< draft-turner-md5-seccon-update-02.txt   draft-turner-md5-seccon-update-03.txt >
Network Working Group S. Turner Network Working Group S. Turner
Internet Draft IECA Internet Draft IECA
Updates: 1321, 2202 (once approved) L. Chen Updates: 1321, 2202 (once approved) L. Chen
Intended Status: Informational NIST Intended Status: Informational NIST
Expires: January 12, 2011 July 12, 2010 Expires: March 23, 2011 September 23, 2010
Updated Security Considerations for the Updated Security Considerations for the
MD5 Message-Digest and the HMAC-MD5 Algorithms MD5 Message-Digest and the HMAC-MD5 Algorithms
draft-turner-md5-seccon-update-02.txt draft-turner-md5-seccon-update-03.txt
Abstract Abstract
This document updates the security considerations for the MD5 message This document updates the security considerations for the MD5 message
digest algorithm. It also updates the security considerations for digest algorithm. It also updates the security considerations for
HMAC-MD5. HMAC-MD5.
Status of this Memo Status of this Memo
This Internet-Draft is submitted in full conformance with the This Internet-Draft is submitted in full conformance with the
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and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt. http://www.ietf.org/ietf/1id-abstracts.txt.
The list of Internet-Draft Shadow Directories can be accessed at The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html. http://www.ietf.org/shadow.html.
This Internet-Draft will expire on January 12, 2011. This Internet-Draft will expire on March 23, 2011.
Copyright Notice Copyright Notice
Copyright (c) 2010 IETF Trust and the persons identified as the Copyright (c) 2010 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of (http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents publication of this document. Please review these documents
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to this document. Code Components extracted from this document must to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License. described in the Simplified BSD License.
1. Introduction 1. Introduction
MD5 [MD5] is a message digest algorithm that takes as input a message 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 of arbitrary length and produces as output a 128-bit "fingerprint" or
"message digest" of the input. The published attacks against MD5 "message digest" of the input. The published attacks against MD5
show and that it is not prudent to use MD5 when collision resistance show that it is not prudent to use MD5 when collision resistance is
is required. This document replaces the security considerations in required. This document replaces the security considerations in RFC
RFC 1321 [MD5]. 1321 [MD5].
[HMAC] defined a mechanism for message authentication using [HMAC] defined a mechanism for message authentication using
cryptographic hash functions. Any message digest algorithm can be cryptographic hash functions. Any message digest algorithm can be
used, but the cryptographic strength of HMAC depends on the used, but the cryptographic strength of HMAC depends on the
properties of the underlying hash function. [HMAC-MD5] defined test properties of the underlying hash function. [HMAC-MD5] defined test
cases for HMAC-MD5. This document updates the security cases for HMAC-MD5. This document updates the security
considerations in [HMAC-MD5]. considerations in [HMAC-MD5].
[HASH-Attack] summarizes the use of hashes in many protocols and [HASH-Attack] summarizes the use of hashes in many protocols and
discusses how attacks against a message digest algorithm's one-way discusses how attacks against a message digest algorithm's one-way
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Psuedo-collisions for the compress function of MD5 were first Psuedo-collisions for the compress function of MD5 were first
described in 1993 [denBBO1993]. In 1996, [DOB1995] demonstrated a described in 1993 [denBBO1993]. In 1996, [DOB1995] demonstrated a
collision pair for the MD5 compression function with a chosen initial collision pair for the MD5 compression function with a chosen initial
value. The first paper that demonstrated two collision pairs for value. The first paper that demonstrated two collision pairs for
regular MD5 was published in 2004 [WFLY2004]. The detailed attack regular MD5 was published in 2004 [WFLY2004]. The detailed attack
techniques for MD5 were published at EUROCRYPT 2005 [WAYU2005]. Since techniques for MD5 were published at EUROCRYPT 2005 [WAYU2005]. Since
then, a lot of research results have been published to improve then, a lot of research results have been published to improve
collision attacks on MD5. The attacks presented in [KLIM2006] can collision attacks on MD5. The attacks presented in [KLIM2006] can
find MD5 collision in about one minute on a standard notebook PC find MD5 collision in about one minute on a standard notebook PC
(Intel Pentium, 1.6 GHz.). In [STEV2007], he claim that it takes 10 (Intel Pentium, 1.6 GHz.). [STEV2007] claims that it takes 10
seconds or less on a 2.6Ghz Pentium4 to find collisions. In seconds or less on a 2.6Ghz Pentium4 to find collisions. In
[STEV2007][SLdeW2007][SSALMOdeW2009][SLdeW2009], the collision [STEV2007][SLdeW2007][SSALMOdeW2009][SLdeW2009], the collision
attacks on MD5 were successfully applied to X.509 certificates. attacks on MD5 were successfully applied to X.509 certificates.
Notice that the collision attack on MD5 can also be applied to Notice that the collision attack on MD5 can also be applied to
password based challenge-and-response authentication protocols such password based challenge-and-response authentication protocols such
as APOP protocol used in post office authentication as presented in as the APOP option in the Post Office Protocol (POP) used in post
[LEUR2007]. office authentication as presented in [LEUR2007].
In fact, more delicate attacks on MD5 to improve the speed of finding In fact, more delicate attacks on MD5 to improve the speed of finding
collisions have published recently. However, the aforementioned collisions have been published recently. However, the aforementioned
results have provided sufficient reason to eliminate MD5 usage in results have provided sufficient reason to eliminate MD5 usage in
applications where collision resistance is required such as digital applications where collision resistance is required such as digital
signatures. signatures.
