Internet Draft IPsec Working Group
June 2002 S. Frankel, NIST
Expiration Date: December 2002 S. Kelly, Black Storm Networks
Category: Experimental
The HMAC-SHA-256-128 Algorithm and Its Use With IPsec
<draft-ietf-ipsec-ciph-sha-256-01.txt>
Status of this Memo
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Abstract
This document describes the use of the HMAC algorithm in conjunction
with the SHA-256 algorithm as an experimental authentication mecha-
nism within the context of the IPsec AH and ESP protocols. This algo-
rithm is intended to provide data origin authentication and integrity
protection. Given the current lack of practical experience with
SHA-256, implementations based on this document will be experimental
in nature, and implementation is not required in order to claim com-
pliance with the IPsec proposed standards. The version of the HMAC-
SHA-256 authenticator described in this document specifies truncation
to 128 bits, and is therefore named HMAC-SHA-256-128.
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Table of Contents
1. Specification of Requirements . . . . . . . . . . . . . . . . . 3
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. The HMAC-SHA-256-128 Algorithm . . . . . . . . . . . . . . . . . 3
3.1 Keying Material . . . . . . . . . . . . . . . . . . . . . . 3
3.2 Padding . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3.3 Truncation . . . . . . . . . . . . . . . . . . . . . . . . . 4
3.4 Interaction with the ESP Cipher Mechanism . . . . . . . . . 4
3.5 Performance . . . . . . . . . . . . . . . . . . . . . . . . 5
3.6 Test Vectors . . . . . . . . . . . . . . . . . . . . . . . . 5
4. IKE Interactions . . . . . . . . . . . . . . . . . . . . . . . . 7
4.1 Phase 1 Identifier . . . . . . . . . . . . . . . . . . . . . 7
4.2 Phase 2 Identifier . . . . . . . . . . . . . . . . . . . . . 7
5. Security Considerations . . . . . . . . . . . . . . . . . . . . 7
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 8
7. Intellectual Property Rights Statement . . . . . . . . . . . . . 8
8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . 8
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
10. Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 9
11. Full Copyright Statement . . . . . . . . . . . . . . . . . . . . 10
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1. Specification of Requirements
The keywords "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" that
appear in this document are to be interpreted as described in
[RFC-2119].
2. Introduction
This document specifies the use of SHA-256 [SHA2-1] combined with
HMAC [HMAC] as an experimental keyed authentication mechanism within
the context of the IPsec AH and ESP protocols. This algorithm is in-
tended to provide data origin authentication and integrity protec-
tion. Given the current lack of practical experience with SHA-256,
implementations based on this document will be experimental in na-
ture, and implementation is not required in order to claim compliance
with the IPsec proposed standards. Furthermore, HMAC-SHA-1-96 [HMAC-
SHA] provides sufficient security at a lower computational cost. The
version of the HMAC-SHA-256 authenticator described in this document
specifies truncation to 128 bits, and is therefore named HMAC-
SHA-256-128. For further information on ESP, refer to [ESP] and
[ROADMAP]. For further information on AH, refer to [AH] and
[ROADMAP].
The goal of HMAC-SHA-256-128 is to ensure that the packet is authen-
tic and cannot be modified in transit. Data integrity and data ori-
gin authentication as provided by HMAC-SHA-256-128 are dependent upon
the scope of the distribution of the secret key. If the key is known
only by the source and destination, this algorithm will provide both
data origin authentication and data integrity for packets sent be-
tween the two parties. In addition, only a party with the identical
key can verify the MAC.
3. The HMAC-SHA-256-128 Algorithm
[SHA2-1] and [SHA2-2] describe the underlying SHA-256 algorithm,
while [HMAC] describes the HMAC algorithm. The HMAC algorithm pro-
vides a framework for inserting various hashing algorithms such as
SHA-256.
The following sections contain descriptions of the various character-
istics and requirements of the HMAC-SHA-256-128 algorithm.
3.1 Keying Material
HMAC-SHA-256-128 is a secret key algorithm. While no fixed key length
is specified in [HMAC], for use with either ESP or AH a fixed key
length of 256-bits MUST be supported. Key lengths other than 256-
bits MUST NOT be supported (i.e. only 256-bit keys are to be used by
HMAC-SHA-256-128). A key length of 256-bits was chosen based on the
recommendations in [HMAC] (i.e. key lengths less than the authentica-
tor length decrease security strength and keys longer than the au-
thenticator length do not significantly increase security strength).
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[HMAC] discusses requirements for key material, which includes a dis-
cussion on requirements for strong randomness. A strong pseudo-random
function MUST be used to generate the required 256-bit key.
