NETWORK WORKING GROUP                                             L. Zhu
Internet-Draft                                                  P. Leach
Updates: 4120 (if approved)                                K. Jaganathan
Expires: January 17, 2007
Intended status: Standards Track                   Microsoft Corporation
                                                           July 16,
Expires: April 14, 2007                                 October 11, 2006

                     Anonymity Support for Kerberos
                       draft-ietf-krb-wg-anon-01
                       draft-ietf-krb-wg-anon-02

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Copyright Notice

   Copyright (C) The Internet Society (2006).

Abstract

   This document defines extensions to the use of anonymous Kerberos tickets protocol for the
   purpose of authenticating
   Kerberos client to authenticate the servers Kerberos Key Distribution Center
   and enabling the Kerberos server, without revealing the client's identity.
   These extensions can be used to secure communication between a client and a server, without identifying the
   anonymous client to and the server.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Conventions Used in This Document  . . . . . . . . . . . . . .  3
   3.  Definitions  . . . . . . . . . . . . . . . . . . . . . . . . .  3
   4.  Protocol Description . . . . . . . . . . . . . . . . . . . . .  5
   5.  GSS-API Implementation Notes . . . . . . . . . . . . . . . . .  7
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . .  8
   7.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . .  8  9
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  8  9
   9.  Normative References . . . . . . . . . . . . . . . . . . . . .  8  9
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 10
   Intellectual Property and Copyright Statements . . . . . . . . . . 11

1.  Introduction

   In certain situations or environments, situations, the Kerberos [RFC4120] client may wish to
   authenticate a server and/or protect communications without revealing
   its 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 its
   identity to the service for privacy reasons.

   To accomplish this, a Kerberos mechanism is

   Extensions to [RFC4120] are specified in this document by which a
   client requests can authenticate the KDC and request an anonymous ticket and ticket.  The
   client can use that the anonymous ticket to authenticate the server and secure
   protect subsequent client-server communications.  This provides  These extensions
   provide Kerberos with functional equivalence to TLS [RFC2246] Transport Layer
   Security (TLS) [RFC4346].

   By using the extensions defined in environments where Kerberos is a more attractive
   authentication mechanism.

   Using this mechanism, specification, the client has to MAY
   reveal its identity in its initial request to its own Key Distribution Center (KDC) [RFC4120],
   and then KDC, but it can
   remain anonymous thereafter to KDCs on the cross-
   realm cross-realm authentication
   path, if any, and to the server with which it communicates.

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 a reserved realm name as defined
   in [KRBNAM] and its based on
   [KRBNAM].  The value is the literal "RESERVED:ANONYMOUS".

   The anonymous Kerberos principal name is a reserved Kerberos
   principal name as defined in [KRBNAM], its based on [KRBNAM].  The value of the name-type [RFC4120] field
   is KRB_NT_RESRVED [KRBNAM], and its the value of the name-string [RFC4120] field is
   a sequence of two KerberosString components: "RESERVED", "ANONYMOUS".

   In

   Note that in this specification, only the client anonymous principal name or the client and
   realm can
   be anonymous; are only applicable to the server name or client in Kerberos messages, the
   server realm can not MUST NOT be
   anonymous. anonymous in any Kerberos message.

   The transited field [RFC4120] of a ticket is an anonymous
   authentication path if the tr-type field of the TransitedEncoding
   type [RFC4120] is NO-TRANSITED-INFO and the contents field is an
   empty OCTET STRING.

           NO-TRANSITED-INFO    TBA

   This transited encoding type indicates means that there is no information
   available about of the authentication path. path is
   disclosed.

