Network Working Group S. Turner Internet Draft IECA Updates: 1320 (once approved) L. Chen Intended Status: Informational NIST Expires: January 12, 2011 July 12, 2010 MD4 to Historic Status draft-turner-md4-to-historic-02.txt Abstract This document recommends the retirement of MD4 and discusses the reasons for doing so. This document recommends RFC 1320 be moved to Historic status. Status of this Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. This document may contain material from IETF Documents or IETF Contributions published or made publicly available before November 10, 2008. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. 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Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. 1. Introduction MD4 [MD4] 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 MD4 be retired. Specifically, this document recommends RFC 1320 [MD4] 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. 2. Rationale MD4 was published in 1992 as an Informational RFC. Since its publication, MD4 has been under attack [denBORBOS1992] [DOBB1995] [DOBB1996] [GLRW2010] [WLDCY2005] [LUER2008]. In fact, RSA, in 1996, suggested that MD4 should not be used [RSA-AdviceOnMD4]. Microsoft also made similar statements [MS-AdviceOnMD4]. In Section 6, this document discusses attacks against MD4 that indicate use of MD4 is no longer appropriate when collision resistance is required. Section 6 also discussed attack against MD4's pre-image and second pre-image resistance. Additionally, attacks against MD4 used in message authentication with a shared secret (i.e., HMAC-MD4) are discussed. 3. Documents that reference RFC 1320 MD4 has been specified in the following RFCs: Internet Standard (IS): o [RFC2289] A One-Time Password System. Turner & Chen Expires January 12, 2011 [Page 2] Internet-Draft MD4 to Historic July 2010 Draft Standard (DS): o [RFC1629] Guidelines for OSI NSAP Allocation in the Internet. Proposed Standard (PS): o [RFC3961] Encryption and Checksum Specifications for Kerberos 5. Best Current Practice: o [RFC4086] Randomness Requirements for Security. Informational: o [RFC1760] The S/KEY One-Time Password System. o [RFC1983] Internet Users' Glossary. o [RFC2433] Microsoft PPP CHAP Extensions. o [RFC2759] Microsoft PPP CHAP Extensions, Version 2. o [RFC3174] US Secure Hash Algorithm 1 (SHA1). o [RFC4757] The RC4-HMAC Kerberos Encryption Types Used by Microsoft Windows. o [RFC5126] CMS Advanced Electronic Signatures (CAdES). There are other RFCs that refer to MD4, but their status is either Historic or Obsoleted. References and discussions about these RFCs are omitted. 4. Impact on Moving MD4 to Historic The impact of moving MD4 to Historic is minimal with one exception, as described below. Concentrating on the standards track RFCs: o The initial One-Time Password systems, based on [RFC2289], have ostensibly been replaced by HMAC based mechanism, as specified in HOTP: An HMAC-Based One-Time Password Algorithm [RFC4226]. [RFC4226] suggests following recommendations in [RFC4086] for random input, and in [RFC4086] weakness of MD4 are discussed. Turner & Chen Expires January 12, 2011 [Page 3] Internet-Draft MD4 to Historic July 2010 o MD4 was used in the Inter-Domain Routing Protocol (IDRP); each IDRP message carries a 16-octet hash that is computed by applying the MD-4 algorithm (RFC 1320) to the context of the message itself. Over time IDRP was replaced by BGP-4. o Kerberos Version 5 [RFC3961] specifies the use of MD4 for DES encryption types and checksum types. They were specified, never really used, and are in the process of being deprecated by [I-D.des-die-die-die]. Further, the mandatory to implement encrypted types and checksum types specified by Kerberos are based on AES-256 and HMAC-SHA1 [RFC3962]. Looking at the informational track RFCs: o Randomness Requirements [RFC4086] does mention MD4, but not in a good way; it