At the time of this writing there are no specified weak keys for use
with HMAC. This does not mean to imply that weak keys do not exist.
[ARCH] describes the general mechanism for obtaining keying material
when multiple keys are required for a single SA (e.g. when an ESP SA
requires a key for confidentiality and a key for authentication).
In order to provide data origin authentication, the key distribution
mechanism must ensure that unique keys are allocated and that they
are distributed only to the parties participating in the communica-
tion.
[HMAC] makes the following recommendation with regard to rekeying:
"Current attacks do not indicate a specific recommended frequency for
key changes ... However, periodic key refreshment is a fundamental
security practice that helps against potential weaknesses of the
function and keys, and limits the damage of an exposed key." Rekey-
ing also reduces the information available to a cryptanalyst.
3.2 Padding
HMAC-SHA-256-128 operates on 512-bit blocks of data. Padding require-
ments are specified in [SHA2-1] and are part of the SHA-256 algo-
rithm. If you build SHA-256 according to [SHA2-1] you do not need to
add any additional padding as far as HMAC-SHA-256-128 is concerned.
With regard to "implicit packet padding" as defined in [AH], no im-
plicit packet padding is required.
3.3 Truncation
HMAC-SHA-256-128 produces a 256-bit authenticator value. This 256-bit
value can be truncated as described in [HMAC]. For use with either
ESP or AH, a truncated value using the first 128 bits MUST be sup-
ported. Upon sending, the truncated value is stored within the au-
thenticator field. Upon receipt, the entire 256-bit value is computed
and the first 128 bits are compared to the value stored in the au-
thenticator field. No other authenticator value lengths are supported
by HMAC-SHA-256-128.
The length of 128 bits was selected because it meets the security re-
quirements described in [HMAC]. [HMAC] discusses the potential addi-
tional security which is provided by the truncation of the resulting
MAC. Specifications which include HMAC are strongly encouraged to
perform this MAC truncation.
3.4 Interaction with the ESP Cipher Mechanism
As of this writing, there are no known issues which preclude the use
of the HMAC-SHA-256-128 with any specific cipher algorithm.
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3.5 Performance
[HASH] states that "(HMAC) performance is essentially that of the un-
derlying hash function". As of this writing no detailed performance
analysis has been done of SHA-256, HMAC or HMAC combined with
SHA-256.
[HMAC] outlines an implementation modification which can improve per-
packet performance without affecting interoperability.
3.6 Test Vectors
The following test cases for HMAC-SHA-256 and HMAC-SHA-256-128 in-
clude the key, the data, and the resulting HMAC. The values of keys
and data are either hexadecimal numbers (prefixed by "0x") or ASCII
character strings (surrounded by double quotes). If a value is an
ASCII character string, then the HMAC computation for the correspond-
ing test case DOES NOT include the trailing null character ('\0') of
the string. The computed HMAC values are all hexadecimal numbers.
These test cases were verified using 3 independent implementations:
an HMAC wrapper on top of Aaron Gifford's SHA256 implementation
(www.aarongifford.com/computers/sha.html), the BeeCrypt crypto li-
brary (www.virtualunlimited.com/products/beecrypt) and the Nettle
cryptographic library (www.lysator.liu.se/~nisse/nettle). Partial
blocks were padded as specified in [SHA2-1].
Test cases 1 and 2 were taken from the SHA-2 FIPS [SHA2-1] and test
cases 4-10 were borrowed from [HMAC-TEST] with some key sizes adjust-
ed for HMAC-SHA-256. These test cases illustrate HMAC-SHA-256 with
various combinations of input and keysize. All test cases include the
computed HMAC-SHA-256; only those with a keysize of 32 bytes (256
bits) also include the truncated HMAC-SHA-256-128.