   The anonymous ticket flag is defined as bit TBA (with the first bit
   being bit 0) in the TicketFlags:

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

   An anonymous ticket is a ticket that has all of the following
   properties:

   o  The cname field [RFC4120] contains the anonymous Kerberos
      principal name.

   o  The crealm field [RFC4120] contains either the client's realm name of the
      client who made the request
      or the anonymous kerberos realm name,
      based on the local policy of the KDC. name.

   o  The transited field [RFC4120] can contain either the client's
      "normal"
      authentication path according to as described in Section 3.3.3.2 of [RFC4120]
      or the anonymous authentication path.

   o  It  The anonymous ticket contains no information that can reveal the
      client's identity.  However the ticket can MAY contain the client
      realm and the realms on the authentication path, and the authorization
      data may that MAY provide
      additional information of related to the client. client's
      identity.  For example, an anonymous principal that is only
      identifiable within a particular group of users can be implemented by
      using authorization data. data and such authorization data, if included
      in the anonymous ticket, shall disclose the client's membership of
      that group.

   o  The anonymous ticket flag is set.

   Notes:

   The anonymous ticket flag MUST NOT be set by implementations
   of this specification if the ticket request-anonymous KDC option is not an anonymous ticket.  The
   server principal name and the server realm in a cross-realm referral
   TGT are not dependent on whether the client is the anonymous
   principal or not.

   The request-anonymous KDC option is defined as bit TBA (with defined as bit TBA (with the
   first bit being bit 0) in the KDCOptions:

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

4.  Protocol Description

   In order to request an anonymous ticket, the client sets the request-
   anonymous KDC option in an Authentication Exchange (AS) or Ticket
   Granting Service (TGS) request [RFC4120].  The client can request an
   anonymous TGT based on a normal TGT.  Note that if  If the client wishes to
   authenticate the KDC anonymously, it sets the client name as
   anonymous 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 PA_PK_AS_REQ
   are empty.  Because the anonymous client does not have an associated
   asymmetric key pair, the client MUST use the Diffie-Hellman key
   agreement method by filling in the Diffie-Hellman domain parameters
   in the clientPublicValue [RFC4556].

   If the ticket in the PA-TGS-REQ [RFC4120] of the TGS request is
   anonymous, or if the client in the AS request is anonymous, the
   request-anonymous KDC option MUST be set in the request.

   When propagating authorization data, care MUST be taken by

   Upon receiving the TGS to
   ensure that AS request with a PA_PK_AS_REQ from the client confidentiality is not violated: anonymous
   client, the TGS MUST
   either fail KDC skips the request or remove authorization data that may reveal checks for the client's identity.  An optional authorization element unknown by signature and the TGS MUST be removed if it can be ignored
   client's public key (such as ones enclosed
   in the AD-IF-RELEVANT or verification of the AD-KDCIssued containers [RFC4120]).  The
   TGS can strip critical unknown authorization data if such data do not
   convey any rights based on binding between
   the requesting client's identity.  Here is
   a table of public key and the known authorization-data elements, flagged with
   whether they interfere with client anonymity name), but performs otherwise-
   applicable checks, and recommendations for
   how proceeds as normal according to process them.

         ad-type          References  Can Breach Confidentiality?
     ------------------------------------------------------------------
     AD-IF-RELEVANT        RFC4120     Yes, remove if unknown
     AD-KDCIssued          RFC4120     Yes, remove if unknown
     AD-AND-OR             RFC4120     Yes, remove if unknown
     AD-MANDATORY-FOR-KDC  RFC4120     Yes, fail [RFC4556].
   For example, the request AS MUST check if unknown

   If it is inappropriate to remove an authorization element from the
   TGS 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 [RFC4120] with the code
   KDC_ERR_PUBLIC_KEY_ENCRYPTION_NOT_SUPPORTED [RFC4556] and there is no
   accompanying e-data.  The client that made the anonymous request can
   authenticate the KDC based on the KDC's signature in the reply.  If
   the KDC does not have an asymmetric key pair, it MAY reply
   anonymously.  In which case, 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 order to produce an anonymous ticket, reply contains the
   name of the TGS.  Upon receipt of an anonymous KDC reply, the client
   MUST
   return an error message reject the returned ticket if it can not authenticate the KDC
   otherwise.

