SASL Working Group                                     L. Nerenberg, Ed.
Internet-Draft                                           Orthanc Systems
Obsoletes: RFC2195                                         July 11, 2008
(if approved)
Intended status: Standards Track
Expires: January 12, 2009

                      The CRAM-MD5 SASL Mechanism

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   This document defines a simple challenge-response authentication
   mechanism, using a keyed MD5 digest, for use with the Simple
   Authentication and Security Layer (SASL).

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  The CRAM-MD5 SASL Mechanism  . . . . . . . . . . . . . . . . .  3
   3.  Formal Grammar . . . . . . . . . . . . . . . . . . . . . . . .  3
   4.  Interoperability Considerations  . . . . . . . . . . . . . . .  4
   5.  Security Considerations  . . . . . . . . . . . . . . . . . . .  5
   6.  References . . . . . . . . . . . . . . . . . . . . . . . . . .  6
     6.1.  Normative References . . . . . . . . . . . . . . . . . . .  6
     6.2.  Informative References . . . . . . . . . . . . . . . . . .  6
   Appendix A.  Examples  . . . . . . . . . . . . . . . . . . . . . .  7
     A.1.  IMAP4  . . . . . . . . . . . . . . . . . . . . . . . . . .  7
       A.1.1.  Example 1: Simple IMAP . . . . . . . . . . . . . . . .  7
       A.1.2.  Example 2: IMAP4 with embedded spaces  . . . . . . . .  8
       A.1.3.  Example 3: IMAP4 with Unicode characters . . . . . . .  8
     A.2.  ACAP . . . . . . . . . . . . . . . . . . . . . . . . . . .  8
       A.2.1.  Example 4: Simple ACAP . . . . . . . . . . . . . . . .  8
   Appendix B.  IANA Considerations . . . . . . . . . . . . . . . . .  9
   Appendix C.  Contributors  . . . . . . . . . . . . . . . . . . . .  9
   Appendix D.  Changes since RFC 2195  . . . . . . . . . . . . . . .  9
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . .  9
   Intellectual Property and Copyright Statements . . . . . . . . . . 10

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1.  Introduction

   This document defines a simple challenge-response authentication
   method, using a keyed MD5 [RFC2104] digest, for use with the Simple
   Security and Authentication Layer (SASL) [RFC4422].  The mechanism
   name associated with CRAM-MD5 is 'CRAM-MD5'.

   This mechanism does not provide a security layer.

   The CRAM-MD5 mechanism is intended to have limited use on the
   Internet.  The mechanism offers inadequate protection against common
   attacks against application-level protocols (see Section 5) and is
   prone to interoperability problems (see Section 4).

2.  The CRAM-MD5 SASL Mechanism

   The mechanism starts with the server issuing a <challenge>.  The data
   contained in the challenge contains a string of random data.

   The client makes note of the data and then responds with a <response>
   consisting of the <username>, a space, and a <digest>.  The digest is
   computed by applying the keyed MD5 algorithm from [RFC2104] 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 a
   restricted hexadecimal format (see the <digest> production in
   Section 3).

   When the server receives this client response, it verifies the digest
   provided.  Since the user name may contain the space character, the
   server MUST ensure the right-most space character is recognised as
   the token separating the user name from the digest.  If the digest is
   correct, the server should consider the client authenticated.

3.  Formal Grammar

   The following grammar specification uses the Augmented Backus-Naur
   Form (ABNF) as specified in [RFC4234], and incorporates by reference
   the Core Rules defined in that document.

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     challenge  = "<" 3*(%x21-3B / %x3D / %x3F-7E) ">"
                  ; a bracketed string of printing ASCII characters, not
                  ; containing embedded "<" or ">"

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

     response   = username SP digest

     username   = 1*OCTET
                  ; SHOULD be well-formed UTF-8

4.  Interoperability Considerations

   The design of CRAM-MD5 [RFC2095] pre-dated any widespread use of
   UTF-8 to encode protocol elements.  It was initially deployed as an
   extension to the IMAP4 protocol at a time when authentication and
   authorization identifiers were almost exclusively encoded in the US-
   ASCII character set, therefore it is silent about the encoding and
   representation of non-US-ASCII data elements.  When sites first began
   using alternate character sets to encode user names (and passwords)
   they simply used the raw 8-bit character representation.  This works
   - for the most part - but only because these enclaves tend to use a
   common character set amongst themselves.  When a second group of
   users using a different character set is introduced into the mix,
   interoperability suffers.