Test Case #1: HMAC-SHA-256 with 3-byte input and 32-byte key
Key_len : 32
Key : 0x0102030405060708090a0b0c0d0e0f10
1112131415161718191a1b1c1d1e1f20
Data_len : 3
Data : "abc"
HMAC-SHA-256 : 0xa21b1f5d4cf4f73a4dd939750f7a066a
7f98cc131cb16a6692759021cfab8181
HMAC-SHA-256-128: 0xa21b1f5d4cf4f73a4dd939750f7a066a
Test Case #2: HMAC-SHA-256 with 56-byte input and 32-byte key
Key_len : 32
Key : 0x0102030405060708090a0b0c0d0e0f10
1112131415161718191a1b1c1d1e1f20
Data_len : 56
Data : "abcdbcdecdefdefgefghfghighijhijk
ijkljklmklmnlmnomnopnopq"
HMAC-SHA-256 : 0x104fdc1257328f08184ba73131c53cae
e698e36119421149ea8c712456697d30
HMAC-SHA-256-128: 0x104fdc1257328f08184ba73131c53cae
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Test Case #3: HMAC-SHA-256 with 112-byte (multi-block) input
and 32-byte key
Key_len : 32
Key : 0x0102030405060708090a0b0c0d0e0f10
1112131415161718191a1b1c1d1e1f20
Data_len : 112
Data : "abcdbcdecdefdefgefghfghighijhijk
ijkljklmklmnlmnomnopnopqabcdbcde
cdefdefgefghfghighijhijkijkljklm
klmnlmnomnopnopq"
HMAC-SHA-256 : 0x470305fc7e40fe34d3eeb3e773d95aab
73acf0fd060447a5eb4595bf33a9d1a3
HMAC-SHA-256-128: 0x470305fc7e40fe34d3eeb3e773d95aab
Test Case #4: HMAC-SHA-256 with 8-byte input and 32-byte key
Key_len : 32
Key : 0x0b repeated 32 times
Data_len : 8
Data : 0x4869205468657265
Data : "Hi There"
HMAC-SHA-256 : 0x198a607eb44bfbc69903a0f1cf2bbdc5
ba0aa3f3d9ae3c1c7a3b1696a0b68cf7
HMAC-SHA-256-128: 0x198a607eb44bfbc69903a0f1cf2bbdc5
Test Case #5: HMAC-SHA-256 with 28-byte input and 4-byte key
Key_len : 4
Key : "Jefe"
Data_len : 28
Data : "what do ya want for nothing?"
HMAC-SHA-256 : 0x5bdcc146bf60754e6a042426089575c7
5a003f089d2739839dec58b964ec3843
Test Case #6: HMAC-SHA-256 with 50-byte input and 32-byte key
Key_len : 32
Key : 0xaa repeated 32 times
Data_len : 50
Data : 0xdd repeated 50 times
HMAC-SHA-256 : 0xcdcb1220d1ecccea91e53aba3092f962
e549fe6ce9ed7fdc43191fbde45c30b0
HMAC-SHA-256-128: 0xcdcb1220d1ecccea91e53aba3092f962
Test Case #7: HMAC-SHA-256 with 50-byte input and 37-byte key
Key_len : 37
Key : 0x0102030405060708090a0b0c0d0e0f10
1112131415161718191a1b1c1d1e1f20
2122232425
Data_len : 50
Data : 0xcd repeated 50 times
HMAC-SHA-256 : 0xd4633c17f6fb8d744c66dee0f8f07455
6ec4af55ef07998541468eb49bd2e917
Test Case #8: HMAC-SHA-256 with 20-byte input and 32-byte key
Key_len : 32
Key : 0x0c repeated 32 times
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Data_len : 20
Data : "Test With Truncation"
HMAC-SHA-256 : 0x7546af01841fc09b1ab9c3749a5f1c17
d4f589668a587b2700a9c97c1193cf42
HMAC-SHA-256-128: 0x7546af01841fc09b1ab9c3749a5f1c17
Test Case #9: HMAC-SHA-256 with 54-byte input and 80-byte key
Key_len : 80
Key : 0xaa repeated 80 times
Data_len : 54
Data : "Test Using Larger Than Block-Size Key -
Hash Key First"
HMAC-SHA-256 : 0x6953025ed96f0c09f80a96f78e6538db
e2e7b820e3dd970e7ddd39091b32352f
Test Case #10: HMAC-SHA-256 with 73-byte (multi-block) input
and 80-byte key
Key_len : 80
Key : 0xaa repeated 80 times
Data_len : 73
Data : "Test Using Larger Than Block-Size Key and
Larger Than One Block-Size Data"
HMAC-SHA-256 : 0x6355ac22e890d0a3c8481a5ca4825bc8
84d3e7a1ff98a2fc2ac7d8e064c3b2e6
4. IKE Interactions
4.1 Phase 1 Identifier
For Phase 1 negotiations, IANA has assigned a Hash Algorithm ID of 4
for SHA2-256.
For further information on the use of Hash Algorithm IDs within IKE,
see [IKE].
4.2 Phase 2 Identifier
For Phase 2 negotiations, IANA has assigned an AH Transform Identifi-
er of 5 for AH_SHA2-256.
For Phase 2 negotiations, IANA has assigned an AH/ESP Authentication
Algorithm Attribute Value of 5 for HMAC-SHA2-256.
For further information on the use of Transform Identifiers and At-
tributes Value within IKE, see [IKE] and [DOI].
5. Security Considerations
The security provided by HMAC-SHA-256-128 is based upon the strength
of SHA-256. At the time of this writing there are no practical cryp-
tographic attacks against SHA-256.
