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Versions: 00                                                            
Network Working Group                                   M. Oehler (NSA)
                                                        R. Glenn (NIST)
Internet Draft                                          March 20, 1997

          HMAC-MD5-96 IP Authentication with Replay Prevention

Status of This Memo

   Distribution of this memo is unlimited.

   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.

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   This document describes a keyed-MD5 transform to be used in
   conjunction with the IP Authentication Header [RFC-1826]. The
   particular transform is based on [RFC-2104].  A replay prevention
   field is also specified.

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   1.  Introduction...................................................3
   1.1    Terminology.................................................3
   1.2    Keys........................................................4
   1.3    Data Size...................................................4
   2.  Packet Format..................................................5
   2.1    Replay Prevention...........................................5
   2.2    Authentication Data Calculation.............................6
   3.  Security Considerations........................................7
   Authors' Addresses.................................................8

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1. Introduction

   The Authentication Header (AH) [RFC-1826] provides integrity and
   authentication for IP datagrams. The transform specified in this
   document uses a keyed-MD5 mechanism [RFC-2104].  The mechanism uses
   the (key-less) MD5 hash function [RFC-1321] which produces a message
   digest. When combined with an AH Key, Authentication Data is
   produced. This value is placed in the Authentication Data field of
   the AH [RFC-1826]. This value is also the basis for the data
   integrity service offered by the AH protocol.

   To provide protection against replay attacks, a Replay Prevention
   field is specified as a transform option.  This field is used to help
   prevent attacks in which a message is stored and re-used later,
   replacing or repeating the original.  The Security Parameters Index
   (SPI) [RFC-1825] is used to determine whether this option is included
   in the AH.

   Familiarity with the following documents is assumed: "Security
   Architecture for the Internet Protocol" [RFC-1825], "IP
   Authentication Header" [RFC-1826], and "HMAC: Keyed Hashing for
   Message Authentication" [RFC-2104].

   All implementations that claim conformance or compliance with the IP
   Authentication Header specification [RFC-1826] MUST implement this
   HMAC-MD5-96 transform.

1.1 Terminology

   In  this  document,  the  words  that  are  used  to   define   the
   significance  of each particular requirement are usually capitalized.
   These words are:

   - MUST

   This word or the adjective "REQUIRED" means that  the  item  is  an
   absolute requirement of the specification.


   This word or the adjective "RECOMMENDED"  means  that  there  might
   exist  valid reasons in particular circumstances to ignore this item,
   but the full implications should be understood and the case carefully
   weighed before taking a different course.

   - MAY

   This word or the adjective "OPTIONAL" means that this item is truly

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   optional.  One vendor might choose to include the item because a
   particular marketplace requires it or because it enhances the product,
   for example; another vendor may omit the same item.

   For the purpose of this specification, the terms conformance and
   compliance are synonymous.

1.2 Keys

   The "AH Key" is used as a shared secret between two communicating
   parties.  The Key is not a "cryptographic key" as used in a
   traditional sense. Instead, the AH key (shared secret) is hashed with
   the transmitted data and thus, assures that an intervening party
   cannot duplicate the Authentication Data.

   Even though an AH key is not a cryptographic key, the rudimentary
   concerns of cryptographic keys still apply. Consider that the
   algorithm and most of the data used to produce the output is known.
   The strength of the transform lies in the singular mapping of the key
   (which needs to be strong) and the IP datagram (which is known) to
   the Authentication Data.  Thus, implementations should, and as
   frequently as possible, change the AH key. Keys need to be chosen at
   random, or generated using a cryptographically strong pseudo-random
   generator seeded with a random seed. [RFC-2104]

   All conforming and compliant implementations MUST support a key
   length of 128 bits or less.  Implementations SHOULD support longer
   key lengths as well.  It is advised that the key length be chosen to
   be the length of the hash algorithm output, which is 128 bits for
   MD5.  For other key lengths the following concerns MUST be

   A key length of zero is prohibited and implementations MUST prevent
   key lengths of zero from being used with this transform, since no
   effective authentication could be provided by a zero-length key.
   Keys having a length less than 128 bits are strongly discouraged as
   it would decrease the security strength of the function.  Keys longer
   than 128 bits are acceptable, but the extra length may not
   significantly increase the function strength.  MD5 operates on 64-
   byte blocks.  Keys longer than 64-bytes are first hashed using MD5.
   The resulting hash is then used to calculate the Authentication Data.

1.3 Data Size

   HMAC-MD5 produces a 128-bit value.  HMAC-MD5-96 uses the first or
   left most 96 bits as the Authentication Data.  This procedure is
   known as truncation.  In the case of this transform, truncation is
   used to help maintain 64-bit packet header alignment, eliminate

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   unnecessary overhead, and potentially provide stronger
   authentication.  [RFC-2104] provides more information on the
   advantages and disadvantages of truncation.

2. Packet Format

        | Next Header   | Length        |           RESERVED            |
        |                              SPI                              |
        |                     Replay Prevention                         |
        |                                                               |
        +                                                               +
        |                     Authentication Data                       |
        +                                                               +
        |                                                               |
         1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8

   The Next Header, RESERVED, and SPI fields are specified in [RFC-
   1826].  The Length field is the length of the Replay Prevention field
   and the Authentication Data in 32-bit words.  The Length field will
   always be set to 4 (128 bits) for HMAC-MD5-96.

2.1 Replay Prevention

   The Replay Prevention field is a 32-bit value used to guarantee that
   each packet exchanged between two parties is different.  Each IPsec
   Security Association specifies whether Replay Prevention is used for
   that Security Association.  The Replay Prevention field is always
   included in the calculation of the Authentication Data. If Replay
   Prevention is NOT in use, the 32-bit value is set to 0, included in
   the calculation of the Authentication Data, and ignored upon receipt
   with regard to checking for replay.  This field is used to help
   prevent attacks in which a message is stored and re-used later,
   replacing or repeating the original.

