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Network Working Group                                         R. HousleyRequest for Comments: 3874                                Vigil SecurityCategory: Informational                                   September 2004A 224-bit One-way Hash Function: SHA-224Status of this Memo   This memo provides information for the Internet community.  It does   not specify an Internet standard of any kind.  Distribution of this   memo is unlimited.Copyright Notice   Copyright (C) The Internet Society (2004).Abstract   This document specifies a 224-bit one-way hash function, called   SHA-224.  SHA-224 is based on SHA-256, but it uses a different   initial value and the result is truncated to 224 bits.1.  Introduction   This document specifies a 224-bit one-way hash function, called   SHA-224.  The National Institute of Standards and Technology (NIST)   announced the FIPS 180-2 Change Notice on February 28, 2004 which   specifies the SHA-224 one-way hash function.  One-way hash functions   are also known as message digests.  SHA-224 is based on SHA-256, the   256-bit one-way hash function already specified by NIST [SHA2].   Computation of a SHA-224 hash value is two steps.  First, the SHA-256   hash value is computed, except that a different initial value is   used.  Second, the resulting 256-bit hash value is truncated to 224   bits.   NIST is developing guidance on cryptographic key management, and NIST   recently published a draft for comment [NISTGUIDE].  Five security   levels are discussed in the guidance: 80, 112, 128, 192, and 256 bits   of security.  One-way hash functions are available for all of these   levels except one.  SHA-224 fills this void.  SHA-224 is a one-way   hash function that provides 112 bits of security, which is the   generally accepted strength of Triple-DES [3DES].   This document makes the SHA-224 one-way hash function specification   available to the Internet community, and it publishes the object   identifiers for use in ASN.1-based protocols.Housley                      Informational                      [Page 1]

RFC 3874        A 224-bit One-way Hash Function: SHA-224  September 20041.1.  Usage Considerations   Since SHA-224 is based on SHA-256, roughly the same amount of effort   is consumed to compute a SHA-224 or a SHA-256 digest message digest   value.  Even though SHA-224 and SHA-256 have roughly equivalent   computational complexity, SHA-224 is an appropriate choice for a   one-way hash function that provides 112 bits of security.  The use of   a different initial value ensures that a truncated SHA-256 message   digest value cannot be mistaken for a SHA-224 message digest value   computed on the same data.   Some usage environments are sensitive to every octet that is   transmitted.  In these cases, the smaller (by 4 octets) message   digest value provided by SHA-224 is important.   These observations lead to the following guidance:   *  When selecting a suite of cryptographic algorithms that all offer      112 bits of security strength, SHA-224 is an appropriate choice      for one-way hash function.   *  When terseness is not a selection criteria, the use of SHA-256 is      a preferred alternative to SHA-224.1.2.  Terminology   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this   document are to be interpreted as described in [STDWORDS].2.  SHA-224 Description   SHA-224 may be used to compute a one-way hash value on a message   whose length less than 2^64 bits.   SHA-224 makes use of SHA-256 [SHA2].  To compute a one-way hash   value, SHA-256 uses a message schedule of sixty-four 32-bit words,   eight 32-bit working variables, and produces a hash value of eight   32-bit words.   The function is defined in the exact same manner as SHA-256, with the   following two exceptions:      First, for SHA-224, the initial hash value of the eight 32-bit      working variables, collectively called H, shall consist of the      following eight 32-bit words (in hex):Housley                      Informational                      [Page 2]

RFC 3874        A 224-bit One-way Hash Function: SHA-224  September 2004         H_0 = c1059ed8               H_4 = ffc00b31         H_1 = 367cd507               H_5 = 68581511         H_2 = 3070dd17               H_6 = 64f98fa7         H_3 = f70e5939               H_7 = befa4fa4      Second, SHA-224 simply makes use of the first seven 32-bit words      in the SHA-256 result, discarding the remaining 32-bit words in      the SHA-256 result.  That is, the final value of H is used as      follows, where || denotes concatenation:         H_0 || H_1 || H_2 || H_3 || H_4 || H_5 || H_63.  Test Vectors   This section includes three test vectors.  These test vectors can be   used to test implementations of SHA-224.3.1.  Test Vector #1   Let the message to be hashed be the 24-bit ASCII string "abc", which   is equivalent to the following binary string:      01100001 01100010 01100011   The SHA-224 hash value (in hex):      23097d22 3405d822 8642a477 bda255b3 2aadbce4 bda0b3f7 e36c9da73.2.  Test Vector #2   Let the message to be hashed be the 448-bit ASCII string   "abcdbcdecdefdefgefghfghighijhijkijkljklmklmnlmnomnopnopq".   The SHA-224 hash value is (in hex):      75388b16 512776cc 5dba5da1 fd890150 b0c6455c b4f58b19 525225253.3.  Test Vector #3   Let the message to be hashed be the binary-coded form of the ASCII   string which consists of 1,000,000 repetitions of the character "a".   The SHA-224 hash value is (in hex):      20794655 980c91d8 bbb4c1ea 97618a4b f03f4258 1948b2ee 4ee7ad67Housley                      Informational                      [Page 3]

