Internet DRAFT - draft-ietf-ipsec-hmac-md5-96
draft-ietf-ipsec-hmac-md5-96
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Network Working Group IPsec Working Group
INTERNET DRAFT C. Madson
Expire in six months Cisco Systems Inc.
R. Glenn
NIST
July 1997
The Use of HMAC-MD5-96 within ESP and AH
<draft-ietf-ipsec-auth-hmac-md5-96-00.txt>
Status of this Memo
This document is a submission to the IETF Internet Protocol Security
(IPSEC) Working Group. Comments are solicited and should be addressed
to the working group mailing list (ipsec@tis.com) or to the editor.
This document is an Internet-Draft. Internet Drafts are working
documents of the Internet Engineering Task Force (IETF), its areas,
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Distribution of this memo is unlimited.
Abstract
This draft describes the use of the HMAC algorithm [RFC-2104] in
conjunction with the MD5 algorithm [RFC-1321] as an authentication
mechanism within the revised IPSEC Encapsulating Security Payload
[ESP] and the revised IPSEC Authentication Header [AH]. HMAC with MD5
provides data origin authentication and integrity protection.
Further information on the other components necessary for ESP and AH
implementations is provided by [Thayer97a].
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1. Introduction
This draft specifies a the use of MD5 [RFC-1321] combined with HMAC
[RFC-2104] as a keyed authentication mechanism within the context of
the Encapsulating Security Payload and the Authentication Header.
The goal of HMAC-MD5-96 is to ensure that the packet is authentic and
cannot be modified in transit.
HMAC is a secret key authentication algorithm. Data integrity and
data origin authentication as provided by HMAC are dependent upon the
scope of the distribution of the secret key. If only the source and
destination know the HMAC key, this provides both data origin
authentication and data integrity for packets sent between the two
parties; if the HMAC is correct, this proves that it must have been
added by the source.
In this draft, HMAC-MD5-96 is used within the context of ESP and AH.
For further information on how the various pieces of ESP - including
the confidentiality mechanism -- fit together to provide security
services, refer to [ESP] and [Thayer97a]. For further information on
AH, refer to [AH] and [Thayer97a].
In this document, the key words "MAY", "MUST", "optional",
"recommended", "required", "SHOULD", and "SHOULD NOT", are to be
interpreted as described in [RFC-2119].
2. Algorithm and Mode
[RFC-1321] describes the underlying MD5 algorithm, while [RFC-2104]
describes the HMAC algorithm. The HMAC algorithm provides a framework
for inserting various hashing algorithms such as MD5.
HMAC-MD5-96 operates on 64-byte blocks of data and produces a 128-bit
authenticator value. This 128-bit value can be truncated as
described in RFC2104. For use with either ESP or AH, a truncated
value using the first 96 bits MUST be supported. Upon sending, the
truncated value is stored within the authenticator field. Upon
receipt, the entire 128-bit value is computed and the first 96 bits
are compared to the value stored in the authenticator field. No
other authenticator value lengths are supported by HMAC-MD5-96.
The length of 96 bits was selected because it is the default
authenticator length as specified in [AH] and meets the security
requirements described in [RFC-2104].
2.1 Performance
[Bellare96a] states that "(HMAC) performance is essentially that of
the underlying hash function". [RFC-1810] provides some performance
analysis and recommendations of the use of MD5 with Internet
protocols. As of this writing no performance analysis has been done
of HMAC or HMAC combined with MD5.
[RFC-2104] outlines an implementation modification which can improve
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per-packet performance without affecting interoperability.
3. Keying Material
HMAC-MD5-96 is a secret key algorithm. While no fixed key length is
specified in [RFC-2104], for use with either ESP or AH a fixed key
length of 128-bits MUST be supported. A key length of 128-bits was
chosen based on the recommendations in [RFC-2104] (i.e. key lengths
less than the authenticator length decrease security strength and
keys longer than the authenticator length do not significantly
increase security strength).
[RFC-2104] discusses requirements for key material, which includes a
discussion on requirements for strong randomness. A strong pseudo-
random function MUST be used to generate the required 128-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.
If, at some point, a set of weak keys for HMAC are identified, the
use of these weak keys must be rejected followed by a request for
replacement keys or a newly negotiated Security Association.
