Internet DRAFT - draft-turner-md2-to-historic



Network Working Group                                          S. Turner
Internet-Draft                                                      IECA
Obsoletes: 1319 (once approved)                                  L. Chen
Intended Status: Informational                                      NIST
Expires: June 28, 2011                                 December 29, 2010

                         MD2 to Historic Status


   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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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

   [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

   Use of 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).


   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.

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   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 on 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 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

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


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   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

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.

   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.


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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


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,

   [MD2] Kaliski, B., "The MD2 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

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

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

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

   [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

   [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.

   [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

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             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

   [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.

Authors' Addresses

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


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


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