Internet Engineering Task Force (IETF) T. Polk
Request for Comments: 6194 L. Chen
Category: Informational NIST
ISSN: 2070-1721 S. Turner
IECA
P. Hoffman
VPN Consortium
March 2011
Security Considerations for
the SHA-0 and SHA-1 Message-Digest Algorithms
Abstract
This document includes security considerations for the SHA-0 and
SHA-1 message digest algorithm.
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/rfc6194.
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
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
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RFC 6194 SHA-0 and SHA-1 Security Consideration March 2011
1. Introduction
The Secure Hash Algorithms are specified in [SHS]. A previous
version of [SHS] also specified SHA-0. SHA-0, first published in
1993, and SHA-1, first published in 1996, are message digest
algorithms, sometimes referred to as hash functions or hash
algorithms, that take as input a message of arbitrary length and
produce as output a 160-bit "fingerprint" or "message digest" of the
input. The published attacks against both algorithms show that it is
not prudent to use either algorithm when collision resistance is
required.
[HASH-Attack] summarizes the use of hashes in Internet 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.
Some may find the guidance for key lengths and algorithm strengths in
[SP800-57] and [SP800-131] useful.
2. SHA-0 Security Considerations
What follows are summaries of recent attacks against SHA-0's
collision, pre-image, and second pre-image resistance. Additionally,
attacks against SHA-0 when used as a keyed-hash (e.g., HMAC-SHA-0)
are discussed.
The U.S. National Institute of Standards and Technology (NIST)
withdrew SHA-0 in 1996. That is, NIST no longer considers it
appropriate to use SHA-0 for any transactions associated with the use
of cryptography by U.S. federal government agencies for the
protection of sensitive, but unclassified information. SHA-0 is
discussed here only for the sake of completeness.
Any use of SHA-0 is strongly discouraged. Analysis of SHA-0
continues today because many see it as a weaker version of SHA-1.
2.1. Collision Resistance
The first attack on SHA-0 was published in 1998 [CHJO1998] and showed
that collisions can be found in 2^61 operations. In 2006,
[NSSYK2006] showed an improved attack that can find collisions in
2^36 operations.
In any case, the known research results indicate that SHA-0 is not as
collision resistant as expected. The collision security strength is
significantly less than an ideal hash function (i.e., 2^36 compared
to 2^80).
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RFC 6194 SHA-0 and SHA-1 Security Consideration March 2011
2.2. Pre-Image and Second Pre-Image Resistance
The pre-image and second pre-image attacks published on reduced
versions of SHA-0 (i.e., less than 80 rounds) indicate that the
security margin of SHA-0 is resistant to these attacks. [deCARE2008]
showed a pre-image attack on 49 out of 80 rounds with complexity of
2^159, and [AOSA2009] showed a pre-image attack on 52 out of 80
rounds with a complexity of 2^156.
2.3. HMAC-SHA-0
The current attack vectors on HMAC can be classified as follows:
distinguishing attacks, existential forgery attacks, and key recovery
attacks. Key recovery attacks are by far the most severe.
Attacks on hash functions can be conducted entirely offline, since
the attacker can generate unlimited message-hash value pairs.
Attacks on HMACs must be online because attackers need a large amount
of HMAC values to deduce the key. The best results for a partial key
recovery attack on HMAC-SHA0 were published at Asiacrypt 2006 with
2^84 queries and 2^60 SHA-0 computations [COYI2006].
3. SHA-1 Security Considerations
What follows are recent attacks against SHA-1's collision, pre-image,
and second pre-image resistance. Additionally, attacks against SHA-1
when used as a keyed-hash (i.e., HMAC-SHA-1) are discussed.
It must be noted that NIST has recommended that SHA-1 not be used for
generating digital signatures after December 31, 2010, and has
specified that it not be used for generating digital signatures by
U.S. federal government agencies "for the protection of sensitive,
but unclassified information" after December 31, 2013 [SP800-131].
3.1. Collision Resistance
The first attack on SHA-1 was published in early 2005 [RIOS2005].
This attack described a theoretical attack on a version of SHA-1
reduced to 53 rounds. The very next month [WLY2005] showed
collisions in the full 80 rounds in 2^69 operations. Since then,
many new analysis methods have been developed to improve the attack
presented in [WLY2005]. However, there are no published results that
improve upon the results found in [WLY2005]. [Man2008/469], which is
the International Association for Cryptologic Research (IACR) ePrint
version of [Man2009], claimed that using the method presented in the
paper, a collision of full SHA-1 can be found in 2^51 hash function
calls. However, this claim is absent from the published conference
paper [Man2009].
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RFC 6194 SHA-0 and SHA-1 Security Consideration March 2011
In any case, the known research results indicate that SHA-1 is not as
collision resistant as expected. The collision security strength is
significantly less than an ideal hash function (i.e., 2^69 compared
to 2^80).
3.2. Pre-Image and Second Pre-Image Resistance
There are no known pre-image or second pre-image attacks that are
specific to the full round SHA-1 algorithm. [KeSch] discovered a
general result for all narrow-pipe Merkle-Damgaard hash functions
(which includes SHA-1), finding a second pre-image takes less than
2^n computations. When n = 160, as is the case for SHA-1, it will
take 2^106 computations to find a second pre-image in a 60-byte
message.
In the absence of full-round attacks, cryptographers consider
reduced-round attacks for clues regarding an algorithm's strength.
