realm that issued the anonymous ticket
 MAY be present in the transited field of a ticket.

4.3. Subsequent Exchanges and Protocol Actions Common to AS and TGS for

    Anonymity Support
 In both AS and TGS exchanges, the realm field in the KDC request is
 always the realm of the target KDC, not the anonymous realm when the
 client requests an anonymous ticket.
 Absent other information, the KDC MUST NOT include any identifier in
 the returned anonymous ticket that could reveal the client's identity
 to the server.
 Unless anonymous PKINIT is used, if a client requires anonymous
 communication, then the client MUST check to make sure that the
 ticket in the reply is actually anonymous by checking the presence of
 the anonymous ticket flag in the flags field of the EncKDCRepPart.
 This is because KDCs ignore unknown KDC options.  A KDC that does not
 understand the anonymous KDC option will not return an error, but
 will instead return a normal ticket.
 The subsequent client and server communications then proceed as
 described in [RFC4120].

Zhu, et al. Standards Track [Page 9] RFC 6112 Kerberos Anonymity Support April 2011

 Note that the anonymous principal name and realm are only applicable
 to the client in Kerberos messages, the server cannot be anonymous in
 any Kerberos message per this specification.
 A server accepting an anonymous service ticket may assume that
 subsequent requests using the same ticket originate from the same
 client.  Requests with different tickets are likely to originate from
 different clients.
 Upon receipt of an anonymous ticket, the transited policy check is
 performed in the same way as that of a normal ticket if the client's
 realm is not the anonymous realm; if the client realm is the
 anonymous realm, absent other information any realm in the
 authentication path is allowed by the cross-realm policy check.

5. Interoperability Requirements

 Conforming implementations MUST support the anonymous principal with
 a non-anonymous realm, and they MAY support the anonymous principal
 with the anonymous realm using anonymous PKINIT.

6. GSS-API Implementation Notes

 GSS-API defines the name_type GSS_C_NT_ANONYMOUS [RFC2743] to
 represent the anonymous identity.  In addition, Section 2.1.1 of
 [RFC1964] defines the single string representation of a Kerberos
 principal name with the name_type GSS_KRB5_NT_PRINCIPAL_NAME.  The
 anonymous principal with the anonymous realm corresponds to the GSS-
 API anonymous principal.  A principal with the anonymous principal
 name and a non-anonymous realm is an authenticated principal; hence,
 such a principal does not correspond to the anonymous principal in
 GSS-API with the GSS_C_NT_ANONYMOUS name type.  The [RFC1964] name
 syntax for GSS_KRB5_NT_PRINCIPAL_NAME MUST be used for importing the
 anonymous principal name with a non-anonymous realm name and for
 displaying and exporting these names.  In addition, this syntax must
 be used along with the name type GSS_C_NT_ANONYMOUS for displaying
 and exporting the anonymous principal with the anonymous realm.
 At the GSS-API [RFC2743] level, an initiator/client requests the use
 of an anonymous principal with the anonymous realm by asserting the
 "anonymous" flag when calling GSS_Init_Sec_Context().  The GSS-API
 implementation MAY provide implementation-specific means for
 requesting the use of an anonymous principal with a non-anonymous
 realm.
 GSS-API does not know or define "anonymous credentials", so the
 (printable) name of the anonymous principal will rarely be used by or
 relevant for the initiator/client.  The printable name is relevant

Zhu, et al. Standards Track [Page 10] RFC 6112 Kerberos Anonymity Support April 2011

 for the acceptor/server when performing an authorization decision
 based on the initiator name that is returned from the acceptor side
 upon the successful security context establishment.
 A GSS-API initiator MUST carefully check the resulting context
 attributes from the initial call to GSS_Init_Sec_Context() when
 requesting anonymity, because (as in the GSS-API tradition and for
 backwards compatibility) anonymity is just another optional context
 attribute.  It could be that the mechanism doesn't recognize the
 attribute at all or that anonymity is not available for some other
 reasons -- and in that case the initiator MUST NOT send the initial
 security context token to the acceptor, because it will likely reveal
 the initiators identity to the acceptor, something that can rarely be
 "un-done".
 Portable initiators are RECOMMENDED to use default credentials
 whenever possible, and request anonymity only through the input
 anon_req_flag [RFC2743] to GSS_Init_Sec_Context().

7. PKINIT Client Contribution to the Ticket Session Key

 The definition in this section was motivated by protocol analysis of
 anonymous PKINIT (defined in this document) in building tunneling
 channels [RFC6113] and subsequent channel bindings.  In order to
 enable applications of anonymous PKINIT to form channels, all
 implementations of anonymous PKINIT need to meet the requirements of
 this section.  There is otherwise no connection to the rest of this
 document.
 PKINIT is useful for constructing tunneling channels.  To ensure that
 an attacker cannot create a channel with a given name, it is
 desirable that neither the KDC nor the client unilaterally determine
 the ticket session key.  To achieve that end, a KDC conforming to
 this definition MUST encrypt a randomly generated key, called the KDC
 contribution key, in the PA_PKINIT_KX padata (defined next in this
 section).  The KDC contribution key is then combined with the reply
 key to form the ticket session key of the returned ticket.  These two
 keys are then combined using the KRB-FX-CF2 operation defined in
 Section 7.1, where K1 is the KDC contribution key, K2 is the reply
 key, the input pepper1 is American Standard Code for Information
 Interchange (ASCII) [ASAX34] string "PKINIT", and the input pepper2
 is ASCII string "KeyExchange".

