Anonymity Support for Kerberos
RFC 6112

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
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   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.

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   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

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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".

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   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]

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   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.

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   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

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   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.

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   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.

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   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].

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   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

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   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".

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   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)

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   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

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   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.

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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.

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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

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