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Network Working Group                                          C. MadsonRequest for Comments: 2403                            Cisco Systems Inc.Category: Standards Track                                       R. Glenn                                                                    NIST                                                           November 1998The Use of HMAC-MD5-96 within ESP and AHStatus 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 inRFC 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 19983. 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 19985.  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 19986.  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 19988.  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.eduMadson & Glenn              Standards Track                     [Page 6]

RFC 2403        The Use of HMAC-MD5-96 within ESP and AH   November 19989.  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.Madson & Glenn              Standards Track                     [Page 7]