Zhu, et al. Standards Track [Page 11] RFC 6112 Kerberos Anonymity Support April 2011

 PA_PKINIT_KX      147
   -- padata for PKINIT that contains an encrypted
   -- KDC contribution key.
 PA-PKINIT-KX  ::= EncryptedData -- EncryptionKey
   -- Contains an encrypted key randomly
   -- generated by the KDC (known as the KDC contribution key).
   -- Both EncryptedData and EncryptionKey are defined in [RFC4120]
 The PA_PKINIT_KX padata MUST be included in the KDC reply when
 anonymous PKINIT is used; it SHOULD be included if PKINIT is used
 with the Diffie-Hellman key exchange but the client is not anonymous;
 it MUST NOT be included otherwise (e.g., when PKINIT is used with the
 public key encryption as the key exchange).
 The padata-value field of the PA-PKINIT-KX type padata contains the
 DER [X.680] [X.690] encoding of the Abstract Syntax Notation One
 (ASN.1) type PA-PKINIT-KX.  The PA-PKINIT-KX structure is an
 EncryptedData.  The cleartext data being encrypted is the DER-encoded
 KDC contribution key randomly generated by the KDC.  The encryption
 key is the reply key and the key usage number is
 KEY_USAGE_PA_PKINIT_KX (44).
 The client then decrypts the KDC contribution key and verifies the
 ticket session key in the returned ticket is the combined key of the
 KDC contribution key and the reply key as described above.  A
 conforming client MUST reject anonymous PKINIT authentication if the
 PA_PKINIT_KX padata is not present in the KDC reply or if the ticket
 session key of the returned ticket is not the combined key of the KDC
 contribution key and the reply key when PA-PKINIT-KX is present in
 the KDC reply.

7.1. Combining Two Protocol Keys

 KRB-FX-CF2() combines two protocol keys based on the pseudo-random()
 function defined in [RFC3961].
 Given two input keys, K1 and K2, where K1 and K2 can be of two
 different enctypes, the output key of KRB-FX-CF2(), K3, is derived as
 follows:
  KRB-FX-CF2(protocol key, protocol key, octet string,
            octet string)  ->  (protocol key)
  PRF+(K1, pepper1) -> octet-string-1
  PRF+(K2, pepper2) -> octet-string-2
  KRB-FX-CF2(K1, K2, pepper1, pepper2) ->
         random-to-key(octet-string-1 ^ octet-string-2)

Zhu, et al. Standards Track [Page 12] RFC 6112 Kerberos Anonymity Support April 2011

 Where ^ denotes the exclusive-OR operation.  PRF+() is defined as
 follows:
 PRF+(protocol key, octet string) -> (octet string)
 PRF+(key, shared-info) -> pseudo-random( key,  1 || shared-info ) ||
              pseudo-random( key, 2 || shared-info ) ||
              pseudo-random( key, 3 || shared-info ) || ...
 Here the counter value 1, 2, 3, and so on are encoded as a one-octet
 integer.  The pseudo-random() operation is specified by the enctype
 of the protocol key.  PRF+() uses the counter to generate enough bits
 as needed by the random-to-key() [RFC3961] function for the
 encryption type specified for the resulting key; unneeded bits are
 removed from the tail.

8. Security Considerations

 Since KDCs ignore unknown options, a client requiring anonymous
 communication needs to make sure that the returned ticket is actually
 anonymous.  This is because a KDC that does not understand the
 anonymous option would not return an anonymous ticket.
 By using the mechanism defined in this specification, the client does
 not reveal the client's identity to the server but the client
 identity may be revealed to the KDC of the server principal (when the
 server principal is in a different realm than that of the client),
 and any KDC on the cross-realm authentication path.  The Kerberos
 client MUST verify the ticket being used is indeed anonymous before
 communicating with the server, otherwise, the client's identity may
 be revealed unintentionally.
 In cases where specific server principals must not have access to the
 client's identity (for example, an anonymous poll service), the KDC
 can define server-principal-specific policy that ensures any normal
 service ticket can NEVER be issued to any of these server principals.
 If the KDC that issued an anonymous ticket were to maintain records
 of the association of identities to an anonymous ticket, then someone
 obtaining such records could breach the anonymity.  Additionally, the
 implementations of most (for now all) KDC's respond to requests at
 the time that they are received.  Traffic analysis on the connection
 to the KDC will allow an attacker to match client identities to
 anonymous tickets issued.  Because there are plaintext parts of the
 tickets that are exposed on the wire, such matching by a third-party
 observer is relatively straightforward.  A service that is
 authenticated by the anonymous principals may be able to infer the

