

Network Working Group                               L. Nerenberg, Editor
Internet Draft: The CRAM-MD5 SASL Mechanism              Orthanc Systems
Document: draft-ietf-sasl-crammd5-01.txt                   November 2003



                      The CRAM-MD5 SASL Mechanism


Status of this Memo

     This document is an Internet Draft and is in full conformance with
     all provisions of Section 10 of RFC 2026.

     Internet Drafts are working documents of the Internet Engineering
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     Copyright 2003, The Internet Society.  All Rights Reserved.

     Please see the Copyright section near the end of this document for
     more information.

Abstract

     This document defines a simple challenge-response authentication
     mechanism, using a keyed-hash digest, for use with the Simple
     Authentication and Security Layer (SASL).

1.  Conventions Used in this Document

     The key words "MUST", "MUST NOT", "SHOULD", "SHOULD NOT", and "MAY"
     in this document are to be interpreted as defined in [KEYWORD].


2.  CRAM-MD5 Authentication Mechanism

     This document defines a simple challenge-response [SASL] authenti-
     cation mechanism, using a [KEYED-MD5] digest, for use with [SASL].
     The mechanism name associated with CRAM-MD5 is 'CRAM-MD5'.

     This mechanism does not provide a security layer.



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     The data encoded in the challenge contains a presumptively arbi-
     trary string of random digits, a time-stamp, and the fully-quali-
     fied primary host name of the server.

     The client makes note of the data and then responds with a string
     consisting of the user name, a space, and a "digest."  The latter
     is computed by applying the keyed MD5 algorithm from [KEYED-MD5]
     where the key is a shared secret and the digested text is the chal-
     lenge (including angle-brackets). The client MUST NOT interpret or
     attempt to validate the contents of the challenge in any way.

     This shared secret is a string known only to the client and server.
     The "digest" parameter itself is a 16-octet value which is sent in
     hexadecimal format, using lower-case US-ASCII characters.

     When the server receives this client response, it verifies the
     digest provided.  Since the user name may contain the space charac-
     ter, the server MUST scan the client response from right to left;
     the first space character encountered separates the digest from the
     user name.  If the digest is correct, the server should consider
     the client authenticated and respond appropriately.

     The client MUST prepare the user name and shared secret strings
     using the [SASLPrep] profile of the [StringPrep] algorithm.  The
     resulting values MUST be encoded as UTF-8 [UTF8].


2.1.  Formal Syntax

     The following syntax specification uses the augmented Backus-Naur
     Form (ABNF) as specified in [ABNF], and incorporates by reference
     the Core Rules defined in that document.

     challenge  = "<" 1*DIGIT "." 1*DIGIT "@" hostname ">"

     digest     = 32(DIGIT / %x61-66)
                  ; A hexadecimal string using only lower-case
                  ; letters

     hostname   = 1*(ALPHA / DIGIT) *("." / "-" / ALPHA / DIGIT)

     response   = user SP digest

     user       = 1*OCTET


2.2.  Examples

     The examples in this section do NOT form part of the specification.
     Where conflicts exist between the examples and the formal grammar
     or specification text, the latter are authoritative.

     These examples show the use of the CRAM-MD5 mechanism with the
     IMAP4 AUTHENTICATE command [IMAP4].  The base64 encoding of the



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     challenges and responses is part of the IMAP4 AUTHENTICATE command,
     not part of the CRAM-MD5 specification itself.

     S: * OK [CAPABILITY IMAP4rev1 STARTTLS LOGINDISABLED AUTH=CRAM-MD5]
     C: A0001 AUTHENTICATE CRAM-MD5
     S: + PDE4OTYuNjk3MTcwOTUyQHBvc3RvZmZpY2UucmVzdG9uLm1jaS5uZXQ+
     C: dGltIGI5MTNhNjAyYzdlZGE3YTQ5NWI0ZTZlNzMzNGQzODkw
     S: A0001 OK CRAM-MD5 authentication successful

     In this example, the shared secret is the string

          tanstaaftanstaaf

     Hence, the Keyed MD5 digest is produced by calculating

          MD5((tanstaaftanstaaf XOR opad),
               MD5((tanstaaftanstaaf XOR ipad),
               <1896.697170952@postoffice.example.net>))

     where ipad and opad are as defined in [KEYED-MD5] and the string
     shown in the challenge is the base64 encoding of
     <1896.697170952@postoffice.reston.mci.net>. The shared secret is
     null-padded to a length of 64 bytes. If the shared secret is longer
     than 64 bytes, the MD5 digest of the shared secret is used as a 16
     byte input to the keyed MD5 calculation.

     This produces a digest value (in hexadecimal) of

          b913a602c7eda7a495b4e6e7334d3890

     The user name is then prepended to it, forming

          tim b913a602c7eda7a495b4e6e7334d3890

     Which is then base64 encoded to meet the requirements of the IMAP4
     AUTHENTICATE command (or the similar POP3 AUTH command), yielding

          dGltIGI5MTNhNjAyYzdlZGE3YTQ5NWI0ZTZlNzMzNGQzODkw



3.  References

3.1.  Normative References

[ABNF]
     Crocker, D., P. Overell, "Augmented BNF for Syntax Specifications:
     ABNF", RFC2234, Internet Mail Consortium and Demon Internet Ltd.,
     November 1997.

[KEYED-MD5]
     Krawczyk, Bellare, Canetti, "HMAC: Keyed-Hashing for Message
     Authentication", RFC 2104, IBM and UCSD, February 1997.




