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Internet Engineering Task Force (IETF) S. Turner
Request for Comments: 6150 IECA
Obsoletes: 1320 L. Chen
Category: Informational NIST
ISSN: 2070-1721 March 2011
MD4 to Historic Status
Abstract
This document retires RFC 1320, which documents the MD4 algorithm,
and discusses the reasons for doing so. This document moves RFC 1320
to Historic status.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for informational purposes.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Not all documents
approved by the IESG are a candidate for any level of Internet
Standard; see Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc6150.
Copyright Notice
Copyright (c) 2011 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. 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.
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RFC 6150 MD2 to Historic Status March 2011
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 retires [MD4].
Specifically, this document moves RFC 1320 [MD4] 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
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 discusses attacks 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
Use of MD4 has been specified in the following RFCs:
Internet Standard (IS):
o [RFC2289] A One-Time Password System.
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 (BCP):
o [RFC4086] Randomness Requirements for Security.
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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 MD2, but they have been either
moved to Historic status or obsoleted by a later RFC. References and
discussions about these RFCs are omitted. The notable exceptions
are:
o [RFC2313] PKCS #1: RSA Encryption Version 1.5.
o [RFC2437] PKCS #1: RSA Cryptography Specifications Version 2.0.
o [RFC3447] Public-Key Cryptography Standards (PKCS) #1: RSA
Cryptography Specifications Version 2.1.
4. Impact of Moving MD4 to Historic
The impact of moving MD4 to Historic is minimal with the one
exception of Microsoft's use of MD4 as part of RC4-HMAC in Windows,
as described below.
Regarding DS, PS, and BCP 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] weaknesses of MD4 are discussed.
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 [RFC4271], which required at
least [MD5].
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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
[DES-DIE]. Further, the mandatory-to-implement encrypted types and
checksum types specified by Kerberos are based on AES-256 and HMAC-
SHA1 [RFC3962].
Regarding Informational RFCs:
o PKCS#1 v1.5 [RFC2313] indicated that there was no reason to not use
MD4. PKCS#1 v2.0 [RFC2437] and v2.1 [RFC3447] recommend against
MD4 due to cryptoanalytic progress having uncovered weaknesses in
the collision resistance of MD4.
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 IETF OTP specification [RFC2289] was based on S/KEY technology.
So S/KEY was replaced by OTP, at least in theory. Additionally,
the S/KEY implementations in the wild have started to use MD5 in
lieu of MD4.
o The CAdES document [RFC5126] lists MD4 as a 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 RFCs describing Microsoft protocols, [RFC2433],
[RFC2759], and [RFC4757], are very widely deployed as MS-CHAP v1,
MS-CHAP v2, and RC4-HMAC, respectively.
o MS-CHAP Version 1 is supported in Microsoft's Windows XP, 2000,
98, 95, NT 4.0, NT 3.51, and NT 3.5, but support has been
dropped in Vista. MS-CHAP Version 2 is supported in Microsoft's
Windows 7, Vista, XP, 2000, 98, 95, and NT 4.0. Both versions
of MS-CHAP are also supported by RADIUS [RFC2548] and the
Extensible Authentication Protocol (EAP) [RFC5281]. In 2007,
[RFC4962] listed MS-CHAP v1 and v2 as flawed and recommended
against their use; these incidents were presented as a strong
indication for the necessity of built-in crypto-algorithm
agility in Authentication, Authorization, and Accounting (AAA)
protocols.
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o The RC4-HMAC is supported in Microsoft's Windows 2000 and later
versions of Windows for backwards compatibility with Windows
2000. As [RFC4757] stated, RC4-HMAC doesn't rely on the
collision resistance property of MD4, but uses it to generate a
key from a password, which is then used as input to HMAC-MD5.
For an attacker to recover the password from RC4-HMAC, the
attacker first needs to recover the key that is used with HMAC-
MD5. As noted in [RFC6151], key recovery attacks on HMAC-MD5
are not yet practical.
5. Other Considerations
rsync [RSYNC], a non-IETF protocol, once specified the use of MD4,
but as of version 3.0.0 published in 2008, it has adopted MD5 [MD5].