   So as not to render existing implementations non-compliant this
   update preserves the existing opaque nature of user names and
   passwords.  However, implementors are strongly encouraged to process
   the user name and password data as described in the next paragraph.
   Doing so prevents interoperability problems caused by incompatible
   character set encodings.

   The client SHOULD prepare the user name and shared secret strings
   using the SASLprep [RFC4013] profile of the Stringprep [RFC3454]
   algorithm.  The resulting values SHOULD be encoded as UTF-8 [RFC3629]
   strings.  The server may store the prepared string instead of, or as
   well as, the unprepared string, so that it does not have to prepare
   it every time it is needed for computation.  However, if the original
   (unprepared) string is not stored, it may render the computed secret
   to be incompatible with a future revisions of SASLprep that support
   currently unassigned code points (see section 7 of [RFC3454]).  It is
   therefor recommended to store the unprepared string in the database.

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5.  Security Considerations

   CRAM-MD5 is no longer considered to provide adequate protection.

   This mechanism is vulnerable to dictionary attack by any passive
   listener able to observe the user name, challenge and response.  An
   attacker can use the user name and challenge to compute a series of
   responses based on a pass-phrase dictionary, looking for a match to
   the response sent by the client.

   CRAM-MD5 does not authenticate the server and does not include a
   client-supplied nonce.  Consequently, 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 or non-random.

   This mechanism does not obscure the user name in any way.
   Accordingly, 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.

   For the reasons described above, CRAM-MD5 SHOULD NOT be used unless
   the application protocol session is protected by an encryption layer,
   such as provided by TLS.

   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 [RFC2104] for pre-computation
   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 "contexts."  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 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 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 network interception of the
   passwords (secrets), then this (and similar) techniques are

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   preferable to clear-text passwords by a wide margin.

   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 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.

6.  References

6.1.  Normative References

   [RFC2104]  Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
              Hashing for Message Authentication", RFC 2104,
              February 1997.

   [RFC3454]  Hoffman, P. and M. Blanchet, "Preparation of
              Internationalized Strings ("stringprep")", RFC 3454,
              December 2002.

   [RFC3629]  Yergeau, F., "UTF-8, a transformation format of ISO
              10646", STD 63, RFC 3629, November 2003.

   [RFC4013]  Zeilenga, K., "SASLprep: Stringprep Profile for User Names
              and Passwords", RFC 4013, February 2005.

   [RFC4234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", RFC 4234, October 2005.

   [RFC4422]  Melnikov, A. and K. Zeilenga, "Simple Authentication and
              Security Layer (SASL)", RFC 4422, June 2006.

6.2.  Informative References

   [RFC2095]  Klensin, J., Catoe, R., and P. Krumviede, "IMAP/POP
              AUTHorize Extension for Simple Challenge/Response",
              RFC 2095, January 1997.

   [RFC2244]  Newman, C. and J. Myers, "ACAP -- Application

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              Configuration Access Protocol", RFC 2244, November 1997.

              4rev1", RFC 3501, March 2003.

   [RFC4616]  Zeilenga, K., "The PLAIN Simple Authentication and
              Security Layer (SASL) Mechanism", RFC 4616, August 2006.

Appendix A.  Examples

   The examples in this appendix DO NOT form part of the specification.
   Where conflicts exist between the examples and the formal grammar or
   the normative text in Section 2, the latter are authoritative.

A.1.  IMAP4

   These examples show the use of the CRAM-MD5 mechanism with the IMAP4
   [RFC3501] AUTHENTICATE command.  The base64 encoding of the
   challenges and responses is part of the IMAP4 AUTHENTICATE command,
   and not part of the CRAM-MD5 specification itself.

