Network Working Group                                         S. Turner
Internet Draft                                                     IECA
Updates: 1319 (once approved)                                   L. Chen
Intended Status: Informational                                     NIST
Expires: June 3, 2011                                  December 3, 2010



                          MD2 to Historic Status
                    draft-turner-md2-to-historic-07.txt

Abstract

   This document recommends the retirement of MD2 and discusses the
   reasons for doing so.  This document recommends RFC 1319 be moved to
   Historic status.

Status of this Memo

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Copyright Notice

   Copyright (c) 2010 IETF Trust and the persons identified as the
   document authors. All rights reserved.





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   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
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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

1. Introduction

   MD2 [MD2] is a message digest algorithm that takes as input a message
   of arbitrary length and produces as output a 128-bit "fingerprint" or
   "message digest" of the input.  This document recommends that MD2 be
   retired.  Specifically, this document recommends RFC 1319 [MD2] be
   moved to Historic status.  The reasons for taking this action are
   discussed.

   [HASH-Attack] summarizes the use of hashes in many protocols and
   discusses how attacks against a message digest algorithm's one-way
   and collision-free properties affect and do not affect Internet
   protocols.  Familiarity with [HASH-Attack] is assumed.

2. Rationale

   MD2 was published in 1992 as an Informational RFC.  Since its
   publication, MD2 has been shown to not be collision-free [ROCH1995]
   [KNMA2005] [ROCH1997], albeit successful collision attacks for
   properly implemented MD2 are not that damaging. MD2 has also been
   shown to have successful pre-image and second-preimage attacks
   [KNMA2005] [MULL2004] [KMM2010].

3. Documents that Reference RFC 1319

   MD2 has been specified in the following RFCs:

   Proposed Standard (PS):

      o [RFC3279] Algorithms and Identifiers for the Internet X.509
        Public Key Infrastructure Certificate and Certificate
        Revocation List (CRL) Profile.

      o [RFC4572] Connection-Oriented Media Transport over the
        Transport Layer Security (TLS) Protocol in the Session
        Description Protocol (SDP).



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   Informational:

      o [RFC1983] Internet Users' Glossary.

      o [RFC2315] PKCS #7: Cryptographic Message Syntax Version 1.5.

      o [RFC2898] PKCS #5: Password-Based Cryptography Specification
        Version 2.0.

      o [RFC3447] Public-Key Cryptography Standards (PKCS) #1: RSA
        Cryptography Specifications Version 2.1.

   Experimental:

      o [RFC2660] The Secure HyperText Transfer Protocol.

   There are other RFCs that refer to MD2, but their status is either
   Historic or Obsoleted.  References and discussions about these RFCs
   are omitted.  The exceptions are:

      o [RFC2313] PKCS #1: RSA Encryption Version 1.5.

      o [RFC2437] PKCS #1: RSA Cryptography Specifications Version 2.0.

4. Impact of Moving MD2 to Historic.

   The impact of moving MD2 to Historic on the RFCs specified in Section
   3 is minimal, as described below.

   Regarding PS RFCs:

      o MD2 support in TLS was dropped in TLS 1.1.

      o MD2 support is optional in [RFC4572], and SHA-1 is specified as
        the preferred algorithm.

      o MD2 is included in the original PKIX certificate profile and
        the PKIX algorithm document [RFC3279] for compatibility with
        older applications, but its use is discouraged.  SHA-1 is
        identified as the preferred algorithm for the Internet PKI.

   Regarding Informational RFCs:

      o The Internet Users' Guide [RFC1983] provided a definition for
        Message Digest and listed MD2 as one example.

      o PKCS#1 v1.5 [RFC2313] stated that there are no known attacks
        against MD2.  PKCS#1 v2.0 [RFC2437] updated this stance to

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        indicate that MD2 should only be supported for backward
        compatibility and to mention the attacks in [ROCH1995].  PKCS#1
        [RFC3447] indicates that support MD2 is only retained for
        compatibility with existing applications.

      o PKCS#5 [RFC2898] recommends that the Password Based Encryption
        Scheme (PBES) that uses MD2 not be used for new applications.

      o PKCS#7 [RFC2315] was replaced by a series of standards track
        publications, "Cryptographic Message Syntax" [RFC2630]
        [RFC3369] [RFC5652] and "Cryptographic Message Syntax (CMS)
        Algorithms" [RFC3370]. Support for MD2 was dropped in
        [RFC3370].

