Request for Comments: 2403
Category: Standards Track
Cisco Systems Inc.
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 © 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 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] 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 HMAC-MD5-96.
[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 be unusable.
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.
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 this document.
[RFC-1321] Rivest, R., "MD5 Digest Algorithm", RFC 1321, April 1992. [RFC-2104] Krawczyk, H., Bellare, M., and R. Canetti, "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., 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 © The Internet Society (1998). All Rights Reserved.
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