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Checking references for intended status: Informational ---------------------------------------------------------------------------- No issues found here. Summary: 0 errors (**), 0 flaws (~~), 1 warning (==), 4 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 1 Network Working Group S. Turner 2 Internet Draft IECA 3 Updates: 1321, 2202 (once approved) L. Chen 4 Intended Status: Informational NIST 5 Expires: January 8, 2011 July 8, 2010 7 Updated Security Considerations for the 8 MD5 Message-Digest Algorithm and HMAC-MD5 9 draft-turner-md5-seccon-update-01.txt 11 Abstract 13 This document updates the security considerations for the MD5 message 14 digest algorithm. It also updates the security considerations for 15 HMAC-MD5. 17 Status of this Memo 19 This Internet-Draft is submitted in full conformance with the 20 provisions of BCP 78 and BCP 79. This document may contain material 21 from IETF Documents or IETF Contributions published or made publicly 22 available before November 10, 2008. 24 Internet-Drafts are working documents of the Internet Engineering 25 Task Force (IETF), its areas, and its working groups. Note that 26 other groups may also distribute working documents as Internet- 27 Drafts. 29 Internet-Drafts are draft documents valid for a maximum of six months 30 and may be updated, replaced, or obsoleted by other documents at any 31 time. It is inappropriate to use Internet-Drafts as reference 32 material or to cite them other than as "work in progress." 34 The list of current Internet-Drafts can be accessed at 35 http://www.ietf.org/ietf/1id-abstracts.txt. 37 The list of Internet-Draft Shadow Directories can be accessed at 38 http://www.ietf.org/shadow.html. 40 This Internet-Draft will expire on January 8, 2011. 42 Copyright Notice 44 Copyright (c) 2010 IETF Trust and the persons identified as the 45 document authors. All rights reserved. 47 This document is subject to BCP 78 and the IETF Trust's Legal 48 Provisions Relating to IETF Documents 49 (http://trustee.ietf.org/license-info) in effect on the date of 50 publication of this document. Please review these documents 51 carefully, as they describe your rights and restrictions with respect 52 to this document. Code Components extracted from this document must 53 include Simplified BSD License text as described in Section 4.e of 54 the Trust Legal Provisions and are provided without warranty as 55 described in the Simplified BSD License. 57 1. Introduction 59 MD5 [MD5] is a message digest algorithm that takes as input a message 60 of arbitrary length and produces as output a 128-bit "fingerprint" or 61 "message digest" of the input. The published attacks against MD5 62 show and that it is not prudent to use MD5 when collision resistance 63 is required. This document replaces the security considerations in 64 RFC 1321 [MD5]. 66 [HMAC] defined a mechanism for message authentication using 67 cryptographic hash functions. Any message digest algorithm can be 68 used, but the cryptographic strength of HMAC depends on the 69 properties of the underlying hash function. [HMAC-MD5] defined test 70 cases for HMAC-MD5. This document updates the security 71 considerations in [HMAC-MD5]. 73 [HASH-Attack] summarizes the use of hashes in many protocols and 74 discusses how attacks against a message digest algorithm's one-way 75 and collision-free properties affect and do not affect Internet 76 protocols. 78 2. Security Considerations 80 MD5 was published in 1992 as an Informational RFC. Since that time, 81 MD5 has been studied extensively. What follows are recent attacks 82 against MD5's collisions, pre-image, and second pre-image resistance. 