RFC 3874        A 224-bit One-way Hash Function: SHA-224  September 20044.  Object Identifier   NIST has assigned an ASN.1 [X.208-88,X.209-88] object identifier for   SHA-224.  Some protocols use object identifiers to name one-way hash   functions.  One example is CMS [CMS].  Implementations of such   protocols that make use of SHA-224 MUST use the following object   identifier.      id-sha224  OBJECT IDENTIFIER  ::=  { joint-iso-itu-t(2)                      country(16) us(840) organization(1) gov(101)                      csor(3) nistalgorithm(4) hashalgs(2) sha224(4) }5.  Security Considerations   One-way hash functions are typically used with other cryptographic   algorithms, such as digital signature algorithms and keyed-hash   message authentication codes, or in the generation of random values.   When a one-way hash function is used in conjunction with another   algorithm, there may be requirements specified elsewhere that require   the use of a one-way hash function with a certain number of bits of   security.  For example, if a message is being signed with a digital   signature algorithm that provides 128 bits of security, then that   signature algorithm may require the use of a one-way hash algorithm   that also provides the same number of bits of security.  SHA-224 is   intended to provide 112 bits of security, which is the generally   accepted strength of Triple-DES [3DES].   This document is intended to provide the SHA-224 specification to the   Internet community.  No independent assertion of the security of this   one-way hash function is intended by the author for any particular   use.  However, as long as SHA-256 provides the expected security,   SHA-224 will also provide its expected level of security.6.  References6.1.  Normative References   [SHA2]      Federal Information Processing Standards Publication               (FIPS PUB) 180-2, Secure Hash Standard, 1 August 2002.   [STDWORDS]  Bradner, S., "Key words for use in RFCs to Indicate               Requirement Levels",BCP 14,RFC 2119, March 1997.Housley                      Informational                      [Page 4]

RFC 3874        A 224-bit One-way Hash Function: SHA-224  September 20046.2.  Informative References   [3DES]      American National Standards Institute.  ANSI X9.52-1998,               Triple Data Encryption Algorithm Modes of Operation.               1998.   [CMS]       Housley, R., "Cryptographic Message Syntax (CMS)",RFC3852, July 2004.   [NISTGUIDE] National Institute of Standards and Technology.  Second               Draft: "Key Management Guideline, Part 1:  General               Guidance."  June 2002.               [http://csrc.nist.gov/encryption/kms/guideline-1.pdf]   [X.208-88]  CCITT Recommendation X.208: Specification of Abstract               Syntax Notation One (ASN.1). 1988.   [X.209-88]  CCITT Recommendation X.209: Specification of Basic               Encoding Rules for Abstract Syntax Notation One (ASN.1).               1988.7.  Acknowledgments   Many thanks to Jim Schaad for generating the test vectors.  A second   implementation by Brian Gladman was used to confirm that the test   vectors are correct.8.  Author's Address   Russell Housley   Vigil Security, LLC   918 Spring Knoll Drive   Herndon, VA 20170   USA   EMail: housley@vigilsec.comHousley                      Informational                      [Page 5]

RFC 3874        A 224-bit One-way Hash Function: SHA-224  September 20049.  Full Copyright Statement   Copyright (C) The Internet Society (2004).   This document is subject to the rights, licenses and restrictions   contained inBCP 78, and except as set forth therein, the authors   retain all their rights.   This document and the information contained herein are provided on an   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/S HE   REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE   INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR   IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF   THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED   WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.Intellectual Property   The IETF takes no position regarding the validity or scope of any   Intellectual Property Rights or other rights that might be claimed to   pertain to the implementation or use of the technology described in   this document or the extent to which any license under such rights   might or might not be available; nor does it represent that it has   made any independent effort to identify any such rights.  Information   on the IETF's procedures with respect to rights in IETF Documents can   be found inBCP 78 andBCP 79.   Copies of IPR disclosures made to the IETF Secretariat and any   assurances of licenses to be made available, or the result of an   attempt made to obtain a general license or permission for the use of   such proprietary rights by implementers or users of this   specification can be obtained from the IETF on-line IPR repository athttp://www.ietf.org/ipr.   The IETF invites any interested party to bring to its attention any   copyrights, patents or patent applications, or other proprietary   rights that may cover technology that may be required to implement   this standard.  Please address the information to the IETF at ietf-   ipr@ietf.org.Acknowledgement   Funding for the RFC Editor function is currently provided by the   Internet Society.Housley                      Informational                      [Page 6]