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HMAC-MD5 IP Authentication with Replay Prevention
RFC 2085

Document Type RFC - Proposed Standard (February 1997)
Authors Michael Oehler , K. Robert Glenn
Last updated 2013-03-02
RFC stream Internet Engineering Task Force (IETF)
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RFC 2085
Network Working Group                                         M. Oehler
Request for Comments: 2085                                          NSA
Category: Standards Track                                      R. Glenn
                                                          February 1997

           HMAC-MD5 IP Authentication with Replay Prevention

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.


   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 [HMAC-MD5].  An option is also
   specified to guard against replay attacks.

Table of Contents

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

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 [HMAC-MD5].  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.

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RFC 2085                        HMAC-MD5                  February 1997

   To provide protection against replay attacks, a Replay Prevention
   field is included 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-MD5: Keyed-MD5 for
   Message Authentication" [HMAC-MD5].

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

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

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RFC 2085                        HMAC-MD5                  February 1997

   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. [HMAC-MD5]

   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 output, which is 128 bits for MD5.  For
   other key lengths the following concerns MUST be considered.

   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.  A longer key may be
   advisable if the randomness of the key is suspect.  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

1.3 Data Size

   MD5 produces a 128-bit value which is used as the authentication
   data.  It is naturally 64 bit aligned and thus, does not need any
   padding for machines with native double words.

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.

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RFC 2085                        HMAC-MD5                  February 1997

2.1 Replay Prevention

   The Replay Prevention field is a 64-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.  If Replay Prevention is NOT in use, then
   the Authentication Data field will directly follow the SPI field.

   The 64-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
   allows the counter to wrap, that is, to transmit more than 2^64
   packets using a single key.

   Upon receipt, the replay value is assured to be increasing.  The
   implementation may accept out of order packets. The number of packets
   to accept out of order is an implementation detail. If an "out of
   order window" is supported, the implementation shall ensure that any
   and all packets accepted out of order are guaranteed not to have
   arrived before. That is, the implementation will accept any packet at
   most once.

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

   [ESP-DES-MD5] provides example code that implements a 32 packet
   replay window and a test routine to show how it works.

2.2 Authentication Data Calculation

   The authentication data is the output of the authentication algorithm
   (MD5).  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 if present, is
   included in the calculation.

   The definition and reference implementation of MD5 appears in [RFC-
   1321].  Let 'text' denote the data to which HMAC-MD5 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.

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RFC 2085                        HMAC-MD5                  February 1997

   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))
    (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) and output
        the result

      This computation is described in more detail, along with example
      code and performance improvements, in [HMAC-MD5]. Implementers
      should consult [HMAC-MD5] 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.  [HMAC-MD5]
   contains a detailed discussion on the strengths and weaknesses of


   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 directly from work
   by Jim Hughes.

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RFC 2085                        HMAC-MD5                  February 1997


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

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


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