Reduced-round attacks, where the number of reduced rounds is not more
than a few less than the full rounds, have not been shown to relate
to full-round attacks. However, the best reduced-round attack
indicates a certain security margin. For example, if the best known
attack is on 60 out of 80 rounds, then the algorithm has about 20
rounds to resist improved attacks. However, the relationship between
the number of rounds an attack can reach and the number of rounds
defined in the algorithm is not linear; it does not provide a
mathematical proof. In other words, reduced-round attacks indicate
how strong the algorithm is with regard to a certain attack, not how
close it is to being broken. Therefore, the following information
about reduced-round attacks is included only for completeness.
The pre-image and second pre-image attacks published on reduced
versions of SHA-1 (i.e., less than 80 rounds) indicate that SHA-1
retains a significant security margin against these attacks.
[AOSA2009] showed a pre-image attack on 48 out of 80 rounds with
complexity of 2^159.
3.3. HMAC-SHA-1
As of today, there is no indication that attacks on SHA-1 can be
extended to HMAC-SHA-1.
4. Conclusions
SHA-1 provides less collision resistance than was originally
expected, and collision resistance has been shown to affect some (but
not all) applications that use digital signatures. Designers of IETF
protocols that use digital signature algorithms should strongly
consider support for a hash algorithm with greater collision
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resistance than that provided by SHA-1. Of course, SHA-0 should
continue to not be used in any IETF protocol.
[Note: Protocol designers should review the current state of the art
to ensure that selected hash algorithms provide sufficient security.
At the time of publication, SHA-256 [SHS] is the most commonly
specified alternative. The known (reduced-round) attacks on the
collision resistance of SHA-256 indicate a significant security
margin, and the longer message digest provides increased strength.]
Nearly all IETF protocols that use signatures assume existing public
key infrastructures, and SHA-1 is still used in signatures nearly
everywhere. Therefore, it is unwise to strictly prohibit the use of
SHA-1 in signature algorithms. Protocols that permit the use of
SHA-1-based digital signatures as an option should strongly consider
referencing this document in the security considerations.
A protocol designer might want to consider the use of SHA-1 with
randomized hashing such as is specified in [SP800-107]. Note that
randomized hashing expands the size of signatures and requires
protocols to carry material that is not needed today. HMAC-SHA-1
remains secure and is the preferred keyed-hash algorithm for IETF
protocol design.
5. Security Considerations
This entire document is about security considerations.
6. Acknowledgements
We'd like to thank Ran Atkinson and Sheila Frankel for their comments
and suggestions.
7. Normative References
[AOSA2009] Aoki, K., and K. Saski, "Meet-in-the-Middle Preimage
Attacks Against Reduced SHA-0 and SHA-1", Crypto 2009.
[deCARE2008] De Canniere, C., and C. Rechberger, "Preimages for
Reduced SHA-0 and SHA-1", Crypto 2008.
[CHJO1998] Chaubad, F., and A. Joux, "Differential Collisions in
SHA-0", Crypto 1998.
[COYI2006] Contini, S., and Y. Lin, "Forgery and Partial Key-
Recovery Attacks on HMAC and NMAC Using Hash
Collisions", Asiacrypt 2006.
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RFC 6194 SHA-0 and SHA-1 Security Consideration March 2011
[HASH-Attack] Hoffman, P. and B. Schneier, "Attacks on Cryptographic
Hashes in Internet Protocols", RFC 4270, November 2005.
[KeSch] Kelsey, J., and B. Schneier, "Second Preimages on n-Bit
Hash Functions for Much Less than 2n Work", In Cramer,
R., ed.: Eurocrypt 2005. Volume 3494 of Lecture Notes
in Computer Science, Springer (2005) 474-490.
[Man2008/469] Manuell, S., "Classification and Generation of
Disturbance Vectors for Collision Attacks against
SHA-1", http://eprint.iacr.org/2008/469.pdf.
[Man2009] Manuell, S., "Classification and Generation of
Disturbance Vectors for Collision Attacks against
SHA-1", International Workshop on Coding and
Cryptography, 2009, Norway.
[NSSYK2006] Naito, Y., Sasaki, Y., Shimoyama, T., Yajima, J.,
Kunihiro, N., and K. Ohta, "Improved Collision Search
for SHA-0", Asiacrypt 2006.
[RIOS2005] Rijmen, V., and E. Oswald, "Update on SHA-1", CT-RSA
2005, Lecture Notes in Computer Science, vol. 3376, pp.
58-71.
[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.
[SP800-107] National Institute of Standards and Technology (NIST),
Special Publication 800-107: Recommendation for
Applications using Approved Hash Algorithms, February
2009.
[SP800-131] National Institute of Standards and Technology (NIST),
Special Publication 800-131A: Recommendation for the
Transitioning of Cryptographic Algorithms and Key
Sizes, January 2011.
[WLY2005] Wang, X., Yin, Y., and H. Yu., "Finding Collisions in
the Full SHA-1", Crypto 2005.
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Authors' Addresses
Tim Polk
National Institute of Standards and Technology
100 Bureau Drive, Mail Stop 8930
Gaithersburg, MD 20899-8930
USA
EMail: tim.polk@nist.gov
Lily Chen
National Institute of Standards and Technology
100 Bureau Drive, Mail Stop 8930
Gaithersburg, MD 20899-8930
USA
EMail: lily.chen@nist.gov
Sean Turner
IECA, Inc.
3057 Nutley Street, Suite 106
Fairfax, VA 22031
USA
EMail: turners@ieca.com
Paul Hoffman
VPN Consortium
EMail: paul.hoffman@vpnc.org
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