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
attacks and second pre-image attacks 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 of 80 bits or longer.
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6.3. HMAC
The attacks on Hash-based Message Authentication Code (HMAC)
algorithms [RFC2104] 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 obsoletes [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]).
RSA Security considers it appropriate to move the MD4 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. MD4 is clearly showing
signs of weakness, and implementations should strongly consider
removing support and migrating to another hash algorithm.
8. Acknowledgements
We'd like to thank RSA for publishing MD4. Obviously, we have to
thank all the cryptographers who produced the results we refer to in
this document. We'd also like to thank Ran Atkinson, Sue Hares, Sam
Hartman, Alfred Hoenes, John Linn, Catherine Meadows, Magnus Nystrom,
and Martin Rex for their input.
9. 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.
[DES-DIE] Astrand, L., "Deprecate DES support for Kerberos", Work
in Progress, July 2010.
[DOBB1995] H. Dobbertin. Alf swindles Ann. CryptoBytes, 1(3): 5,
1995.
Turner & Chen Informational [Page 6]
RFC 6150 MD2 to Historic Status March 2011
[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.
[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/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., Ed., "Internet Users' Glossary", FYI 18, RFC
1983, August 1996.
[RFC2104] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
Hashing for Message Authentication", RFC 2104, February
1997.
[RFC2289] Haller, N., Metz, C., Nesser, P., and M. Straw, "A One-
Time Password System", STD 61, RFC 2289, February 1998.
[RFC2313] Kaliski, B., "PKCS #1: RSA Encryption Version 1.5", RFC
2313, March 1998.
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[RFC2433] Zorn, G. and S. Cobb, "Microsoft PPP CHAP Extensions",
RFC 2433, October 1998.
[RFC2437] Kaliski, B., and J. Staddon, "PKCS #1: RSA Cryptography
Specifications Version 2.0", RFC 2437, October 1998.
[RFC2548] Zorn, G., "Microsoft Vendor-specific RADIUS Attributes",
RFC 2548, March 1999.
[RFC2759] Zorn, G., "Microsoft PPP CHAP Extensions, Version 2", RFC
2759, January 2000.
[RFC3174] Eastlake 3rd, D. and P. Jones, "US Secure Hash Algorithm
1 (SHA1)", RFC 3174, September 2001.
[RFC3447] Jonsson, J. and B. Kaliski, "Public-Key Cryptography
Standards (PKCS) #1: RSA Cryptography Specifications
Version 2.1", RFC 3447, February 2003.
[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] Eastlake 3rd, D., Schiller, J., and S. Crocker,
"Randomness Requirements for Security", BCP 106, RFC
4086, June 2005.
[RFC4226] M'Raihi, D., Bellare, M., Hoornaert, F., Naccache, D.,
and O. Ranen, "HOTP: An HMAC-Based One-Time Password
Algorithm", RFC 4226, December 2005.
[RFC4271] Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A
Border Gateway Protocol 4 (BGP-4)", RFC 4271, January
2006.
[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 B. Aboba, "Guidance for Authentication,
Authorization, and Accounting (AAA) Key Management", BCP
132, RFC 4962, July 2007.
[RFC5126] Pinkas, D., Pope, N., and J. Ross, "CMS Advanced
Electronic Signatures (CAdES)", RFC 5126, March 2008.
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[RFC5281] Funk, P. and S. Blake-Wilson, "Extensible Authentication
Protocol Tunneled Transport Layer Security Authenticated
Protocol Version 0 (EAP-TTLSv0)", RFC 5281, August 2008.
[RFC6151] Turner, S. and L. Chen, "Updated Security Considerations
for the MD5 Message-Digest and the HMAC-MD5 Algorithms",
RFC 6151, March 2011.
[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] rsync web pages, 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.
[WLDCY2005] X. Wang, X. Lai, D. Feng, H. Chen, and X. Yu,
Cryptanalysis of Hash Functions MD4 and RIPEMD, LNCS
3944, Advances in Cryptology - EUROCRYPT2005, Springer,
2005.
[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.
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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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