

NETWORK WORKING GROUP                                             L. Zhu
Internet-Draft                                                  P. Leach
Updates: 4120, 4121 and 4556                       Microsoft Corporation
(if approved)                                            October 8, 2008
Intended status: Standards Track
Expires: April 11, 2009


                     Anonymity Support for Kerberos
                       draft-ietf-krb-wg-anon-10

Status of this Memo

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   This Internet-Draft will expire on April 11, 2009.

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 which allow a
   Kerberos client to obtain anonymous credentials without revealing its
   identity to the Kerberos Key Distribution Center (KDC).  This
   document updates RFC 4120, RFC 4121, and RFC 4556.






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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.  Combinging Two protocol Keys . . . . . . . . . . . . . . . 12
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 13
   9.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 13
   10. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 14
   11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 14
     11.1. Normative References . . . . . . . . . . . . . . . . . . . 14
     11.2. Informative References . . . . . . . . . . . . . . . . . . 15
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 15
   Intellectual Property and Copyright Statements . . . . . . . . . . 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
   which provides read access to a research database, and which permits
   queries by arbitrary requestors.  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



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   name based on [KRBNAM], and the value of this well-known realm name
   is the literal "WELLKNOWN:ANONYMOUS".

   The anonymous Kerberos principal name is defined as a well-known
   Kerberos principal name based on [KRBNAM].  The value of the name-
   type field is KRB_NT_WELLKNOWN [KRBNAM], 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 14 (with the first bit
   being bit 0) in the TicketFlags:

           TicketFlags     ::= KerberosFlags
             -- anonymous(14)
             -- TicketFlags and KerberosFlags are defined in [RFC4120]

   This is a new ticket flag that is used to indicate 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 14 (with the first bit
   being bit 0) in the KDCOptions:

           KDCOptions      ::= KerberosFlags
             -- anonymous(14)
             -- KDCOptions and KerberosFlags are defined in [RFC4120]

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



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   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 an 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 exchange.  These are then followed by the decription 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, or the
   client's X.509 certificates [RFC4556], or any other preauthenication
   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 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 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.

   The AD-INITIAL-VERIFIED-CAS authorization data as defined in
   [RFC4556] contains the issuer name of the client certificate.  This



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   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, request an anonymous TGT in the AS request.  And 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 anonymity 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 a PA_PK_AS_REQ pre-authentication
   data [RFC4556] where both the signerInfos field and the certificates
   field of the SignedData [RFC3852] of the PA_PK_AS_REQ are empty.
   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 a PA_PK_AS_REP [RFC4556] pre-authentication data is
   included in the KDC reply according to [RFC4556].  If the KDC does
   not have an asymmetric key pair, it MAY reply anonymously or reject



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   the authentication attempt.  If the KDC replies anonymously, both the
   signerInfos field and the certificates field of the SignedData
   [RFC3852] of PA_PK_AS_REP in the reply are empty.  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 a
   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].

   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.

   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



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   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's 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.

   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



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   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].

   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
   preformed 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.






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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.

   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
   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



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   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 [FAST] 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 can 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".

   PA_PKINIT_KX      135
     -- 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-Helleman 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 [X680] [X690] encoding of the Abstract Syntax Notation One
   (ASN.1) type PA-PKINIT-KX.  The PA-PKINIT-KX structure is a
   EncryptedData.  The clear text data being encrypted is the DER
   encoded Kerberos session key randomly generated by the KDC.  The



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   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.  Combinging 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)

   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.







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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 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 insure 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
   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.

   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.




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   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.


10.  IANA Considerations

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


11.  References

11.1.  Normative References

   [ASAX34]   American Standard Code for Information Interchange, 
              ASA X3.4-1963, American Standards Association, June 17, 
              1963.

   [KRBNAM]   Zhu, L., "Additional Kerberos Naming Constraints",
              draft-ietf-krb-wg-naming (work in progress), 2008.

   [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.

   [RFC3852]  Housley, R., "Cryptographic Message Syntax (CMS)",
              RFC 3852, July 2004.

   [RFC3961]  Raeburn, K., "Encryption and Checksum Specifications for
     


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              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.


   [X680]     ITU-T Recommendation X.680 (2002) | ISO/IEC 8824-1:2002,
              Information technology - Abstract Syntax Notation One
              (ASN.1): Specification of basic notation.
   
   [X690]     ITU-T Recommendation X.690 (2002) | ISO/IEC 8825-1:2002,
              Information technology - ASN.1 encoding Rules:
              Specification of Basic Encoding Rules (BER), Canonical
              Encoding Rules (CER) and Distinguished Encoding Rules
              (DER).        
              
11.2.  Informative References

   [FAST]     Zhu, L. and S. Hartman, "A Generalized Framework for
              Kerberos Pre-Authentication",
              draft-ietf-krb-wg-preauth-framework (work in progress),
              2008.


Authors' Addresses

   Larry Zhu
   Microsoft Corporation
   One Microsoft Way
   Redmond, WA  98052
   US

   Email: lzhu@microsoft.com


   Paul Leach
   Microsoft Corporation
   One Microsoft Way
   Redmond, WA  98052
   US

   Email: paulle@microsoft.com
















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Full Copyright Statement

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