A.1.1.  Example 1: Simple IMAP

   In this example the shared secret is the string 'tanstaaftanstaaf'.

     S: + PDE4OTYuNjk3MTcwOTUyQHBvc3RvZmZpY2UuZXhhbXBsZS5uZXQ+
     C: am9lIDNkYmM4OGYwNjI0Nzc2YTczN2IzOTA5M2Y2ZWI2NDI3
     S: A0001 OK CRAM-MD5 authentication successful

   Hence, the keyed MD5 digest is produced by calculating

     MD5((SASLprep(tanstaaftanstaaf) XOR opad),
         MD5((SASLprep(tanstaaftanstaaf) XOR ipad),

   where ipad and opad are as defined in RFC 2104 and the string shown
   in the challenge is the base64 encoding of
   '<>'.  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
   '3dbc88f0624776a737b39093f6eb6427'.  The user name is then prepended

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   to it, forming 'joe 3dbc88f0624776a737b39093f6eb6427', which is then
   base64 encoded to meet the requirements of the IMAP4 AUTHENTICATE
   command yielding 'am9lIDNkYmM4OGYwNjI0Nzc2YTczN2IzOTA5M2Y2ZWI2NDI3'.

A.1.2.  Example 2: IMAP4 with embedded spaces

   This example uses the user name 'Ali Baba' and the shared secret
   'Open, Sesame'.  It illustrates that both user names and passwords
   may contain non-alphanumeric characters.

     S: <68451038525716401353.0@localhost>
     C: Ali Baba 6fa32b6e768f073132588e3418e00f71

A.1.3.  Example 3: IMAP4 with Unicode characters

   This example demonstrates the processing of Unicode strings.  The raw
   user name is 'Al<U+00AA>dd<U+00AD>in<U+00AE>' where <U+00AA> is the
   Unicode Latin symbol <FEMININE ORDINAL INDICATOR>, <U+00AD> is <SOFT
   HYPHEN>, and <U+00AE> is the <REGISTERED SIGN>.  Preparing the raw
   user name with SASLprep returns 'Aladdin<U+00AE>' which we then
   encode into the UTF-8 string 'Aladdin\xC2\xAE' (shown here and below
   using C-style string format notation).  As before, the shared secret
   is 'Open, Sesame'.

     S: <92230559549732219941.0@localhost>
     C: Aladdin\xC2\xAE 9950ea407844a71e2f0cd3284cbd912d

A.2.  ACAP

   An example of using CRAM-MD5 with ACAP [RFC2244].

A.2.1.  Example 4: Simple ACAP

   This example uses the user name 'joe' and the shared secret

    S: * ACAP (IMPLEMENTATION "Infotrope ACAP Server, version 0.1.3,
        Copyright 2002-2004 Dave Cridland <>")
    S: + {43}
    S: <>
    C: {36+}
    C: joe 2aa383bf320a941d8209a7001ef6aeb6
    S: AUTH OK "You're logged in as joe. Frooby."

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Appendix B.  IANA Considerations

   It is requested that the Internet Assigned Numbers Authority (IANA)
   update the SASL Mechanism Registry entry for CRAM-MD5 to refer to
   this document.

   Subject: Updated Registration of SASL CRAM-MD5 mechanism.

   SASL mechanism name: CRAM-MD5
   Security considerations: See RFC XXXX
   Published specification: RFC XXXX
   Person & email address to contact for further information:
       Lyndon Nerenberg <>
       IETF SASL WG     <>

Appendix C.  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, and
   they are the original authors of the majority of the text in this
   document.  This memo serves only to re-state CRAM-MD5 within the
   formal context of SASL, which specification it preceded by several

   Dave Cridland and Simon Josefsson contributed updated examples.

Appendix D.  Changes since RFC 2195

   The syntax of the <challenge> has been relaxed.

   A section on interoperability concerns has been added.

   The security considerations have been updated to reflect the current
   views of the security community.

Author's Address

   Lyndon Nerenberg (editor)
   Orthanc Systems


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