As is true with any cryptographic algorithm, part of its strength
lies in the correctness of the algorithm implementation, the security
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of the key management mechanism and its implementation, the strength
of the associated secret key, and upon the correctness of the imple-
mentation in all of the participating systems. This draft contains
test vectors to assist in verifying the correctness of HMAC-
SHA-256-128 code.
6. IANA Considerations
IANA has assigned Hash Algorithm ID 4 to SHA2-256.
IANA has assigned AH Transform Identifier 5 to AH_SHA2-256.
IANA has assigned AH/ESP Authentication Algorithm Attribute Value 5
to HMAC-SHA2-256.
7. Intellectual Property Rights Statement
Pursuant to the provisions of [RFC-2026], the authors represent that
they have disclosed the existence of any proprietary or intellectual
property rights in the contribution that are reasonably and personal-
ly known to the authors. The authors do not represent that they per-
sonally know of all potentially pertinent proprietary and intellectu-
al property rights owned or claimed by the organizations they repre-
sent or third parties.
The IETF takes no position regarding the validity or scope of any in-
tellectual property or other rights that might be claimed to pertain
to the implementation or use of the technology described in this doc-
ument or the extent to which any license under such rights might or
might not be available; neither does it represent that it has made
any effort to identify any such rights. Information on the IETF's
procedures with respect to rights in standards-track and standards-
related documentation can be found in BCP-11. Copies of claims of
rights made available for publication 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 Secretariat.
8. Acknowledgments
Portions of this text were unabashedly borrowed from [HMAC-SHA].
Thanks to Hugo Krawczyk for his comments and recommendations.
9. References
[AH] Kent, S. and R. Atkinson, "IP Authentication Header",
RFC 2402, November 1998.
[ARCH] Kent, S. and R. Atkinson, "Security Architecture for
the Internet Protocol", RFC 2401, November 1998.
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[DOI] Piper, D., "The Internet IP Security Domain of
Interpretation for ISAKMP,"
[ESP] Kent, S. and R. Atkinson, "IP Encapsulating Security
Payload (ESP)", RFC 2406, November 1998.
[HASH] Bellare, M., R. Canetti and H. Krawczyk, "Keying Hash
Functions for Message Authentication," Advances in
Cryptography, Crypto96 Proceedings, June 1996.
[HMAC] Krawczyk, H., M. Bellare and R. Canetti, "HMAC: Keyed-
Hashing for Message Authentication," RFC 2104, February
1997.
[HMAC-SHA] Madson, C. and R. Glenn, "The Use of HMAC-SHA-1-96
within ESP and AH," RFC 2404, November 1998.
[HMAC-TEST] Cheng, P. and R. Glenn, "Test Cases for HMAC-MD5 and
HMAC-SHA-1", RFC 2202, September 1997.
[IKE] Harkins, D. and D. Carrel, "The Internet Key Exchange
(IKE)", RFC 2409, November 1998.
[RFC-2026] Bradner, S., "The Internet Standards Process --
Revision 3", RFC2026, October 1996.
[RFC-2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", RFC-2119, March 1997.
[ROADMAP] Thayer, R., N. Doraswamy, and R. Glenn, "IP Security
Document Roadmap", RFC 2411, November 1998.
[SHA2-1] NIST, Draft FIPS PUB 180-2 "Specifications for the
Secure Hash Standard," May 2001.
http://csrc.nist.gov/encryption/shs/dfips-180-2.pdf
[SHA2-2] "Descriptions of SHA-256, SHA-384, and SHA-512."
http://csrc.nist.gov/cryptval/shs/sha256-384-512.pdf
10. Authors' Addresses
Sheila Frankel
NIST
820 West Diamond Ave.
Room 680
Gaithersburg, MD 20899
Phone: +1 (301) 975-3297
Email: sheila.frankel@nist.gov
Scott Kelly
Black Storm Networks
250 Cambridge Ave
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Palo Alto CA 94304
Phone: +1 (650) 617-2934
Email: scott@bstormnetworks.com
The IPsec working group can be contacted through the chairs:
Barbara Fraser
Cisco Systems Inc.
Email: byfraser@cisco.com
Theodore Ts'o
Massachusetts Institute of Technology
Email: tytso@mit.edu
11. Full Copyright Statement
Copyright (C) The Internet Society (1998). All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this doc-
ument itself may not be modified in any way, such as by removing the
copyright notice or references to the Internet Society or other In-
ternet organizations, except as needed for the purpose of developing
Internet standards in which case the procedures for copyrights de-
fined in the Internet Standards process must be followed, or as re-
quired to translate it into languages other than English.
The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assigns.
This document and the information contained herein is provided on an
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HERE-
IN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MER-
CHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
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