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 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