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+Network Working Group C. Madson
+Request for Comments: 2403 Cisco Systems Inc.
+Category: Standards Track R. Glenn
+ NIST
+ November 1998
+
+
+ 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 Notice
+
+ Copyright (C) The Internet Society (1998). All Rights Reserved.
+
+Abstract
+
+ 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].
+
+1. Introduction
+
+ 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.
+
+
+
+
+Madson & Glenn Standards Track [Page 1]
+
+RFC 2403 The Use of HMAC-MD5-96 within ESP and AH November 1998
+
+
+ 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].
+
+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.
+
+
+
+
+
+Madson & Glenn Standards Track [Page 2]
+
+RFC 2403 The Use of HMAC-MD5-96 within ESP and AH November 1998
+
+
+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.
+
+
+
+
+
+
+
+
+Madson & Glenn Standards Track [Page 3]
+
+RFC 2403 The Use of HMAC-MD5-96 within ESP and AH November 1998
+
+
+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.
+
+
+
+
+
+
+
+
+Madson & Glenn Standards Track [Page 4]
+
+RFC 2403 The Use of HMAC-MD5-96 within ESP and AH November 1998
+
+
+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.
+
+ We would also like to thank Hugo Krawczyk for his comments and
+ recommendations regarding some of the cryptographic specific text in
+ this document.
+
+7. References
+
+ [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.
+
+
+
+
+
+
+
+Madson & Glenn Standards Track [Page 5]
+
+RFC 2403 The Use of HMAC-MD5-96 within ESP and AH November 1998
+
+
+8. Editors' Address
+
+ Cheryl Madson
+ Cisco Systems, Inc.
+
+ EMail: cmadson@cisco.com
+
+
+ Rob Glenn
+ NIST
+
+ EMail: <rob.glenn@nist.gov>
+
+ The IPsec working group can be contacted through the chairs:
+
+ Robert Moskowitz
+ ICSA
+
+ EMail: rgm@icsa.net
+
+
+ Ted T'so
+ Massachusetts Institute of Technology
+
+ EMail: tytso@mit.edu
+
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+Madson & Glenn Standards Track [Page 6]
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+RFC 2403 The Use of HMAC-MD5-96 within ESP and AH November 1998
+
+
+9. 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
+ document itself may not be modified in any way, such as by removing
+ the copyright notice or references to the Internet Society or other
+ Internet organizations, except as needed for the purpose of
+ developing Internet standards in which case the procedures for
+ copyrights defined in the Internet Standards process must be
+ followed, or as required 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
+ HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
+ MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
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+Madson & Glenn Standards Track [Page 7]
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