Security Considerations for the SHA-0 and SHA-1 Message-Digest Algorithms
RFC 6194

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.

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   document authors.  All rights reserved.

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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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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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   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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   [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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