Network Working Group                                         S. Turner
Internet Draft                                                     IECA
Updates: 1321 (once approved)                                   L. Chen
Intended Status: Informational                                     NIST
Expires: January 5, 2011                                   July 5, 2010

   Updated Security Considerations for the MD5 Message-Digest Algorithm


   This document updates the security considerations for the MD5 message
   digest algorithm.

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   ( in effect on the date of
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1. Introduction

   MD5 [MD5] 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.  The published attacks against MD5
   show and that it is not prudent to use MD5 when collision resistance
   is required.  This document replaces the security considerations in
   RFC 1321 [MD5].

   [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

2. Security Considerations

   MD5 was published in 1992 as an Informational RFC.  Since that time,
   MD5 has been studied extensively.  What follows are recent attacks
   against MD5's collisions, pre-image, and second pre-image resistance.
   Additionally, attacks against MD5 used in message authentication with
   a shared secret (i.e., HMAC-MD5) are discussed.

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

2.1. Collision Resistance

   The first paper that demonstrates actual collisions of MD5 was
   published in 2004 [MD5-Analysis1]. The detailed attack techniques for
   MD5 were published at EUROCRYPT 2005 [MD5-Analysis2]. Since then, a
   lot of research results have been published to improve collision
   attacks on MD5. The attacks presented in [MD5-Analysis3] can find MD5
   collision in about one minute on a standard notebook PC (Intel
   Pentium, 1.6 GHz.). In [MD5-Analysis4], the collision attack on MD5
   was successfully applied to X.509 certificates.

   Notice that the collision attack on MD5 can also be applied to
   password based challenge-and-response authentication protocols such
   as APOP protocol used in post office authentication as presented in

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   In fact, more delicate attacks on MD5 to improve the speed of finding
   collisions have published recently. However, the aforementioned
   results have provided sufficient reason to eliminate MD5 usage in
   applications where collision resistance is required such as digital

2.2. Pre-image and Second Pre-image Resistance

   Even though the best result can find a pre-image attack of MD5 faster
   than exhaustive search as presented in [MD5-Analysis6], the
   complexity 2^123.4 is still pretty high.

2.3. HMAC

   The cryptanalysis of HMAC-MD5 usually conducted together with NMAC
   (Nested MAC) since they are closely related. NMAC uses two
   independent keys K1 and K2 such that NMAC(K1, K2, M) = H(K1, H(K2,
   M), where K1 and K2 are used as secret IVs for hash functions
   H(IV,M). If we re-write HMAC equation using two secret IVs such that
   IV2 = H(K Xor ipad) and IV1 = H(K Xor opad), then HMAC(K, M) =
   NMAC(IV1, IV2, M).  Here it is very important to notice that IV1 and
   IV2 are not independently selected.

   The first analysis was explored on NMAC-MD5 using related keys in
   [HMAC-Analysis1]. The partial key recovery attack cannot be extended
   to HMAC-MD5, since for HMAC, recovering partial secret IVs can hardly
   lead to recovering (partial) key K. Another paper presented at Crypto
   2007 [HMAC-Analysis2] extended results of [HMAC-Analysis1] to a full
   key recovery attack on NMAC-MD5. Since it also uses related key
   attack, it does not seem applicable to HMAC-MD5.

   A EUROCRYPT 2009 paper presented a distinguishing attack on HMAC-MD5
   [HMAC-Analysis3] without using related keys. It can distinguish an
   instantiation of HMAC with MD5 from an instantiation with a random
   function with 2^97 queries with probability 0.87. This is called
   distinguishing-H. Using the distinguishing attack, it can recover
   some bits of the intermediate status of the second block. However, as
   it is pointed in [HMAC-Analysis3], it cannot be used to recover the
   (partial) inner key H(K Xor ipad).  It is not obvious how the attack
   can be used to form a forgery attack either.

   The attacks on HMAC-MD5 do not seem to indicate a practical
   vulnerability when used as a message authentication code. Considering
   that the distinguishing-H attack is different from distinguishing-R
   attack, which distinguishes an HMAC from a random function, the
   practical impact on HMAC usage as a PRF such as in a key derivation
   function is not well understood.

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   Therefore, it may not be urgent to remove HMAC-MD5 from the existing
   protocols.  However, since MD5 must not be used for digital
   signatures, for a new protocol design, a ciphersuite with HMAC-MD5
   should not be included.

3. IANA Considerations


4. Normative References

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

   [HMAC-Analysis1]  S. Contini, Y.L. Yin. Forgery and partial key-
                     recovery attacks on HMAC and NMAC using hash
                     collisions. ASIACRYPT 2006. LNCS 4284, Springer,

   [HMAC-Analysis2]  Fouque, P.-A., Leurent, G., Nguyen, P.Q.: Full key-
                     recovery attacks on HMAC/NMAC-MD4 and NMAC-MD5.
                     CRYPTO 2007. LNCS, 4622, Springer, 2007.

   [HMAC-Analysis3]  X. Wang, H. Yu, W. Wang, H. Zhang, and T. Zhan.
                     Cryptanalysis of HMAC/NMAC-MD5 and MD5-MAC. LNCS
                     5479. Advances in Cryptology - EUROCRYPT2009,
                     Springer 2009.

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

   [MD5-Analysis1]   X. Wang, D. Feng, X. Lai, H. Yu, Collisions for
                     Hash Functions MD4, MD5, HAVAL-128 and RIPEMD,

   [MD5-Analysis2]   X. Wang and H. Yu. How to Break MD5 and other Hash
                     Functions. LNCS 3494. Advances in Cryptology -
                     EUROCRYPT2005, Springer 2005.

   [MD5-Analysis3]   V. Klima. Tunnels in Hash Functions: MD5 Collisions
                     within a Minute. Cryptology ePrint Archive, Report
                     2006/105 (2006),

   [MD5-Analysis4]   Stevens, M., Lenstra, A., de Weger, B., Target
                     Collisions for MD5 and Colliding X.509
                     Certificates for Different Identities. Cryptology

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                     ePrint Archive, Report 2006/360 (2006),

   [MD5-Analysis5]   G. Leurent, Message freedom in MD4 and MD5
                     collisions: Application to APOP. Proceedings of
                     FSE 2007. Lecture Notes in Computer Science 4715.
                     Springer 2007.

   [MD5-Analysis6]   Y. Sasaki and K. Aoki. Finding preimages in full
                     MD5 faster than exhaustive search. Advances in
                     Cryptology - EUROCRYPT 2009, LNCS 5479 of Lecture
                     Notes in Computer Science, Springer, 2009.

   [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.

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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