explains how the algorithm works and that there have been a number of attacks found against it. o The Internet Users' Glossary [RFC1983] provided a definition for Message Digest and listed MD4 as one example. o The S/Key implementations in the wild have started to use MD5 in lieu of MD4. o The CAdES document [RFC5126] lists MD4 as hash algorithm, disparages it, and then does not mention it again. o The SHA-1 document [RFC3174] mentions MD4 in the acknowledgements section. o The three Microsoft RFCs, [RFC2433], [RFC2759], and [RFC4757], are very widely deployed. MS-CHAP Version 1 is supported Microsoft's Windows XP, 2000, 98, 95, NT 4.0, NT 3.51, NT 3.5, but support has been dropped in Vista. MS-CHAP Version 2 is supported in Microsoft's Windows 7, XP, 2000, 98, 98, and NT 4.0. Both versions of MS-CHAP are also supported by RADIUS [RFC2548], AAA [RFC4962], and EAP [RFC5281]. The RC4-HMAC is supported in Microsoft's Windows 2000 and later. EDITOR'S NOTE: Need to verify the last bullet and make sure it doesn't have additional legs. 5. Other Considerations rsync [RSYNC], a non-IETF protocol, once specified the use of MD4, but as of version 3.0.0 published in '08 it has adopted MD5 [MD5]. Turner & Chen Expires January 12, 2011 [Page 4] Internet-Draft MD4 to Historic July 2010 6. Security Considerations This section addresses attacks against MD4's collisions, pre-image, and second pre-image resistance. Additionally, attacks against HMAC- MD4 are discussed. Some may find the guidance for key lengths and algorithm strengths in [SP800-57] and [SP800-131] useful. 6.1. Collision Resistance A practical attack on MD4 was shown by Dobbertin in 1996 with complexity 2^20 of MD4 hash computations [DOBB1996]. In 2004, a more devastating result presented by Xiaoyun Wang showed that the complexity can be reduced to 2^8 of MD4 hash operations. At the Rump Session of Crypto 2004, Wang said that as a matter of fact, finding a collision of MD4 can be accomplished with a pen on a piece of paper. The formal result was presented at EUROCRYPT 2005 in [WLDCY2005]. 6.2. Pre-image and Second Pre-image Resistance The first pre-image attack on full MD4 was accomplished in [LUER2008] with complexity 2^100. Some improvements are shown on pre-image attack and second pre-image attack of MD4 with certain pre- computations [GLRW2010], where complexity is reduced to 2^78.4 and 2^69.4 for pre-image and second pre-image, respectively. The pre- image attacks on MD4 are practical. It cannot be used as a one-way function. For example, it must not be used to hash a cryptographic key 80 bits or longer. 6.3. HMAC The attacks on HMAC presented so far can be classified in three types: distinguishing attacks, existential forgery attacks, and key recovery attacks. Of course, among all these attacks, key recovery attacks are the most severe attacks. The best results on key recovery attacks on HMAC-MD4 were published at EUROCRYPT 2008 with 2^72 queries and 2^77 MD4 computations [WOK2008]. 7. Recommendation Despite MD4 seeing some deployment on the Internet, this specification recommends obsoleting MD4 because MD4 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]). Turner & Chen Expires January 12, 2011 [Page 5] Internet-Draft MD4 to Historic July 2010 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. MD4 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 Obviously, we have to thank all the cryptographers who produced the results we refer to in this document. We'd also like to thank Sam Hartman and Sue Hares for their input. 