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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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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