Network Working Group C. Madson
Request for Comments: 2403 Cisco Systems Inc.
Category: Standards Track R. Glenn
The Use of HMAC-MD5-96 within ESP and AH
Status of this Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright (C) The Internet Society (1998). All Rights Reserved.
This memo 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].
This memo 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 memo, 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 RFC 2104. 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].
[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
[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
[RFC-2104] makes the following recommendation with regard to
rekeying. Current attacks do not indicate a specific recommended
frequency for key changes as these attacks are practically
infeasible. However, periodic key refreshment is a fundamental
security practice that helps against potential weaknesses of the
function and keys, reduces the information avaliable to a
cryptanalyst, and limits the damage of an exposed key.
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 practical cryptographic attacks against
[RFC-2104] states that for "minimally reasonable hash functions" the
"birthday attack", the strongest attack know against HMAC, 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
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
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.
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.
We would also like to thank Hugo Krawczyk for his comments and
recommendations regarding some of the cryptographic specific text in
[RFC-1321] Rivest, R., "MD5 Digest Algorithm", RFC 1321, April
[RFC-2104] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC:
Keyed-Hashing for Message Authentication", RFC 2104,
[RFC-1810] Touch, J., "Report on MD5 Performance", RFC 1810, June
[Bellare96a] Bellare, M., Canetti, R., and H. Krawczyk, "Keying Hash
Functions for Message Authentication", Advances in
Cryptography, Crypto96 Proceeding, June 1996.
[ARCH] Kent, S., and R. Atkinson, "Security Architecture for
the Internet Protocol", RFC 2401, November 1998.
[ESP] Kent, S., and R. Atkinson, "IP Encapsulating Security
Payload", RFC 2406, November 1998.
[AH] Kent, S., and R. Atkinson, "IP Authentication Header",
RFC 2402, November 1998.
[Thayer97a] Thayer, R., Doraswamy, N., and R. Glenn, "IP Security
Document Roadmap", RFC 2411, November 1998.
[RFC-2202] Cheng, P., and R. Glenn, "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", BCP 14, RFC 2119, March 1997.
8. Editors' Address
Cisco Systems, Inc.
The IPsec working group can be contacted through the chairs:
Massachusetts Institute of Technology
9. Full Copyright Statement
Copyright (C) The Internet Society (1998). All Rights Reserved.
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