   The client can use its keys to mutually authenticate with the code KDC_ERR_POLICY KDC,
   and 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].

   For the TGS exchange, the reply key is selected as normal according
   to Section 3.3.3 of [RFC4120].

   When policy allows, the KDC issues an anonymous ticket.  The  Based on
   local policy, the client realm in the anonymous ticket can be the
   anonymous realm name based
   on local policy.  The or the realm of the KDC.  However, in all cases,
   the client name and the client realm in the EncKDCRepPart of the
   reply [RFC4120] MUST match with the corresponding client name and the
   client realm of the anonymous reply
   ticket. ticket in the reply.  The client then MUST
   use the client name and the client realm returned in the
   EncKDCRepPart in subsequent message exchanges when using that the obtained
   anonymous ticket.

   If there is a key known by both the client and the KDC for encrypting
   the KDC reply, the cname field in

   During the request [RFC4120] can TGS request, when propagating authorization data, care
   MUST be
   anonymous.  If taken by the TGS to ensure that the client confidentiality is anonymous and the KDC does
   not have a
   key to encrypt the reply, the KDC violated.  The TGS MUST return an error message with either fail the code KDC_ERR_NULL_KEY [RFC4120].  For AS exchange, if request or remove
   authorization data that may reveal the reply
   key is selected from client's identity.  An
   optional authorization element unknown by the client keys (for example, TGS MUST be removed if
   it can be ignored (such as described ones enclosed in
   Section 3.1.3 of [RFC4120]), then the client principal MUST NOT be
   anonymous. AD-IF-RELEVANT
   structure).  The client TGS can use only strip critical unknown authorization
   data if the client keys ticket does not convey any rights such as those conveyed
   by a KDCIssued authorization data element.  If a ticket contains a
   KDCIssued authorization data element, then no other authorization
   data elements may be removed if they could serve to request an
   anonymous TGT in limit the AS request.  The anonymous client name, for
   example, can be used in conjunction rights
   conveyed by the KDCIssued element.  Here is a table of the known
   authorization-data elements, tagged with whether they interfere with PKINIT [RFC4556].  An
   anonymous PKINIT
   client can authenticate the KDC based on anonymity and recommendations for how to process them:

         ad-type          References  Can Breach Confidentiality?
     ------------------------------------------------------------------
     AD-IF-RELEVANT        RFC4120     Yes, remove if unknown
     AD-KDCIssued          RFC4120     Yes, fail the KDC
   certificate.  For TGS exchange, request if unknown
     AD-AND-OR             RFC4120     Yes, remove if unknown
     AD-MANDATORY-FOR-KDC  RFC4120     Yes, fail the reply key is selected according
   to Section 3.3.3 of [RFC4120] as normal. request if unknown

   The KDC fills out the transited field of the anonymous ticket in the
   reply as follows: If the service ticket in a TGS request is an
   anonymous ticket with a "normal" authentication path, then the
   authentication path in the reply ticket MUST also contain a "normal"
   authentication path: path, the TGS MUST add the name of the previous realm.
   However, if the service ticket in a TGS request is an anonymous
   ticket with an anonymous authentication path, then the reply ticket
   can contain either an anonymous authentication path or a "normal"
   authentication path, based on the local policy of the KDC.  Thus a
   "normal" authentication path in an anonymous ticket can be a partial
   path:
   path, it may not include all the intermediate realms on the
   authentication path.

   The KDC fills out the authtime field of the anonymous ticket in the
   reply as follows: If the anonymous ticket is returned in an AS
   exchange, the authtime field of the ticket contains the request time.

   If the anonymous ticket is returned in a TGS exchange, the authtime
   field contains the time authtime of the initial authentication for the
   principal who has made ticket in the request. PA-TGS-REQ
   [RFC4120].  An anonymous ticket can be renewed, and the authtime
   field of a renewed ticket is the authtime in the anonymous ticket that on
   which the renewed ticket was based on. based.