[ESP] describes the general mechanism to obtain keying material for
the ESP transform. The derivation of the key from some amount of
keying material does not differ between the manual and automatic key
management mechanisms.
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
communication.
[RFC-2104] states that for "minimally reasonable hash functions" the
"birthday attack" is impractical. For a 64-byte block hash such as
HMAC-MD5-96, an attack involving the successful processing of 2**64
blocks would be infeasible unless it were discovered that the
underlying hash had collisions after processing 2**30 blocks. (A
hash with such weak collision-resistance characteristics would
generally be considered to be unusable.) No time-based attacks are
discussed in the document.
As it it still cryptographically prudent to perform frequent
rekeying, the recommended key lifetimes are 2**32 blocks and ????
seconds.
4. Interaction with the ESP Cipher Mechanism
As of this writing, there are no known issues which preclude the use
of the HMAC-MD5-96 algorithm with any specific cipher algorithm.
5. Security Considerations
The security provided by HMAC-MD5-96 is based upon the strength of
HMAC, and to a lesser degree, the strength of MD5. [RFC-2104] claims
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that HMAC does not depend upon the property of strong collision
resistance, which is important to consider when evaluating the use of
MD5, an algorithm which has, under recent scrutiny, been shown to be
much less collision-resistant than was first thought.
It is also important to consider that while MD5 was never developed
to be used as a keyed hash algorithm, HMAC had that criteria from the
onset. While the use of MD5 in the context of data security is
undergoing reevaluation, the combined HMAC with MD5 algorithm has
held up to cryptographic scrutiny.
[RFC-2104] also discusses the potential additional security which is
provided by the truncation of the resulting hash. Specifications
which include HMAC are strongly encouraged to perform this hash
truncation.
As [RFC-2104] provides a framework for incorporating various hash
algorithms with HMAC, it is possible to replace MD5 with other
algorithms such as SHA-1. [RFC-2104] contains a detailed discussion
on the strengths and weaknesses of HMAC algorithms.
As is true with any cryptographic algorithm, part of its strength
lies in the correctness of the algorithm implementation, the security
of the key management mechanism and its implementation, the strength
of the associated secret key, and upon the correctness of the
implementation in all of the participating systems. [HMAC-TESTS]
contains test vectors and example code to assist in verifying the
correctness of HMAC-MD5-96 code.
6. Acknowledgments
This document is derived in part from previous works by Jim Hughes,
those people that worked with Jim on the combined DES/CBC+HMAC-MD5
ESP transforms, the ANX bakeoff participants, and the members of the
IPsec working group.
7. References
[RFC-1321] Rivest, R., "MD5 Digest Algorithm", RFC-1321, April 1992
[RFC-2104] Krawczyk, H., Bellare, M., Canetti, R., "HMAC: Keyed-
Hashing for Message Authentication", RFC-2104,
February, 1997
[RFC-1810] Touch, J. "Report on MD5 Performance", RFC-1810,
June 1995.
[Bellare96a] Bellare, M., Canetti, R., Krawczyk, H., "Keying
Hash Functions for Message Authentication", Advances in
Cryptography, Crypto96 Proceeding, June 1996.
[ESP] Kent, S., Atkinson, R., "IP Encapsulating Security
Payload", draft-ietf-ipsec-esp-04.txt, work in progress,
March 26, 1997
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[AH] Kent, S., Atkinson, R., "IP Authentication Header",
draft-ietf-ipsec-auth-05.txt, work in progress,
March 26, 1997
[Thayer97a] Thayer, R., Doraswamy, N., Glenn, R., "IP Security
Document Framework",
draft-ietf-ipsec-ff-Doc-Framework-00.txt, work in progress,
June 1997.
[HMAC-TESTS] Cheng, P., Glenn, R., "Test Cases for HMAC-MD5 and
HMAC-SHA-1", draft-cheng-hmac-test-cases-00.txt, work
in progress, March 1997.
8. Editors' Address
Cheryl Madson
<cmadson@cisco.com>
Cisco Systems, Inc.
Rob Glenn
<rob.glenn@nist.gov>
NIST
The IPsec working group can be contacted through the chairs:
Robert Moskowitz
<rgm3@chrysler.com>
Chrysler Corporation
Ted T'so
tytso@mit.edu
Massachusetts Institute of Technology
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