   RFC 2818 "HTTP Over TLS", which does not reference MD2, largely
   supplanted implementation of [RFC2660].  [RFC2660] specified MD2 for
   use both as a digest algorithm as a MAC algorithm [RFC2104].  Note
   that this is the only reference to HMAC-MD2 found in the RFC
   repository.

5. Other Considerations

   MD2 has also fallen out of favor because it is slower than both MD4
   [MD4] and MD5 [MD5].  This is because MD2 was optimized for 8-bit
   machines while MD4 and MD5 were optimized for 32-bit machines.  MD2
   is also slower than the Secure Hash Standard (SHS) [SHS] algorithms:
   SHA-1, SHA-224, SHA-256, SHA-384, and SHA-512.

6. Security Considerations

   MD2 is different from MD4 and MD5 in that is not a straight Merkle-
   Damgaard design. For a padded message with t blocks, it generates a
   nonlinear checksum as its t+1 block.  The checksum is considered as
   the final block input of MD2.

   As confirmed in 1997 by Rogier et. al. [ROCH1997], the collision
   resistance property of MD2 highly depends on the nonlinear checksum.
   Without the checksum, a collision can be found in 2^12 MD2 operations
   according, while with the checksum, the best collision attack takes
   2^63.3 operations with 2^50 memory complexity [MULL2004], which is
   not significantly better than the birthday attack.

   Even though collision attacks on MD2 are not significantly more
   powerful than the birthday attack, MD2 was found not to be one-way.
   In [KMM2010], a pre-image can be found with 2^104 MD2 operations. In
   an improved attack described in [KMM2010], a pre-image can be found
   in 2^73 MD2 operations.  Because of this "invertible" property of
   MD2, when using MD2 in HMAC, it may leak information of the keys.

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   Obviously, the pre-image attack can be used to find a second pre-
   image.  The second pre-image attack is even more severe than a
   collision attack to digital signatures.  Therefore, MD2 must not be
   used for digital signatures.

   Some may find the guidance for key lengths and algorithm strengths in
   [SP800-57] and [SP800-131] useful.

7. Recommendation

   Despite MD2 seeing some deployment on the Internet, this
   specification recommends obsoleting MD2.  MD2 is not a reasonable
   candidate for further standardization and should be deprecated in
   favor of one or more existing hash algorithms (e.g., SHA-256 [SHS]).

   RSA Security considers it appropriate to move the MD2 algorithm to
   Historic status.

   It takes a number of years to deploy crypto and it also takes a
   number of years to withdraw it.  Algorithms need to be withdrawn
   before a catastrophic break is discovered.  MD2 is clearly showing
   signs of weakness and implementations should strongly consider
   removing support and migrating to another hash algorithm.

8. IANA Considerations

   None.

9. Acknowledgements

   We'd like to thank RSA for publishing MD2.  We'd also like to thank
   all the cryptographers who studied the algorithm.  For his
   contribution to this draft we'd like to thank Ran Atkinson, Alfred
   Hoenes, John Linn, and Martin Rex.

10. Informative References

   [HASH-Attack]    Hoffman, P., and B. Schneier, "Attacks on
                    Cryptographic Hashes in Internet Protocols", RFC
                    4270, November 2005.

   [KNMA2005]       Knudsen, L., and J. Mathiassen, "Preimage and
                    Collision Attacks on MD2," FSE 2005.

   [KMM2010]        Knudsen, L., Mathiassen, J., Muller, F., and
                    Thomsen, S., "Cryptanalysis of MD2", Journal of
                    Cryptology, 23(1):72-90, 2010.


