

Network Working Group                               L. Nerenberg, Editor
Internet Draft: The CRAM-MD5 SASL Mechanism              Orthanc Systems
Document: draft-nerenberg-sasl-crammd5-03.txt              November 2002



                      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
     Task Force (IETF), its areas, and its working groups.  Note that
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     A revised version of this draft document will be submitted to the
     RFC editor as a Proposed Standard for the Internet Community.
     Discussion and suggestions for improvement are requested.
     Distribution of this draft is unlimited.


Copyright Notice

     Copyright (C) The Internet Society (2002).  All Rights Reserved.

How to Read This Document

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


1.  Introduction

     This document defines a simple challenge-response [SASL]
     authentication mechanism, using a [KEYED-MD5] digest.






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2.  CRAM-MD5 Authentication Mechanism

     The mechanism name associated with CRAM-MD5 is 'CRAM-MD5'.

     This mechanism does not provide a security layer.

     The data encoded in the challenge contains a presumptively
     arbitrary string of random digits, a timestamp, and the fully-
     qualified 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
     challenge (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
     character, 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 user name and shared secret MUST be represented in the Unicode
     character set [UNICODE], and MUST be normalised using the Unicode
     Normalisation Form KC [NFKC]. 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






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

          S: * OK IMAP4rev1 Server
          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.reston.mci.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











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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.

     [KEYWORDS]
          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
          Laboratory for Computer Science and RSA Data Security, Inc.,
          April 1992.

     [NFKC]
          Davis, M., M. Durst, "Unicode Standard Annex #15: Unicode
          Normalisation Forms", An integral part of The Unicode
          Standard, Version 3.2.0
          (http://www.unicode.org/reports/tr15/).

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

     [UNICODE]
          The Unicode Consortium, The Unicode Standard, Version 3.2.0,
          defined by: The Unicode Standard, Version 3.0 (Reading, MA,
          Addison-Wesley, 2000. ISBN 0-201-61633-5), as amended by the
          Unicode Standard Annex #27: Unicode 3.1
          (http://www.unicode.org/reports/tr27/) and the Unicode
          Standard Annex #28: Unicode 3.2
          (http://www.unicode.org/reports/tr28/)

     [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," Work in progress (son of RFC2060)






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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.
     Accordingly, a server that implements both a cleartext 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
     precomputation of intermediate results make it possible to avoid
     explicit cleartext storage of the shared secret on the server
     system by instead storing the intermediate results which are known
     as "contexts."

     While the saving, on the server, of the MD5 "context" is marginally
     better than saving the shared secrets in cleartext, it is not
     sufficient to protect the secrets if the server itself is
     compromised.  Consequently, servers that store the secrets or
     contexts must both be protected to a level appropriate to the
     potential information value in the data and services protected by
     this mechanism.  In other words, techniques like this one involve a
     tradeoff 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 cleartext
     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
     network interception of the passwords (secrets), then this (and
     similar) techniques are preferable to clear-text passwords by a
     wide margin.

     As the length of the shared secret increases, so does the
     difficulty 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
     limited 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 cleartext 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.




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5.  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 preceeded by several
     months.


6.  Intellectual Property

     The IETF takes no position regarding the validity or scope of any
     intellectual property 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; 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
     proprietary rights by implementors 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
     Executive Director.


7.  Authors' Address

     Lyndon Nerenberg
     Orthanc Systems
     508 - 11025 Jasper Avenue
     Edmonton, Alberta
     Canada T5K 0K7
     Email: lyndon@orthanc.ab.ca















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8.  Full Copyright Statement

     Copyright (C) The Internet Society (2002). 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 implmentation may be prepared, copied,
     published and distributed, in whole or in part, without restriction
     of any kind, provided that the above copyright notice and this
     paragraph are included on all such copies and derivative works.
     However, this document itself may not be modified in any way, such
     as by removing the copyright notice or references to the Internet
     Society or other Internet organizations, except as needed for the
     purpose of developing Internet standards in which case the
     procedures for copyrights defined in the Internet Standards process
     must be followed, or as required to translate it into languages
     other than English.
     The limited permissions granted above are perpetual and will not be
     revoked by the Internet Society or its successors or assigns.

     This document and the information contained herein is provided on
     an "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET
     ENGINEERING TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR
     IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF
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     WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.































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