Internet DRAFT - draft-ietf-ipsec-auth-hmac-md5-96
draft-ietf-ipsec-auth-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
February 1998
The Use of HMAC-MD5-96 within ESP and AH
<draft-ietf-ipsec-auth-hmac-md5-96-02.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,
and its working Groups. Note that other groups may also distribute
working documents as Internet Drafts.
Internet-Drafts draft documents are valid for a maximum of six months
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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 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].
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 [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. Padding requirements
are specified in [RFC-1321] and are part of the MD5 algorithm. If
MD5 is built according to [RFC-1321], there is no need to add any
additional padding as far as HMAC-MD5-96 is concerned. With regard
to "implicit packet padding" as defined in [AH], no implicit packet
padding is required.
HMAC-MD5-96 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].
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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
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. Key lengths other than
128-bits MUST NOT be supported (i.e. only 128-bit keys are to be used
by HMAC-MD5-96). 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.
[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
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.
While it it still cryptographically prudent to perform frequent
rekeying, current literature does not include any recommended key
lifetimes for HMAC-MD5-96 (i.e. there are too many variables involved
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to propose a general recommendation). When any recommendations for
HMAC-MD5-96 key lifetimes become available they will be included in a
revised version of this document.
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
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. At the time of
this writing there are no known cryptographic attacks against HMAC-
MD5-96.
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. [RFC-2202]
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.
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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.
[ARCH] Kent, S., Atkinson, R., "Security Architecture for
the Internet Protocol", draft-ietf-ipsec-arch-sec-02.txt,
work in progress, November 1997.
[ESP] Kent, S., Atkinson, R., "IP Encapsulating Security
Payload", draft-ietf-ipsec-esp-v2-02.txt, work in progress,
November 1997.
[AH] Kent, S., Atkinson, R., "IP Authentication Header",
draft-ietf-ipsec-auth-header-03.txt, work in progress,
November 1997.
[Thayer97a] Thayer, R., Doraswamy, N., Glenn, R., "IP Security
Document Roadmap",
draft-ietf-ipsec-doc-roadmap-02.txt, work in progress,
November 1997.
[RFC-2202] Cheng, P., Glenn, R., "Test Cases for HMAC-MD5 and
HMAC-SHA-1", RFC-2202, March 1997.
[RFC-2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", RFC-2119, March 1997.
8. Editors' Address
Cheryl Madson
Cisco Systems, Inc.
e-mail: <cmadson@cisco.com>
Rob Glenn
NIST
e-mail: <rob.glenn@nist.gov>
The IPsec working group can be contacted through the chairs:
Robert Moskowitz
ICSA
e-mail: <rgm@icsa.net>
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Ted T'so
Massachusetts Institute of Technology
e-mail: <tytso@mit.edu>
Madson, Glenn [Page 6]