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   [MD2]            Rivest, R., "The MD5 Message-Digest Algorithm", RFC
                    1319, April 1992.

   [MULL2004]       Muller, F., "The MD2 Hash Function Is Not One-Way",
                    ASIACRYPT, LNCS 3329, pp. 214-229, Springer, 2004.

   [MD4]            Rivest, R., "The MD4 Message-Digest Algorithm", RFC
                    1320, April 1992.

   [MD5]            Rivest, R., "The MD5 Message-Digest Algorithm", RFC
                    1321, April 1992.

   [RFC1983]        Malkin, G., "Internet Users' Glossary", RFC 1983,
                    August 1996.

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

   [RFC2313]        Kaliski, B., "PKCS #1: RSA Encryption Version 1.5",
                    RFC 2313, March 1998.

   [RFC2315]        Kaliski, B., "PKCS #7: Cryptographic Message Syntax
                    Version 1.5," RFC 2315, March 1998.

   [RFC2437]        Kaliski, B., and J. Staddon, "PKCS #1: RSA
                    Cryptography Specifications Version 2.0", RFC 2437,
                    October 1998.

   [RFC2630]        Housley, R., "Cryptographic Message Syntax", RFC
                    2630, June 1999.

   [RFC2660]        Rescorla, E., and A. Schiffman, "The Secure
                    HyperText Transfer Protocol", RFC 2660, August 1999.

   [RFC2898]        Kaliski, B., "PKCS #5: Password-Based Cryptography
                    Specification Version 2.0", RFC 2898, September
                    2000.

   [RFC3279]        Polk, W., Housley, R., and L. Bassham, "Algorithms
                    and Identifiers for the Internet X.509 Public Key
                    Infrastructure Certificate and Certificate
                    Revocation List (CRL) Profile", RFC 3279, April
                    2002.

   [RFC3369]        Housley, R., "Cryptographic Message Syntax (CMS)",
                    RFC 3369, August 2002.


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   [RFC3370]        Housley, R., "Cryptographic Message Syntax (CMS)
                    Algorithms", RFC 3370, August 2002.

   [RFC3447]        Jonsson, J. and B. Kaliski, "Public-Key Cryptography
                    Standards (PKCS) #1: RSA Cryptography Specifications
                    Version 2.1" RFC 3447, February 2003.

   [RFC4572]        Lennox, J., "Connection-Oriented Media Transport
                    over the Transport Layer Security (TLS) Protocol in
                    the Session Description Protocol (SDP)", RFC 4572,
                    July 2006.

   [RFC5652]        Housley, R., "Cryptographic Message Syntax (CMS)",
                    STD 70, RFC 5652, August 2002.

   [ROCH1995]       Rogier, N., and P. Chauvaud, "The compression
                    function of MD2 is not collision free", Presented at
                    Selected Areas in Cryptography '95, Carleton
                    University, Ottawa, Canada. May 18-19, 1995.

   [ROCH1997]       Rogier, N. and P. Chauvaud, "MD2 is not secure
                    without the checksum byte", Des. Codes Cryptogr.
                    12(3), 245-251 (1997).

   [SP800-57]       National Institute of Standards and Technology
                    (NIST), Special Publication 800-57: Recommendation
                    for Key Management - Part 1 (Revised), March 2007.

   [SP800-131]      National Institute of Standards and Technology
                    (NIST), Special Publication 800-131: DRAFT
                    Recommendation for the Transitioning of
                    Cryptographic Algorithms and Key Sizes, June 2010.

   [SHS]            National Institute of Standards and Technology
                    (NIST), FIPS Publication 180-3: Secure Hash
                    Standard, October 2008.













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Authors' Addresses

   Sean Turner
   IECA, Inc.
   3057 Nutley Street, Suite 106
   Fairfax, VA 22031
   USA

   EMail: turners@ieca.com

   Lily Chen
   National Institute of Standards and Technology
   100 Bureau Drive, Mail Stop 8930
   Gaithersburg, MD 20899-8930
   USA

   EMail: lily.chen@nist.gov
































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