Zhu, et al. Standards Track [Page 13] RFC 6112 Kerberos Anonymity Support April 2011

 identity of the client by examining and linking quasi-static protocol
 information such as the IP address from which a request is received,
 or by linking multiple uses of the same anonymous ticket.
 Two mechanisms, the FAST facility with the hide-client-names option
 in [RFC6113] and the Kerberos5 starttls option [STARTTLS], protect
 the client identity so that an attacker would never be able to
 observe the client identity sent to the KDC.  Transport or network
 layer security between the client and the server will help prevent
 tracking of a particular ticket to link a ticket to a user.  In
 addition, clients can limit how often a ticket is reused to minimize
 ticket linking.
 The client's real identity is not revealed when the client is
 authenticated as the anonymous principal.  Application servers MAY
 reject the authentication in order to, for example, prevent
 information disclosure or as part of Denial of Service (DoS)
 prevention.  Application servers MUST avoid accepting anonymous
 credentials in situations where they must record the client's
 identity; for example, when there must be an audit trail.

9. Acknowledgements

 JK Jaganathan helped editing early revisions of this document.
 Clifford Neuman contributed the core notions of this document.
 Ken Raeburn reviewed the document and provided suggestions for
 improvements.
 Martin Rex wrote the text for GSS-API considerations.
 Nicolas Williams reviewed the GSS-API considerations section and
 suggested ideas for improvements.
 Sam Hartman and Nicolas Williams were great champions of this work.
 Miguel Garcia and Phillip Hallam-Baker reviewed the document and
 provided helpful suggestions.
 In addition, the following individuals made significant
 contributions: Jeffrey Altman, Tom Yu, Chaskiel M Grundman, Love
 Hornquist Astrand, Jeffrey Hutzelman, and Olga Kornievskaia.

Zhu, et al. Standards Track [Page 14] RFC 6112 Kerberos Anonymity Support April 2011

10. IANA Considerations

 This document defines a new 'anonymous' Kerberos well-known name and
 a new 'anonymous' Kerberos well-known realm based on [RFC6111].  IANA
 has added these two values to the Kerberos naming registries that are
 created in [RFC6111].

11. References

11.1. Normative References

 [ASAX34]    American Standards Institute, "American Standard Code for
             Information Interchange", ASA X3.4-1963, June 1963.
 [RFC1964]   Linn, J., "The Kerberos Version 5 GSS-API Mechanism",
             RFC 1964, June 1996.
 [RFC2119]   Bradner, S., "Key words for use in RFCs to Indicate
             Requirement Levels", BCP 14, RFC 2119, March 1997.
 [RFC2743]   Linn, J., "Generic Security Service Application Program
             Interface Version 2, Update 1", RFC 2743, January 2000.
 [RFC3961]   Raeburn, K., "Encryption and Checksum Specifications for
             Kerberos 5", RFC 3961, February 2005.
 [RFC4120]   Neuman, C., Yu, T., Hartman, S., and K. Raeburn, "The
             Kerberos Network Authentication Service (V5)", RFC 4120,
             July 2005.
 [RFC4556]   Zhu, L. and B. Tung, "Public Key Cryptography for Initial
             Authentication in Kerberos (PKINIT)", RFC 4556,
             June 2006.
 [RFC5652]   Housley, R., "Cryptographic Message Syntax (CMS)",
             STD 70, RFC 5652, September 2009.
 [RFC6111]   Zhu, L., "Additional Kerberos Naming Constraints",
             RFC 6111, April 2011.
 [X.680]     "Abstract Syntax Notation One (ASN.1): Specification of
             Basic Notation", ITU-T Recommendation X.680: ISO/IEC
             International Standard 8824-1:1998, 1997.
 [X.690]     "ASN.1 encoding rules: Specification of Basic Encoding
             Rules (BER), Canonical Encoding Rules (CER) and
             Distinguished Encoding Rules (DER)", ITU-T Recommendation
             X.690  ISO/IEC International Standard 8825-1:1998, 1997.

Zhu, et al. Standards Track [Page 15] RFC 6112 Kerberos Anonymity Support April 2011

11.2. Informative References

 [RFC6113]   Hartman, S. and L. Zhu, "A Generalized Framework for
             Kerberos Pre-Authentication", RFC 6113, April 2011.
 [STARTTLS]  Josefsson, S., "Using Kerberos V5 over the Transport
             Layer Security (TLS) protocol", Work in Progress,
             August 2010.

Authors' Addresses

 Larry Zhu
 Microsoft Corporation
 One Microsoft Way
 Redmond, WA  98052
 US
 EMail: larry.zhu@microsoft.com
 Paul Leach
 Microsoft Corporation
 One Microsoft Way
 Redmond, WA  98052
 US
 EMail: paulle@microsoft.com
 Sam Hartman
 Painless Security
 EMail: hartmans-ietf@mit.edu

Zhu, et al. Standards Track [Page 16]

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