2.2. Pre-image and Second Pre-image Resistance 2.2. Pre-image and Second Pre-image Resistance
Even though the best result can find a pre-image attack of MD5 faster Even though the best result can find a pre-image attack of MD5 faster
than exhaustive search as presented in [SAAO2009], the complexity than exhaustive search as presented in [SAAO2009], the complexity
2^123.4 is still pretty high. 2^123.4 is still pretty high.
2.3. HMAC 2.3. HMAC
The cryptanalysis of HMAC-MD5 usually conducted together with NMAC The cryptanalysis of HMAC-MD5 is usually conducted together with NMAC
(Nested MAC) since they are closely related. NMAC uses two (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, 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 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 H(IV,M). If we re-write the HMAC equation using two secret IVs such
IV2 = H(K Xor ipad) and IV1 = H(K Xor opad), then HMAC(K, M) = 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 NMAC(IV1, IV2, M). Here it is very important to notice that IV1 and
IV2 are not independently selected. IV2 are not independently selected.
The first analysis was explored on NMAC-MD5 using related keys in The first analysis was explored on NMAC-MD5 using related keys in
[COYI2006]. The partial key recovery attack cannot be extended to [COYI2006]. The partial key recovery attack cannot be extended to
HMAC-MD5, since for HMAC, recovering partial secret IVs can hardly HMAC-MD5, since for HMAC, recovering partial secret IVs can hardly
lead to recovering (partial) key K. Another paper presented at Crypto lead to recovering (partial) key K. Another paper presented at Crypto
2007 [FLN2007] extended results of [COYI2006] to a full key recovery 2007 [FLN2007] extended results of [COYI2006] to a full key recovery
attack on NMAC-MD5. Since it also uses related key attack, it does attack on NMAC-MD5. Since it also uses related key attack, it does
not seem applicable to HMAC-MD5. not seem applicable to HMAC-MD5.
A EUROCRYPT 2009 paper presented a distinguishing attack on HMAC-MD5 A EUROCRYPT 2009 paper presented a distinguishing attack on HMAC-MD5
[WYWZZ2009] without using related keys. It can distinguish an [WYWZZ2009] without using related keys. It can distinguish an
instantiation of HMAC with MD5 from an instantiation with a random instantiation of HMAC with MD5 from an instantiation with a random
function with 2^97 queries with probability 0.87. This is called function with 2^97 queries with probability 0.87. This is called
distinguishing-H. Using the distinguishing attack, it can recover distinguishing-H. Using the distinguishing attack, it can recover
some bits of the intermediate status of the second block. However, as some bits of the intermediate status of the second block. However, as
it is pointed in [WYWZZ2009], it cannot be used to recover the it is pointed out in [WYWZZ2009], it cannot be used to recover the
(partial) inner key H(K Xor ipad). It is not obvious how the attack (partial) inner key H(K Xor ipad). It is not obvious how the attack
can be used to form a forgery attack either. can be used to form a forgery attack either.
The attacks on HMAC-MD5 do not seem to indicate a practical The attacks on HMAC-MD5 do not seem to indicate a practical
vulnerability when used as a message authentication code. Considering vulnerability when used as a message authentication code. Considering
that the distinguishing-H attack is different from distinguishing-R that the distinguishing-H attack is different from a distinguishing-R
attack, which distinguishes an HMAC from a random function, the attack, which distinguishes an HMAC from a random function, the
practical impact on HMAC usage as a PRF such as in a key derivation practical impact on HMAC usage as a PRF such as in a key derivation
function is not well understood. function is not well understood.
Therefore, it may not be urgent to remove HMAC-MD5 from the existing Therefore, it may not be urgent to remove HMAC-MD5 from the existing
protocols. However, since MD5 must not be used for digital protocols. However, since MD5 must not be used for digital
signatures, for a new protocol design, a ciphersuite with HMAC-MD5 signatures, for a new protocol design, a ciphersuite with HMAC-MD5
should not be included. Options include HMAC-SHA256 [HMAC][HMAC- should not be included. Options include HMAC-SHA256 [HMAC][HMAC-
SHA256] and [AES-CMAC] when AES is more readily available than a hash SHA256] and [AES-CMAC] when AES is more readily available than a hash
function. function.
3. IANA Considerations 3. IANA Considerations
None. None.
4. Acknowledgements 4. Acknowledgements
Obviously, we have to thank all the cryptographers who produced the Obviously, we have to thank all the cryptographers who produced the
results we refer to in this document. We'd also like to thank Martin results we refer to in this document. We'd also like to thank Alfred
Rex and Benne de Weger for their comments. Hoenes, Martin Rex, and Benne de Weger for their comments.
5. Normative References 5. Normative References
[AES-CMAC] Song, J., Poovendran, R., Lee., J., and T. Iwata, [AES-CMAC] Song, J., Poovendran, R., Lee., J., and T. Iwata,
"The AES-CMAC Algorithm", RFC 4493, June 2006. "The AES-CMAC Algorithm", RFC 4493, June 2006.
[COYI2006] S. Contini, Y.L. Yin. Forgery and partial key- [COYI2006] S. Contini, Y.L. Yin. Forgery and partial key-
recovery attacks on HMAC and NMAC using hash recovery attacks on HMAC and NMAC using hash
collisions. ASIACRYPT 2006. LNCS 4284, Springer, collisions. ASIACRYPT 2006. LNCS 4284, Springer,
2006. 2006.
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