83 Additionally, attacks against MD5 used in message authentication with 84 a shared secret (i.e., HMAC-MD5) are discussed. 86 Some may find the guidance for key lengths and algorithm strengths in 87 [SP800-57] and [SP800-131] useful. 89 2.1. Collision Resistance 91 The first paper that demonstrates actual collisions of MD5 was 92 published in 2004 [WFLY2004]. The detailed attack techniques for MD5 93 were published at EUROCRYPT 2005 [WAYU2005]. Since then, a lot of 94 research results have been published to improve collision attacks on 95 MD5. The attacks presented in [KLIM2006] can find MD5 collision in 96 about one minute on a standard notebook PC (Intel Pentium, 1.6 GHz.). 97 In [STEV2007], he claim that it takes 10 seconds or less on a 2.6Ghz 98 Pentium4 to find collisions. In 99 [STEV2007][SLdeW2007][SSALMOdeW2009][SLdeW2009], the collision 100 attacks on MD5 were successfully applied to X.509 certificates. 102 Notice that the collision attack on MD5 can also be applied to 103 password based challenge-and-response authentication protocols such 104 as APOP protocol used in post office authentication as presented in 105 [LEUR2007]. 107 In fact, more delicate attacks on MD5 to improve the speed of finding 108 collisions have published recently. However, the aforementioned 109 results have provided sufficient reason to eliminate MD5 usage in 110 applications where collision resistance is required such as digital 111 signatures. 113 2.2. Pre-image and Second Pre-image Resistance 115 Even though the best result can find a pre-image attack of MD5 faster 116 than exhaustive search as presented in [SAAO2009], the complexity 117 2^123.4 is still pretty high. 119 2.3. HMAC 121 The cryptanalysis of HMAC-MD5 usually conducted together with NMAC 122 (Nested MAC) since they are closely related. NMAC uses two 123 independent keys K1 and K2 such that NMAC(K1, K2, M) = H(K1, H(K2, 124 M), where K1 and K2 are used as secret IVs for hash functions 125 H(IV,M). If we re-write HMAC equation using two secret IVs such that 126 IV2 = H(K Xor ipad) and IV1 = H(K Xor opad), then HMAC(K, M) = 127 NMAC(IV1, IV2, M). Here it is very important to notice that IV1 and 128 IV2 are not independently selected. 130 The first analysis was explored on NMAC-MD5 using related keys in 131 [COYI2006]. The partial key recovery attack cannot be extended to 132 HMAC-MD5, since for HMAC, recovering partial secret IVs can hardly 133 lead to recovering (partial) key K. Another paper presented at Crypto 134 2007 [FLN2007] extended results of [COYI2006] to a full key recovery 135 attack on NMAC-MD5. Since it also uses related key attack, it does 136 not seem applicable to HMAC-MD5. 138 A EUROCRYPT 2009 paper presented a distinguishing attack on HMAC-MD5 139 [WYWZZ2009] without using related keys. It can distinguish an 140 instantiation of HMAC with MD5 from an instantiation with a random 141 function with 2^97 queries with probability 0.87. This is called 142 distinguishing-H. Using the distinguishing attack, it can recover 143 some bits of the intermediate status of the second block. However, as 144 it is pointed in [WYWZZ2009], it cannot be used to recover the 145 (partial) inner key H(K Xor ipad). It is not obvious how the attack 146 can be used to form a forgery attack either. 148 The attacks on HMAC-MD5 do not seem to indicate a practical 149 vulnerability when used as a message authentication code. Considering 150 that the distinguishing-H attack is different from distinguishing-R 151 attack, which distinguishes an HMAC from a random function, the 152 practical impact on HMAC usage as a PRF such as in a key derivation 153 function is not well understood. 