


Network Working Group                                       S. Josefsson
Internet-Draft                                                       SJD
Intended status: Standards Track                         October 4, 2006
Expires: April 7, 2007


   Using Kerberos V5 over the Transport Layer Security (TLS) protocol
                 draft-josefsson-kerberos5-starttls-01

Status of this Memo

   By submitting this Internet-Draft, each author represents that any
   applicable patent or other IPR claims of which he or she is aware
   have been or will be disclosed, and any of which he or she becomes
   aware will be disclosed, in accordance with Section 6 of BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
   other groups may also distribute working documents as Internet-
   Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/ietf/1id-abstracts.txt.

   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html.

   This Internet-Draft will expire on April 7, 2007.

Copyright Notice

   Copyright (C) The Internet Society (2006).














Josefsson                 Expires April 7, 2007                 [Page 1]

Internet-Draft       Protecting Kerberos V5 with TLS        October 2006


Abstract

   This document specify how the Kerberos V5 protocol can be transported
   over the Transport Layer Security (TLS) protocol, to provide
   additional security features.


Table of Contents

   1.  Introduction and Background  . . . . . . . . . . . . . . . . .  3
   2.  Kerberos V5 STARTTLS Extension . . . . . . . . . . . . . . . .  5
   3.  Examples . . . . . . . . . . . . . . . . . . . . . . . . . . .  6
   4.  STARTTLS aware KDC Discovery . . . . . . . . . . . . . . . . .  7
   5.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  8
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . .  9
   7.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
     7.1.  Normative References . . . . . . . . . . . . . . . . . . . 10
     7.2.  Informative References . . . . . . . . . . . . . . . . . . 10
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 11
   Intellectual Property and Copyright Statements . . . . . . . . . . 12































Josefsson                 Expires April 7, 2007                 [Page 2]

Internet-Draft       Protecting Kerberos V5 with TLS        October 2006


1.  Introduction and Background

   This document describe how a Kerberos V5 [2] implementation may
   upgrade communication between clients and Key Distribution Centers
   (KDCs) to use the Transport Layer Security (TLS) [4] protocol.

   The TLS protocol offer integrity and privacy protected exchanges that
   can be authentication using X.509 certificates, OpenPGP keys [7], and
   user name and passwords via SRP [6].

   There are several reasons to use Kerberos V5 over TLS.

   o  Kerberos exchanges are privacy protected.  Part of many Kerberos
      packets are transfered without privacy protection (i.e.,
      encryption).  That part contains information, such as the client
      principal name, the server principal name, the encryption types
      supported by the client, the lifetime of tickets, etc.  Revealing
      such information is, in some threat models, considered a problem.


   o  Prevents downgrade attacks affecting encryption types.  The
      encryption type of the ticket in KDC-REQ are sent in the clear in
      Kerberos 5.  This allows an attacker to replace the encryption
      type with a compromised mechanisms, e.g., 56-bit DES.  Since
      clients in general cannot know the encryption types other servers
      support, it is difficult for the client to detect if there was a
      man-in-the-middle or if the remote server simply did not support a
      stronger mechanism.  Clients could chose to refuse, e.g., 56-bit
      DES altogether, but in some environments this leads to operational
      difficulties.


   o  Additional authentication against the KDC.  In some situations,
      users are equipped with smart cards with a RSA authentication key.
      In others, users have a OpenPGP client on their desktop, with a
      public OpenPGP key known to the server.  In some situations, the
      policy may be that password authentication may only be done
      through SRP.


   o  The TLS protocol has been studied by many parties.  In some threat
      models, the designer prefer to reduce the number of protocols that
      can hurt the overall system security if they are compromised.


   o  Explicit server authentication of the KDC to the client.  In
      traditional Kerberos 5, authentication of the KDC is proved as a
      side effect that the KDC knows your encryption key (i.e., your



Josefsson                 Expires April 7, 2007                 [Page 3]

Internet-Draft       Protecting Kerberos V5 with TLS        October 2006


      password).

   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 RFC 2119 [1].














































Josefsson                 Expires April 7, 2007                 [Page 4]

Internet-Draft       Protecting Kerberos V5 with TLS        October 2006


2.  Kerberos V5 STARTTLS Extension

   The STARTTLS extension uses the Kerberos V5 TCP extension mechanism
   [3].  The extension uses bit #TBD in the extension bitmask.

   The protocol is as follows.  After the server has sent the 4-octet
   value 0x00000000 to indicate support of this extension, the stream
   will be controlled by the TLS protocol and its framing.  The TLS
   protocol is initiated by the client.

   Typically, the client initiate the TLS handshake protocol by sending
   a client hello, and the server responds, and the handshake continues
   until it either succeed or fails.

   If for any reason the handshake fails, the STARTTLS protocol will
   also fail, and the TLS error is used as the error indication.

   If the handshake succeeds, the Kerberos V5 authentication protocol is
   performed within the protected TLS channel, like a normal TCP
   Kerberos V5 exchange.  In particular, this means that every Kerberos
   V5 packet will be prefixed by a 4-octet length field, that indicate
   the length of the Kerberos V5 packet.





























Josefsson                 Expires April 7, 2007                 [Page 5]

Internet-Draft       Protecting Kerberos V5 with TLS        October 2006


3.  Examples

   A complete packet flow for a successful AS-REQ/REP exchange protected
   by this mechanism will be as follows.  The "STARTTLS-bit" is a
   4-octet value with only the bit allocated for this extension set.