   If it is inappropriate to remove an authorization element from the
   TGS request in order to produce an anonymous ticket, the KDC MUST
   return an error message with the code KDC_ERR_POLICY [RFC4120].

   If the client is anonymous and the KDC does not have a key to encrypt
   the reply, the KDC MUST return an error message with the code
   KDC_ERR_NULL_KEY [RFC4120] and there is no accompanying e-data.

   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.  Because  This is
   because KDCs ignore unknown KDC options, a options.  A KDC that does not
   understand the request-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].  No transited policy checking is needed for
   the anonymous authentication path.  However, transited policy checks
   defined in Section 2.7 of [RFC4120] would apply to an anonymous
   ticket that contains a "normal" authentication path.

   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.

   Interoperability and backward-compatibility notes: the KDC is given
   the task of rejecting a request for an anonymous ticket when the
   anonymous ticket is not acceptable by the server.

5.  GSS-API Implementation Notes

   At the GSS-API [RFC2743] level, the use of an anonymous principal by
   the initiator/client requires a software change of the initiator/
   client software (to initiator/client to assert the
   "anonymous" flag when calling GSS_Init_Sec_Context().

   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 initator/client. initiator/client.  The printable name is relevant
   for the acceptor/server when performing an authorization decision
   based on the name that pops up from GSS_Accept_Sec_Context() upon
   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".

   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.  For
   the anonymous principals, the name component within the exportable
   name as defined in Section 2.1.3 of [RFC1964] MUST signify the realm
   name according to Section 2.1.1 of [RFC1964].  In  Note that in this
   specification only the client/initiator can be the anonymous identity. anonymous.

   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().

6.  Security Considerations

   Since KDCs ignore unknown options [RFC4120], a client requiring
   anonymous communication needs to make sure that the ticket is
   actually anonymous.  A  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 its identity to the server but its 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 cross-realm KDC or the server, otherwise the client's identity may be revealed to the server
   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
   implementation
   implementations of most (for now all) KDC's respond to requests at
   the time that they are received.  Traffic analasys analysis on the connection
   to the KDC will allow an attacket 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 straigtforward. straightforward.

7.  Acknowledgements

   The authors would like to thank

   Clifford Neuman contributed the following individuals core notions of this document.

   Martin Rex wrote the text for their
   insightful comments GSS-API considerations.

   Nicolas Williams reviewed the GSS-API considerations section and fruitful discussions:
   suggested ideas for improvements.

   Sam Hartman, Clifford
   Neuman, Martin Rex, Hartman and Nicolas Williams, Williams were great champions of this work.

   In addition, the following individuals made significant
   contributions: Jeffery Altman, Tom Yu, Chaskiel M Grundman, Love
   Hoernquist Aestrand, and Jeffery Hutzelman.

8.  IANA Considerations

   No

   Section 3 defines the anonymous Kerberos name and the anonymous
   Kerberos realm based on [KRBNAM].  The IANA actions are required registry for [KRBNAM]
   need to be updated to add references to this document.

9.  Normative References

   [KRBNAM]   Zhu, L., "Additonal Kerberos Naming Contraints",
              draft-ietf-krb-wg-naming, work in progress.

   [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.

   [RFC2246]  Dierks, T. and C. Allen, "The TLS Protocol Version 1.0",
              RFC 2246, January 1999.

   [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.

   [RFC4120]  Neuman, C., Yu, T., Hartman, S., and K. Raeburn, "The
              Kerberos Network Authentication Service (V5)", RFC 4120,
              July 2005.

   [RFC4346]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.1", RFC 4346, April 2006.

   [RFC4556]  Zhu, L. and B. Tung, "Public Key Cryptography for Initial
              Authentication in Kerberos (PKINIT)", RFC 4556, June 2006.

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

   Karthik Jaganathan
   Microsoft Corporation
   One Microsoft Way
   Redmond, WA  98052
   US

   Email: karthikj@microsoft.com

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   Copyright (C) The Internet Society (2006).

   This document is subject to the rights, licenses and restrictions
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   ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
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   INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
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