155 Therefore, it may not be urgent to remove HMAC-MD5 from the existing 156 protocols. However, since MD5 must not be used for digital 157 signatures, for a new protocol design, a ciphersuite with HMAC-MD5 158 should not be included. 160 3. IANA Considerations 162 None. 164 4. Normative References 166 [COYI2006] S. Contini, Y.L. Yin. Forgery and partial key- 167 recovery attacks on HMAC and NMAC using hash 168 collisions. ASIACRYPT 2006. LNCS 4284, Springer, 169 2006. 171 [FLN2007] Fouque, P.-A., Leurent, G., Nguyen, P.Q.: Full key- 172 recovery attacks on HMAC/NMAC-MD4 and NMAC-MD5. 173 CRYPTO 2007. LNCS, 4622, Springer, 2007. 175 [HASH-Attack] Hoffman, P., and B. Schneier, "Attacks on 176 Cryptographic Hashes in Internet Protocols", RFC 177 4270, November 2005. 179 [HMAC] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: 180 Keyed-Hashing for Message Authentication", RFC 181 2104, February 1997. 183 [HMAC-MD5] Cheng, P., and R. Glenn, "Test Cases for HMAC-MD5 184 and HMAC-SHA-1", RC 2201, September 1997. 186 [KLIM2006] V. Klima. Tunnels in Hash Functions: MD5 Collisions 187 within a Minute. Cryptology ePrint Archive, Report 188 2006/105 (2006), http://eprint.iacr.org/2006/105. 190 [LEUR2007] G. Leurent, Message freedom in MD4 and MD5 191 collisions: Application to APOP. Proceedings of 192 FSE 2007. Lecture Notes in Computer Science 4715. 193 Springer 2007. 195 [MD5] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 196 1321, April 1992. 198 [SAAO2009] Y. Sasaki and K. Aoki. Finding preimages in full 199 MD5 faster than exhaustive search. Advances in 200 Cryptology - EUROCRYPT 2009, LNCS 5479 of Lecture 201 Notes in Computer Science, Springer, 2009. 203 [SLdeW2007] Stevens, M., Lenstra, A., de Weger, B., Chosen- 204 prefix Collisions for MD5 and Colliding X.509 205 Certificates for Different Identities. EuroCrypt 206 2007. 208 [SLdeW2009] Stevens, M., Lenstra, A., de Weger, B., "Chosen- 209 prefix Collisions for MD5 and Applications", 210 Journal of Cryptology, 2009. 211 http://deweger.xs4all.nl/papers/%5B42%5DStLedW- 212 MD5-JCryp%5B2009%5D.pdf. 214 [SSALMOdeW2009] Stevens, M., Sotirov, A., Appelbaum, J., Lenstra, 215 A., Molnar, D., Osvik, D., and B. de Weger. Short 216 chosen-prefix collisions for MD5 and the creation 217 of a rogue CA certificate, Crypto 2009. 219 [SP800-57] National Institute of Standards and Technology 220 (NIST), Special Publication 800-57: Recommendation 221 for Key Management - Part 1 (Revised), March 2007. 223 [SP800-131] National Institute of Standards and Technology 224 (NIST), Special Publication 800-131: DRAFT 225 Recommendation for the Transitioning of 226 Cryptographic Algorithms and Key Sizes, June 2010. 228 [STEV2007] Stevens, M., On Collisions for MD5. 229 http://www.win.tue.nl/hashclash/On%20Collisions%20 230 for%20MD5%20-%20M.M.J.%20Stevens.pdf. 232 [WAYU2005] X. Wang and H. Yu. How to Break MD5 and other Hash 233 Functions. LNCS 3494. Advances in Cryptology - 234 EUROCRYPT2005, Springer 2005. 236 [WFLY2004] X. Wang, D. Feng, X. Lai, H. Yu, Collisions for 237 Hash Functions MD4, MD5, HAVAL-128 and RIPEMD, 238 2004, http://eprint.iacr.org/2004/199.pdf 240 [WYWZZ2009] X. Wang, H. Yu, W. Wang, H. Zhang, and T. Zhan. 241 Cryptanalysis of HMAC/NMAC-MD5 and MD5-MAC. LNCS 242 5479. Advances in Cryptology - EUROCRYPT2009, 243 Springer 2009. 245 Authors' Addresses 247 Sean Turner 248 IECA, Inc. 249 3057 Nutley Street, Suite 106 250 Fairfax, VA 22031 251 USA 253 EMail: turners@ieca.com 255 Lily Chen 256 National Institute of Standards and Technology 257 100 Bureau Drive, Mail Stop 8930 258 Gaithersburg, MD 20899-8930 259 USA 261 EMail: lily.chen@nist.gov