Network Working Group R. Housley
Request for Comments: 3874 Vigil Security
Category: Informational September 2004
A 224bit Oneway Hash Function: SHA224
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 224bit oneway hash function, called
SHA224. SHA224 is based on SHA256, but it uses a different
initial value and the result is truncated to 224 bits.
1. Introduction
This document specifies a 224bit oneway hash function, called
SHA224. The National Institute of Standards and Technology (NIST)
announced the FIPS 1802 Change Notice on February 28, 2004 which
specifies the SHA224 oneway hash function. Oneway hash functions
are also known as message digests. SHA224 is based on SHA256, the
256bit oneway hash function already specified by NIST [SHA2].
Computation of a SHA224 hash value is two steps. First, the SHA256
hash value is computed, except that a different initial value is
used. Second, the resulting 256bit 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. Oneway hash functions are available for all of these
levels except one. SHA224 fills this void. SHA224 is a oneway
hash function that provides 112 bits of security, which is the
generally accepted strength of TripleDES [3DES].
This document makes the SHA224 oneway hash function specification
available to the Internet community, and it publishes the object
identifiers for use in ASN.1based protocols.
1.1. Usage Considerations
Since SHA224 is based on SHA256, roughly the same amount of effort
is consumed to compute a SHA224 or a SHA256 digest message digest
value. Even though SHA224 and SHA256 have roughly equivalent
computational complexity, SHA224 is an appropriate choice for a
oneway hash function that provides 112 bits of security. The use of
a different initial value ensures that a truncated SHA256 message
digest value cannot be mistaken for a SHA224 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 SHA224 is important.
These observations lead to the following guidance:
* When selecting a suite of cryptographic algorithms that all offer
112 bits of security strength, SHA224 is an appropriate choice
for oneway hash function.
* When terseness is not a selection criteria, the use of SHA256 is
a preferred alternative to SHA224.
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. SHA224 Description
SHA224 may be used to compute a oneway hash value on a message
whose length less than 2^64 bits.
SHA224 makes use of SHA256 [SHA2]. To compute a oneway hash
value, SHA256 uses a message schedule of sixtyfour 32bit words,
eight 32bit working variables, and produces a hash value of eight
32bit words.
The function is defined in the exact same manner as SHA256, with the
following two exceptions:
First, for SHA224, the initial hash value of the eight 32bit
working variables, collectively called H, shall consist of the
following eight 32bit words (in hex):
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, SHA224 simply makes use of the first seven 32bit words
in the SHA256 result, discarding the remaining 32bit words in
the SHA256 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 SHA224.
3.1. Test Vector #1
Let the message to be hashed be the 24bit ASCII string "abc", which
is equivalent to the following binary string:
01100001 01100010 01100011
The SHA224 hash value (in hex):
23097d22 3405d822 8642a477 bda255b3 2aadbce4 bda0b3f7 e36c9da7
3.2. Test Vector #2
Let the message to be hashed be the 448bit ASCII string
"abcdbcdecdefdefgefghfghighijhijkijkljklmklmnlmnomnopnopq".
The SHA224 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 binarycoded form of the ASCII
string which consists of 1,000,000 repetitions of the character "a".
The SHA224 hash value is (in hex):
20794655 980c91d8 bbb4c1ea 97618a4b f03f4258 1948b2ee 4ee7ad67
4. Object Identifier
NIST has assigned an ASN.1 [X.20888, X.20988] object identifier for
SHA224. Some protocols use object identifiers to name oneway hash
functions. One example is CMS [CMS]. Implementations of such
protocols that make use of SHA224 MUST use the following object
identifier.
idsha224 OBJECT IDENTIFIER ::= { jointisoitut(2)
country(16) us(840) organization(1) gov(101)
csor(3) nistalgorithm(4) hashalgs(2) sha224(4) }
5. Security Considerations
Oneway hash functions are typically used with other cryptographic
algorithms, such as digital signature algorithms and keyedhash
message authentication codes, or in the generation of random values.
When a oneway hash function is used in conjunction with another
algorithm, there may be requirements specified elsewhere that require
the use of a oneway 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 oneway hash algorithm
that also provides the same number of bits of security. SHA224 is
intended to provide 112 bits of security, which is the generally
accepted strength of TripleDES [3DES].
This document is intended to provide the SHA224 specification to the
Internet community. No independent assertion of the security of this
oneway hash function is intended by the author for any particular
use. However, as long as SHA256 provides the expected security,
SHA224 will also provide its expected level of security.
6. References
6.1. Normative References
[SHA2] Federal Information Processing Standards Publication
(FIPS PUB) 1802, 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.
6.2. Informative References
[3DES] American National Standards Institute. ANSI X9.521998,
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/guideline1.pdf]
[X.20888] CCITT Recommendation X.208: Specification of Abstract
Syntax Notation One (ASN.1). 1988.
[X.20988] 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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