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[KEYWORD]
     Bradner, S., "Key words for use in RFCs to Indicate Requirement
     Levels", BCP 14, RFC2119, Harvard University, March 1997.

[MD5]
     Rivest, R., "The MD5 Message Digest Algorithm", RFC 1321, MIT Labo-
     ratory for Computer Science and RSA Data Security, Inc., April
     1992.

[SASL]
     Myers, J., "Simple Authentication and Security Layer (SASL)," RFC
     2222, Netscape Communications, October 1997.

[SASLPrep]
     Zeilenga, K., "SASL String Preparation Profiles", draft-ietf-sasl-
     saslprep (work in progress)

[StringPrep]
     Hoffman, P., M. Blanchet, "Preparation of Internationalized Strings
     (stringprep)", RFC 3454, IMC and Viagenie, December 2002.

[UTF8]
     Yergeau, F., "UTF-8, a transformation format of ISO 10646", RFC
     2279, Alis Technologies, January 1998.

3.2.  Informative References

[IMAP4]
     Crispin, M., "Internet Message Access Protocol - Version 4rev1,"
     RFC 3501, University of Washington, March 2003.


4.  Security Considerations

     It is conjectured that use of the CRAM-MD5 authentication mechanism
     provides replay protection for a session.

     This mechanism does not obscure the user name in any way.  Accord-
     ingly, a server that implements both a clear-text password command
     and this authentication type should not allow both methods of
     access for a given user name.

     Keyed MD5 is chosen for this application because of the greater
     security imparted to authentication of short messages. In addition,
     the use of the techniques described in [KEYED-MD5] for pre-computa-
     tion of intermediate results make it possible to avoid explicit
     clear-text storage of the shared secret on the server system by
     instead storing the intermediate results which are known as "con-
     texts."  While the saving, on the server, of the MD5 "context" is
     marginally better than saving the shared secrets in clear-text, it
     is not sufficient to protect the secrets if the server itself is
     compromised.  Consequently, servers that store the secrets or con-
     texts must both be protected to a level appropriate to the poten-
     tial information value in the data and services protected by this



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     mechanism.  In other words, techniques like this one involve a
     trade-off between vulnerability to network sniffing and I/O buffer
     snooping and vulnerability of the server host's databases.  If one
     believes that the host and its databases are subject to compromise,
     and the network is not, this technique (and all others like it) is
     unattractive.  It is perhaps even less attractive than clear-text
     passwords, which are typically stored on hosts in one-way hash
     form.  On the other hand, if the server databases are perceived as
     reasonably secure, and one is concerned about client-side or net-
     work interception of the passwords (secrets), then this (and simi-
     lar) techniques are preferable to clear-text passwords by a wide
     margin.

     As the length of the shared secret increases, so does the diffi-
     culty of deriving it.

     While there are now suggestions in the literature that the use of
     MD5 and keyed MD5 in authentication procedures probably has a lim-
     ited effective lifetime, the technique is now widely deployed and
     widely understood.  It is believed that this general understanding
     may assist with the rapid replacement, by CRAM-MD5, of the current
     uses of permanent clear-text passwords in many protocols.  This
     document has been deliberately written to permit easy upgrading to
     use SHA (or whatever alternatives emerge) when they are considered
     to be widely available and adequately safe.

     Even with the use of CRAM-MD5, users are still vulnerable to active
     attacks.  An example of an increasingly common active attack is
     'TCP Session Hijacking' as described in CERT Advisory CA-95:01.

     CRAM-MD5 does not authenticate the server and does not include a
     client-supplied nonce.  As a result, it is possible to construct a
     server with a fixed challenge string that has pre-computed the
     hashes for all possible passwords up to a certain length (or from a
     dictionary).  Such a server could then immediately determine the
     user's password if it is sufficiently short.


5.  IANA Considerations

     The SASL Mechanism Registry entry for CRAM-MD5 must be updated to
     reference this specification.


6.  Contributors

     The CRAM-MD5 mechanism was originally specified in RFC 2095,
     IMAP/POP AUTHorize Extension for Simple Challenge/Response.  The
     authors of that document -- John C. Klensin, Paul Krumviede, and
     Randy Catoe -- are to be credited with the design and specification
     of CRAM-MD5. This memo serves only to re-state CRAM-MD5 within the
     formal context of SASL, which specification it preceded by several
     months.




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7.  Intellectual Property

     The IETF takes no position regarding the validity or scope of any
     intellectual property or other rights that might be claimed to per-
     tain 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; neither does it represent that it
     has made any effort to identify any such rights.  Information on
     the IETF's procedures with respect to rights in standards-track and
     standards-related documentation can be found in BCP-11.  Copies of
     claims of rights made available for publication 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 pro-
     prietary rights by implementers or users of this specification can
     be obtained from the IETF Secretariat.

     The IETF invites any interested party to bring to its attention any
     copyrights, patents or patent applications, or other proprietary
     rights which may cover technology that may be required to practice
     this standard.  Please address the information to the IETF Execu-
     tive Director.


8.  Editors' Address

     Lyndon Nerenberg
     Orthanc Systems
     1606 - 10770 Winterburn Road
     Edmonton, Alberta
     Canada T5S 1T6
     Email: lyndon+rfc-crammd5@orthanc.ca


























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9.  Full Copyright Statement

     Copyright 2003, The Internet Society. All Rights Reserved.

     This document and translations of it may be copied and furnished to
     others, and derivative works that comment on or otherwise explain
     it or assist in its implementation may be prepared, copied, pub-
     lished and distributed, in whole or in part, without restriction of
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     ever, this document itself may not be modified in any way, such as
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     This document and the information contained herein is provided on
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