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.
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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):
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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_6
3. 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 e36c9da7
3.2. 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 52522525
3.3. 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 4ee7ad67
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4. 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. References
6.1. 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.
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6.2. 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)", RFC
3852, 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.com
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RFC 3874 A 224-bit One-way Hash Function: SHA-224 September 2004
9. Full Copyright Statement
Copyright (C) The Internet Society (2004).
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Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
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