Internet Engineering Task Force (IETF)                       A. Melnikov
Request for Comments: 6331                                 Isode Limited
Obsoletes: 2831                                                July 2011
Category: Informational
ISSN: 2070-1721


                    Moving DIGEST-MD5 to Historic

Abstract

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

Status of This Memo

  This document is not an Internet Standards Track specification; it is
  published for informational purposes.

  This document is a product of the Internet Engineering Task Force
  (IETF).  It represents the consensus of the IETF community.  It has
  received public review and has been approved for publication by the
  Internet Engineering Steering Group (IESG).  Not all documents
  approved by the IESG are a candidate for any level of Internet
  Standard; see Section 2 of RFC 5741.

  Information about the current status of this document, any errata,
  and how to provide feedback on it may be obtained at
  http://www.rfc-editor.org/info/rfc6331.

Copyright Notice

  Copyright (c) 2011 IETF Trust and the persons identified as the
  document authors.  All rights reserved.

  This document is subject to BCP 78 and the IETF Trust's Legal
  Provisions Relating to IETF Documents
  (http://trustee.ietf.org/license-info) in effect on the date of
  publication of this document.  Please review these documents
  carefully, as they describe your rights and restrictions with respect
  to this document.  Code Components extracted from this document must
  include Simplified BSD License text as described in Section 4.e of
  the Trust Legal Provisions and are provided without warranty as
  described in the Simplified BSD License.





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  This document may contain material from IETF Documents or IETF
  Contributions published or made publicly available before November
  10, 2008.  The person(s) controlling the copyright in some of this
  material may not have granted the IETF Trust the right to allow
  modifications of such material outside the IETF Standards Process.
  Without obtaining an adequate license from the person(s) controlling
  the copyright in such materials, this document may not be modified
  outside the IETF Standards Process, and derivative works of it may
  not be created outside the IETF Standards Process, except to format
  it for publication as an RFC or to translate it into languages other
  than English.

Table of Contents

  1.    Introduction and Overview . . . . . . . . . . . . . . . . . . 2
  2.    Security Considerations . . . . . . . . . . . . . . . . . . . 5
  3.    IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5
  4.    Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . 5
  5.    References  . . . . . . . . . . . . . . . . . . . . . . . . . 5
  5.1.  Normative References  . . . . . . . . . . . . . . . . . . . . 5
  5.2.  Informative References  . . . . . . . . . . . . . . . . . . . 5

1.  Introduction and Overview

  [RFC2831] defines 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, email, the
  Lightweight Directory Access Protocol (LDAP), and other protocols.
  While it can be argued that it is an improvement over CRAM-MD5, many
  implementors commented that the additional complexity of DIGEST-MD5
  makes it difficult to implement fully and securely.

  Below is an incomplete list of problems with the DIGEST-MD5 mechanism
  as specified in [RFC2831]:

  1.  The mechanism has too many options and modes.  Some of them are
      not well described and are not widely implemented.  For example,
      DIGEST-MD5 allows the "qop" directive to contain multiple values,
      but it also allows for multiple qop directives to be specified.
      The handling of multiple options is not specified, which results
      in minor interoperability problems.  Some implementations
      amalgamate multiple qop values into one, while others treat
      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 is not clear on whether



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      the authzid must be omitted or if it can be specified with an
      empty value to convey this.  The requirement for backward
      compatibility with HTTP Digest means that the situation is even
      worse.  For example, DIGEST-MD5 requires all usernames/passwords
      that can be entirely represented in the ISO-8859-1 charset to be
      down converted from UTF-8 [RFC3629] to ISO-8859-1 [ISO-8859-1].
      Another example is the use of quoted strings.  Handling of
      characters that need escaping is not properly described, and the
      DIGEST-MD5 document has no examples to demonstrate correct
      behavior.

  2.  The DIGEST-MD5 document uses 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 a
      more common ABNF defined in [RFC5234] is confusing for some
      implementors.  As a result, many implementations do not accept
      folding whitespace in many places where it is 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 does not provide any
      guidance on how realms should be named and, more importantly, how
      they can be entered in User Interfaces (UIs).  As a result, many
      DIGEST-MD5 clients have confusing UIs, do not allow users to
      enter a realm, and/or do not allow users to pick one of the
      server-supported realms.

  4.  Use of username in the inner hash is problematic.  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.  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 is rarely done in practice.  First,
      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 [DES] and RC4 security layers are
      inadequate to produce independently developed interoperable
      implementations.  In the DES/3DES case, this is partly a problem
      with existing DES APIs.

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



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

      A.  Channel bindings [RFC5056].

      B.  Hash agility (i.e., no easy way to replace the MD5 hash
          function with another one).

      C.  Support for SASLPrep [RFC4013] or any other type of Unicode
          character normalization of usernames and passwords.  The
          original DIGEST-MD5 document predates SASLPrep and does not
          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 [RFC6151].

      B.  The RC4 algorithm is prone to attack when used as the
          security layer without discarding the initial key stream
          output [RFC6229].

      C.  The DES cipher for the security layer is considered insecure
          due to its small key space [RFC3766].

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

  Because DIGEST-MD5 is defined as an extensible mechanism, it is
  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 is not worth the cost.  In
  addition, an implementation of a "fixed" DIGEST-MD5 specification
  would likely either not interoperate with any existing implementation
  of [RFC2831] 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.

  The Salted Challenge Response Authentication Mechanism (SCRAM) family
  of SASL mechanisms [RFC5802] has been developed to provide similar
  features as DIGEST-MD5 but with a better design.





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2.  Security Considerations

  Security issues are discussed throughout this document.

3.  IANA Considerations

  IANA has changed 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:

     http://www.iana.org/assignments/sasl-mechanisms

4.  Acknowledgements

  The author gratefully acknowledges the feedback provided by Chris
  Newman, Simon Josefsson, Kurt Zeilenga, Sean Turner, and Abhijit
  Menon-Sen.  Various text was copied from other RFCs, in particular,
  from [RFC2831].

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.

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

5.2.  Informative References

  [DES]         National Institute of Standards and Technology, "Data
                Encryption Standard (DES)", FIPS PUB 46-3,
                October 1999.

  [ISO-8859-1]  International Organization for Standardization,
                "Information technology - 8-bit single-byte coded
                graphic character sets - Part 1: Latin alphabet No. 1",
                ISO/IEC 8859-1, 1998.

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






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

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

  [RFC3766]     Orman, H. and P. Hoffman, "Determining Strengths For
                Public Keys Used For Exchanging Symmetric Keys",
                BCP 86, RFC 3766, April 2004.

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

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

  [RFC5056]     Williams, N., "On the Use of Channel Bindings to Secure
                Channels", RFC 5056, November 2007.

  [RFC5234]     Crocker, D. and P. Overell, "Augmented BNF for Syntax
                Specifications: ABNF", STD 68, RFC 5234, January 2008.

  [RFC5802]     Newman, C., Menon-Sen, A., Melnikov, A., and N.
                Williams, "Salted Challenge Response Authentication
                Mechanism (SCRAM) SASL and GSS-API Mechanisms",
                RFC 5802, July 2010.

  [RFC6151]     Turner, S. and L. Chen, "Updated Security
                Considerations for the MD5 Message-Digest and the HMAC-
                MD5 Algorithms", RFC 6151, March 2011.

  [RFC6229]     Strombergson, J. and S. Josefsson, "Test Vectors for
                the Stream Cipher RC4", RFC 6229, May 2011.

Author's Address

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

  EMail: [email protected]
  URI:   http://www.melnikov.ca/





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