10. Informative References [denBORBOS1992] B. den Boer and A. Bosselaers. An attack on the last two rounds of MD4. In Advances in Cryptology -Crypto '91, pages 194-203, Springer-Verlag, 1992. [DOBB1995] H. Dobbertin. Alf swindles Ann. CryptoBytes, 1(3): 5, 1995. [DOBB1996] H. Dobbertin. Cryptanalysis of MD4. In Proceedings of the 3rd Workshop on Fast Software Encryption, Cambridge, U.K., pages 53-70, Lecture Notes in Computer Science 1039, Springer-Verlag, 1996. [GLRW2010] Guo, J., Ling, S., Rechberger, C., and H. Wang, "Advanced Meet-in-the-Middle Preimage Attacks: First Results on Full Tiger, and Improved Results on MD4 and SHA-2", http://eprint.iacr.org/2010/016.pdf. [HASH-Attack] Hoffman, P., and B. Schneier, "Attacks on Cryptographic Hashes in Internet Protocols", RFC 4270, November 2005. [LUER2008] G. Leurent. MD4 is Not One-Way. Fast Software Encryption 2008, Lausanne, Switzerland, February 10-13, 2008, LNCS 5086. Springer, 2008. [MD4] Rivest, R., "The MD4 Message-Digest Algorithm", RFC 1320, April 1992. Turner & Chen Expires January 12, 2011 [Page 6] Internet-Draft MD4 to Historic July 2010 [MD5] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321, April 1992. [MS-AdviceOnMD4] Howard, M., "Secure Habits: 8 Simple Rules For Developing More Secure Code", http://msdn.microsoft.com/en- us/magazine/dvdarchive/cc163518.aspx#S6 [RFC1629] Colella, R., Callon, R., Gardner, E., and Y. Rekhter, "Guidelines for OSI NSAP Allocation in the Internet", RFC 1629, May 1994. [RFC1760] Haller, N., "The S/Key One-Time Password System", RFC 1760, February 1995. [RFC1983] Malkin, G., "Internet Users' Glossary", FYI 18, RFC 1983, August 1996. [RFC2289] Haller, N., Metz, C., Nesser, P. and M. Straw, "A One-Time Password System", RFC 2289, February 1998. [RFC2433] Zorn, G. and S. Cobb, "Microsoft PPP CHAP Extensions", RFC 2433, October 1998. [RFC2548] Zorn, G., "Microsoft Vendor-specific RADIUS Attributes", RFC 2548, March 1998. [RFC2759] Zorn, G., "Microsoft PPP CHAP Extensions, Version 2", RFC 2759, January 2000. [RFC3174] Eastlake, D. and P. Jones, "US Secure Hash Algorithm 1 (SHA1)", RFC 3174, September 2001. [RFC3961] Raeburn, K., "Encryption and Checksum Specifications for Kerberos 5", RFC 3961, February 2005. [RFC3962] Raeburn, K., "Advanced Encryption Standard (AES) Encryption for Kerberos 5", RFC 3962, February 2005. [RFC4086] R Eastlake, D., 3rd, Schiller, J., and S. Crocker, "Randomness Requirements for Security", BCP 106, RFC 4086, June 2005. [RFC4226] Nikander, P., Arkko, J., Aura, T., Montenegro, G., and E. Nordmark, "Mobile IP Version 6 Route Turner & Chen Expires January 12, 2011 [Page 7] Internet-Draft MD4 to Historic July 2010 Optimization Security Design Background", RFC 4226, December 2005. [RFC4757] Jaganathan, K., Zhu, L., and J. Brezak, "The RC4- HMAC Kerberos Encryption Types Used by Microsoft Windows," RFC 4757, December 2006. [RFC4962] Housley, R., and Aboba, B., "Guidance for Authentication, Authorization, and Accounting (AAA) Key Management", RFC 4962, July 2007. [RFC5126] Pinkas, D., Pope, N., and J. Ross, "CMS Advanced Electronic Signatures (CAdES)", RFC 5126, February 2008. [RFC5281] Funk, P., and S. Blake-Wilson, "Extensible Authentication Protocol Tunneled Transport Layer Security Authenticated Protocol Version 0 (EAP- TTLSv0)", RFC 5281, August 2008. [RSA-AdviceOnMD4] Robshaw, M.J.B., "On Recent Results for MD2, MD4 and MD5", November 1996, ftp://ftp.rsasecurity.com/pub/pdfs/bulletn4.pdf [RSYNC] http://www.samba.org/rsync/ [SHS] National Institute of Standards and Technology (NIST), FIPS Publication 180-3: Secure Hash Standard, October 2008. [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. [I-D.des-die-die-die] Astrand, L.H., "Deprecate DES support for Kerberos", draft-lha-des-die-die-die-05, work-in- progress. [WLDCY2005] X. Wang, X. Lai, D. Feng, H. Chen, and X. Yu. Cryptanalysis of Hash Functions MD4 and RIPEMD. LNCS 3494. Advances in Cryptology - EUROCRYPT2005, Springer 2005. Turner & Chen Expires January 12, 2011 [Page 8] Internet-Draft MD4 to Historic July 2010 [WOK2008] L. Wang, K. Ohta, and N. Kunihiro. New Key- recovery Attacks on HMAC/NMAC-MD4 and NMAC-MD5. EUROCRYPT 2008.LNCS 4965, Springer, 2008. 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 Turner & Chen Expires January 12, 2011 [Page 9]