   Replay Prevention SHOULD be implemented.  If Replay Prevention is not
   implemented, the 32-bit field remains are part of the AH and is
   treated as if Replay Prevention is NOT in use (i.e. the 32-bit value
   is set to 0, included in the calculation of the Authentication Data,
   and ignored upon receipt with regard to checking for replay.

   The 32-bit field is an up counter starting at a value of 1.

   The secret shared key MUST NOT be used for a period of time that

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   allows the counter to wrap, that is, to transmit more than 2^32
   packets using a single key.

   Upon receipt, the replay value is assured to be increasing.  An
   implementation MAY accept out of order packets.  If an "out of order
   window" is supported, an implementation MUST guarantee that any and
   all packets accepted out of order have not arrived before. That is,
   an implementation MUST accept any packet, at most, once.  The size of
   the window is a negotiated value specified by the IPsec Security

   [ESP-DES-MD5] provides more information on negotiated windows sizes,
   example code that implements a 32 packet replay window, and a test
   routine to show how it could be implemented.

   When the destination address is a multicast address and more than one
   sender is sharing the same IPsec Security Association to that
   multicast destination address, then Replay Prevention SHOULD NOT be
   enabled.  When Replay Prevention is desired for a multicast session
   having multiple senders to the same multicast destination address,
   each sender SHOULD have its own IPsec Security Association.

2.2 Authentication Data Calculation

   The Authentication Data is the output of the MD5 authentication
   algorithm. This value is calculated over the entire IP datagram.
   Fields within the datagram that are variant during transit and the
   Authentication Data field itself, must contain all zeros prior to the
   computation [RFC-1826].  The Replay Prevention field, used or not, is
   always included in the calculation.

   The definition and reference implementation of MD5 appears in [RFC-
   1321].  Let 'text' denote the data to which HMAC-MD5-96 is to be
   applied and K be the message authentication secret key shared by the
   parties.  If K is longer than 64-bytes it MUST first be hashed using
   MD5.  In this case, K is the resulting hash.

   We define two fixed and different strings ipad and opad as follows
   (the 'i' and 'o' are mnemonics for inner and outer):

      ipad = the byte 0x36 repeated 64 times
      opad = the byte 0x5C repeated 64 times.

   To compute HMAC-MD5 over the data `text' we perform

      MD5(K XOR opad, MD5(K XOR ipad, text))

   The result of which is truncated to 96 bits (retaining the left most

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   bits) to produce HMAC-MD5-96.

   The calculation of the Authentication Data consists of the following

   (1) append zeros to the end of K to create a 64 byte string (e.g., if
       K is of length 16 bytes it will be appended with 48 zero bytes 0x00)
   (2) XOR (bitwise exclusive-OR) the 64 byte string computed in
       step (1) with ipad
   (3) append the data stream 'text' to the 64 byte string resulting
       from step (2)
   (4) apply MD5 to the stream generated in step (3)
   (5) XOR (bitwise exclusive-OR) the 64 byte string computed in
       step (1) with opad
   (6) append the MD5 result from step (4) to the 64 byte string
       resulting from step (5)
   (7) apply MD5 to the stream generated in step (6)
   (8) use the left most 96 bits of the result obtained in (7) as the final

   A similar computation is described in more detail, along with example
   code and performance improvements, in [RFC-2104]. Implementers should
   consult [RFC-2104] for more information on this technique for keying
   a cryptographic hash function.

3. Security Considerations

   The security provided by this transform is based on the strength of
   MD5, the correctness of the algorithm's implementation, the security
   of the key management mechanism and its implementation, the strength
   of the associated secret key, and upon the correctness of the
   implementations in all of the participating systems.  [RFC-2104]
   contains a detailed discussion on the strengths and weaknesses of
   HMAC algorithms. [HMAC-TESTS] contains test vectors and example code
   to assist in verifying the correctness of HMAC-MD5 code.


   This document is largely based on text written by Hugo Krawczyk.  The
   format used was derived from work by William Simpson and Perry
   Metzger.  The text on replay prevention is derived from work by Jim

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   [RFC-1825]    R. Atkinson, "Security Architecture for the Internet
                 Protocol", RFC-1852, Naval Research Laboratory, July 1995.
   [RFC-1826]    R. Atkinson, "IP Authentication Header",
                 RFC-1826, August 1995.
   [RFC-1828]    P. Metzger, W. A. Simpson, "IP Authentication using Keyed MD5",
                 RFC-1828, August 1995.
   [RFC-1321]    R. Rivest, "The MD5 Message-Digest Algorithm",
                 RFC-1321, April 1992.
   [RFC-2104]    H. Krawczyk, M. Bellare, R. Canetti, "HMAC: Keyed Hashing
                 for Message Authentication", RFC-2104, February, 1997.
   [ESP-DES-MD5] J. Hughes, "Combined DES-CBC, MD5, and Replay Prevention
                 Security Transform", Internet Draft, September 1996.
   [HMAC-TESTS]  P. Cheng, R. Glenn, "Test Cases for HMAC-MD5 and HMAC-SHA-1",
                 Internet Draft, March 1997.

Authors' Addresses

   Michael J. Oehler
   National Security Agency
   Atn: R23, INFOSEC Research and Development
   9800 Savage Road
   Fort Meade, MD 20755


   Robert Glenn
   Building 820, Room 455
   Gaithersburg, MD 20899


Oehler, Glenn                                                   [Page 8]