ftp.cc.uoc.gr
rfc6194
This is a purely informative rendering of an RFC that includes verified errata. This rendering may not be used as a reference.

The following 'Verified' errata have been incorporated in this document: EID 3683
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
   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.

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).

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].

   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, the estimated computational 
 complexity of finding a second preimage of any given message of 
about 2^60 bytes in length is 2^106 (compression function 
executions) which is significantly less than 2^160. 
EID 3683 (Verified) is as follows:

Section: 3.2

Original Text:

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.

Corrected Text:

When n = 160, as is the case for SHA-1, the estimated computational
 complexity of finding a second preimage of any given message of 
about 2^60 bytes in length is 2^106 (compression function 
executions) which is significantly less than 2^160. 
Notes:
Clarification.

spt: I replaced 2^55 blocks with 2^60 bytes after some consultation with Lily and Quynh.
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 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. [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. 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