SASL Working Group                                           A. Melnikov
Internet-Draft                                             Isode Limited
Intended status: Informational                             July 29, 2008
Expires: January 30, 2009

                     Moving DIGEST-MD5 to Historic

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   This memo describes problems with the DIGEST-MD5 Simple
   Authentication and Security Layer (SASL) mechanism as specified in
   RFC 2831.  It recommends that DIGEST-MD5 to be marked as OBSOLETE in
   the IANA Registry of SASL mechanisms, and that RFC 2831 be moved to
   Historic status.


   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, and should be sent to

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Table of Contents

   1.    Overview  . . . . . . . . . . . . . . . . . . . . . . . . . . 3

   2.    Security Considerations . . . . . . . . . . . . . . . . . . . 5

   3.    IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5

   4.    Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . 5

   5.    References  . . . . . . . . . . . . . . . . . . . . . . . . . 5
   5.1.  Normative References  . . . . . . . . . . . . . . . . . . . . 5
   5.2.  Informative References  . . . . . . . . . . . . . . . . . . . 6

         Author's Address  . . . . . . . . . . . . . . . . . . . . . . 6
         Intellectual Property and Copyright Statements  . . . . . . . 7

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1.  Overview

   [RFC2831] defined how HTTP Digest Authentication [RFC2617] can be
   used as a Simple Authentication and Security Layer (SASL) [RFC4422]
   mechanism for any protocol that has a SASL profile.  It was intended
   both as an improvement over CRAM-MD5 [RFC2195] and as a convenient
   way to support a single authentication mechanism for web, mail, LDAP,
   and other protocols.  While it can be argued that it was an
   improvement over CRAM-MD5, many implementors commented that the
   additional complexity of DIGEST-MD5 made it difficult to implement
   fully and securely.

   Below is an incomplete list of problems with DIGEST-MD5 mechanism as
   specified in RFC 2831:

   1.  The mechanism had too many options and modes.  Some of them were
       not well described and were not widely implemented.  For example,
       DIGEST-MD5 allowed the "qop" directive to contain multiple
       values, but it also allowed for multiple qop directives to be
       specified.  The handling of multiple options was not specified,
       which resulted in minor interoperability problems.  Some
       implementations amalgamated multiple qop values into one, while
       others treated multiple qops as an error.  Another example is the
       use of an empty authorization identity.  In SASL an empty
       authorization identity means that the client is willing to
       authorize as the authentication identity.  The document was not
       clear on whether the authzid must be omitted or can be specified
       with the empty value to convey this.  The requirement for
       backward compatibility with HTTP Digest meant that the situation
       was even worse.  For example DIGEST-MD5 required all usernames/
       passwords which can be entirely represented in ISO-8859-1 charset
       to be down converted from UTF-8 to ISO-8859-1.  Another example
       is use of quoted strings.  Handling of characters that needed
       escaping was not properly described and the DIGEST-MD5 document
       had no examples to demonstrate correct behavior.

   2.  The document used ABNF from RFC 822 [RFC0822], which allows an
       extra construct and allows for "implied folding whitespace" to be
       inserted in many places.  The difference from ABNF [RFC4234] was
       confusing for some implementors.  As a result, many
       implementations didn't accept folding whitespace in many places
       where it was allowed.

   3.  The DIGEST-MD5 document uses the concept of a "realm" to define a
       collection of accounts.  A DIGEST-MD5 server can support one or
       more realms.  The DIGEST-MD5 document didn't provide any guidance
       on how realms should be named, and, more importantly, how they
       can be entered in User Interfaces (UIs).  As the result many

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       DIGEST-MD5 clients had confusing UIs, didn't allow users to enter
       a realm and/or didn't allow users to pick one of the server
       supported realms.

   4.  Use of username in the inner hash.  The inner hash of DIGEST-MD5
       is an MD5 hash of colon separated username, realm and password.
       Implementations may choose to store inner hashes instead of clear
       text passwords.  While this has some useful properties, such as
       protection from compromise of authentication databases containing
       the same username and password on other servers, if a server with
       the username and password is compromised, however this was rarely
       done in practice.  Firstly, the inner hash is not compatible with
       widely deployed Unix password databases, and second, changing the
       username would invalidate the inner hash.

   5.  Description of DES/3DES and RC4 security layers are inadequate to
       produce independently-developed interoperable implementations.
       In the DES/3DES case this was partly a problem with existing DES

   6.  DIGEST-MD5 outer hash (the value of the "response" directive)
       didn't protect the whole authentication exchange, which made the
       mechanism vulnerable to "man in the middle" (MITM) attacks, such
       as modification of the list of supported qops or ciphers.

   7.  The following features are missing from DIGEST-MD5, which make it
       insecure or unsuitable for use in protocols:

       A.  Lack of channel bindings.

       B.  Lack of hash agility.

       C.  Lack of SASLPrep [RFC4013] support.  The original DIGEST-MD5
           document predates SASLPrep and doesn't recommend any Unicode
           character normalization.

   8.  The cryptographic primitives in DIGEST-MD5 are not up to today's
       standards, in particular:

       A.  The MD5 hash is sufficiently weak to make a brute force
           attack on DIGEST-MD5 easy with common hardware.

       B.  Using the RC4 algorithm for the security layer without
           discarding the initial key stream output is prone to attack.

   Note that most of the problems listed above are already present in
   the HTTP Digest authentication mechanism.

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   Because DIGEST-MD5 was defined as an extensible mechanism, it would
   be possible to fix most of the problems listed above.  However this
   would increase implementation complexity of an already complex
   mechanism even further, so the effort would not be worth the cost.
   In addition, an implementation of a "fixed" DIGEST-MD5 specification
   would likely either not interoperate with any existing implementation
   of RFC 2831, or would be vulnerable to various downgrade attacks.

   Note that despite DIGEST-MD5 seeing some deployment on the Internet,
   this specification recommends obsoleting DIGEST-MD5 because DIGEST-
   MD5, as implemented, is not a reasonable candidate for further
   standardization and should be deprecated in favor of one or more new
   password-based mechanisms currently being designed.

2.  Security Considerations

   Security issues are discussed through out this document.

3.  IANA Considerations

   IANA is requested to change the "Intended usage" of the DIGEST-MD5
   mechanism registration in the SASL mechanism registry to OBSOLETE.
   The SASL mechanism registry is specified in [RFC4422] and is
   currently available at:

4.  Acknowledgements

   The author gratefully acknowledges the feedback provided by Chris
   Newman, Simon Josefsson, Kurt Zeilenga and Abhijit Menon-Sen.
   [[anchor3: Various text was copied from other RFCs.]]

5.  References

5.1.  Normative References

   [RFC2617]  Franks, J., Hallam-Baker, P., Hostetler, J., Lawrence, S.,
              Leach, P., Luotonen, A., and L. Stewart, "HTTP
              Authentication: Basic and Digest Access Authentication",
              RFC 2617, June 1999.

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   [RFC2831]  Leach, P. and C. Newman, "Using Digest Authentication as a
              SASL Mechanism", RFC 2831, May 2000.

5.2.  Informative References

   [RFC0822]  Crocker, D., "Standard for the format of ARPA Internet
              text messages", STD 11, RFC 822, August 1982.

   [RFC2195]  Klensin, J., Catoe, R., and P. Krumviede, "IMAP/POP
              AUTHorize Extension for Simple Challenge/Response",
              RFC 2195, September 1997.

   [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.

Author's Address

   Alexey Melnikov
   Isode Limited
   5 Castle Business Village
   36 Station Road
   Hampton, Middlesex  TW12 2BX


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