       Client                                               Server

        [ Kerberos V5 TCP extension mechanism negotiation starts ]

       [0x70000000 & STARTTLS-bit]  -------->
                                                       [0x00000000]
                                    <--------

                            [ TLS negotiation starts ]


       ClientHello                  -------->
                                                       ServerHello
                                                      Certificate*
                                                ServerKeyExchange*
                                               CertificateRequest*
                                    <--------      ServerHelloDone
       Certificate*
       ClientKeyExchange
       CertificateVerify*
       [ChangeCipherSpec]
       Finished                     -------->
                                                [ChangeCipherSpec]
                                    <--------             Finished

                       [ Kerberos V5 negotiation starts ]

       Kerberos V5 AS-REQ           -------->
                                                Kerberos V5 AS-REP
                                    <--------

       * Indicates optional or situation-dependent messages that are not
         always sent.












Josefsson                 Expires April 7, 2007                 [Page 6]

Internet-Draft       Protecting Kerberos V5 with TLS        October 2006


4.  STARTTLS aware KDC Discovery

   Section 7.2.3 of Kerberos V5 [2] describe how Domain Name System
   (DNS) SRV records [5] can be used to find the address of an KDC.
   Using the terminology of Section 7.2.3 of RFC 4120, we define a new
   Proto of "tls" to indicate that the particular KDC is intended to
   support this STARTTLS extension.  The Service, Realm, TTL, Class,
   SRV, Priority, Weight, Port and Target have the same meaning as in
   RFC 4120.

   For example:

   _kerberos._tls.EXAMPLE.COM. IN SRV 0 0 88 kdc1.example.com.
   _kerberos._tls.EXAMPLE.COM. IN SRV 1 0 88 kdc2.example.com.





































Josefsson                 Expires April 7, 2007                 [Page 7]

Internet-Draft       Protecting Kerberos V5 with TLS        October 2006


5.  IANA Considerations

   The IANA is requested to allocate a bit in the "Kerberos TCP
   Extensions" registry for the extension described in this document, as
   per [3].














































Josefsson                 Expires April 7, 2007                 [Page 8]

Internet-Draft       Protecting Kerberos V5 with TLS        October 2006


6.  Security Considerations

   The security considerations in Kerberos V5, TLS, and the extension
   mechanism framework are inherited.

   To protect against the inherent downgrade attack in the extension
   framework, it is suggested that implementations offer a policy to
   require that this extension is successfully negotiated.  For
   interoperability with implementations that do not support this
   extension, it is suggested that the policy is disabled by default.









































Josefsson                 Expires April 7, 2007                 [Page 9]

Internet-Draft       Protecting Kerberos V5 with TLS        October 2006


7.  References

7.1.  Normative References

   [1]  Bradner, S., "Key words for use in RFCs to Indicate Requirement
        Levels", BCP 14, RFC 2119, March 1997.

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

   [3]  Josefsson, S., "Extended Kerberos Version 5 Key Distribution
        Center (KDC) Exchanges Over  TCP",
        draft-ietf-krb-wg-tcp-expansion-01 (work in progress),
        September 2006.

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

   [5]  Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
        specifying the location of services (DNS SRV)", RFC 2782,
        February 2000.

7.2.  Informative References

   [6]  Taylor, D., "Using SRP for TLS Authentication",
        draft-ietf-tls-srp-12 (work in progress), June 2006.

   [7]  Mavroyanopoulos, N., "Using OpenPGP keys for TLS
        authentication", draft-ietf-tls-openpgp-keys-10 (work in
        progress), June 2006.





















Josefsson                 Expires April 7, 2007                [Page 10]

Internet-Draft       Protecting Kerberos V5 with TLS        October 2006


Author's Address

   Simon Josefsson
   SJD

   Email: simon@josefsson.org













































Josefsson                 Expires April 7, 2007                [Page 11]

Internet-Draft       Protecting Kerberos V5 with TLS        October 2006


Full Copyright Statement

   Copyright (C) The Internet Society (2006).

   This document is subject to the rights, licenses and restrictions
   contained in BCP 78, and except as set forth therein, the authors
   retain all their rights.

   This document and the information contained herein are provided on an
   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
   OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
   ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
   INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
   INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
   WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.


Intellectual Property

   The IETF takes no position regarding the validity or scope of any
   Intellectual Property Rights or other rights that might be claimed to
   pertain to the implementation or use of the technology described in
   this document or the extent to which any license under such rights
   might or might not be available; nor does it represent that it has
   made any independent effort to identify any such rights.  Information
   on the procedures with respect to rights in RFC documents can be
   found in BCP 78 and BCP 79.

   Copies of IPR disclosures made to the IETF Secretariat and any
   assurances of licenses to be made available, or the result of an
   attempt made to obtain a general license or permission for the use of
   such proprietary rights by implementers or users of this
   specification can be obtained from the IETF on-line IPR repository at
   http://www.ietf.org/ipr.

   The IETF invites any interested party to bring to its attention any
   copyrights, patents or patent applications, or other proprietary
   rights that may cover technology that may be required to implement
   this standard.  Please address the information to the IETF at
   ietf-ipr@ietf.org.


Acknowledgment

   Funding for the RFC Editor function is provided by the IETF
   Administrative Support Activity (IASA).





Josefsson                 Expires April 7, 2007                [Page 12]

