# A 224-bit One-way Hash Function: SHA-224

RFC 3874

Document | Type | RFC - Informational (September 2004; No errata) | |
---|---|---|---|

Last updated | 2013-03-02 | ||

Stream | IETF | ||

Formats | plain text pdf html | ||

Stream | WG state | (None) | |

Consensus | Unknown | ||

Document shepherd | No shepherd assigned | ||

IESG | IESG state | RFC 3874 (Informational) | |

Telechat date | |||

Responsible AD | Steven Bellovin | ||

Send notices to | kent@bbn.com, wpolk@nist.gov, housley@vigilsec.com |

Network Working Group R. Housley Request for Comments: 3874 Vigil Security Category: Informational September 2004 A 224-bit One-way Hash Function: SHA-224 Status 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 2004 1.1. 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 inShow full document text