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Versions: 00 01 02 03 04                                                
Network Working Group                                   S. Chang (NIST)
                                                        R. Glenn (NIST)
                                                        May 1, 1996
Internet Draft


           HMAC-SHA IP Authentication with Replay Prevention
                 <draft-ietf-ipsec-ah-hmac-sha-00.txt>


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,
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Abstract

   This document describes a keyed-SHA 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.










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Contents

   1.  Introduction...................................................3
   1.1    Keys........................................................3
   1.2    Data Size...................................................4
   2   Packet Format..................................................4
   2.1    Replay Prevention...........................................4
   2.2    Authentication Data Calculation.............................5
   3.  Security Considerations........................................6
   ACKNOWLEDGMENTS....................................................6
   REFERENCES.........................................................6
   CONTACTS...........................................................6







































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1. Introduction

   The IP Authentication Header (AH) provides integrity and
   authentication for IP datagrams [RFC-1826]. The transform specified
   in this document uses a keyed-SHA mechanism based on [HMAC-MD5].  The
   mechanism uses the (key-less) SHA hash function [FIPS-180-1] which
   produces a message authentication code. 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 included as a transform option.  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].

1.1 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. [HMAC-MD5]

   There is no mandated key size for the HMAC-SHA transform.
   Implementations must support a key length of any size, except zero.
   It is advised that keys be chosen as the length of the hash output,
   or 160-bits for SHA. For other key lengths, the following concerns
   must be considered.

   A key length of zero is prohibited and implementations should provide
   an alert, since the authentication data would be identical to that of



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   SHA, solely.  SHA operates on 64-byte blocks.  Keys longer than 64
   bytes are first hashed using SHA.  The resulting hash is then used to
   calculated the authentication data.

1.2 Data Size

   SHA generates a message digest of 160 bits, which is automatically
   aligned on a 32-bit word boundary. However, some implementations may
   require 64-bit alignment of the IP headers, in which case, 32 zero
   bits are appended as padding to the SHA output. The length of the
   Authentication Data, specified in the Length field of the AH in 32-
   bit words, should include the padding bits. Therefore, an
   implementation that appends a 32-bit padding to the SHA output will
   have a length of six 32-bit words.  The padded bits are ignored at
   the receiving end.

2. Packet Format

        +---------------+---------------+---------------+---------------+
        | Next Header   | Length        |           RESERVED            |
        +---------------+---------------+---------------+---------------+
        |                              SPI                              |
        +---------------+---------------+---------------+---------------+
        |                     Replay Prevention (optional)              |
        +---------------+---------------+---------------+---------------+
        |                                                               |
        +                     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.

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.  This field
   is similar to the one specified in [ESP-DES-MD5].  The SPI is used to
   determine whether or not the field is included in the packet (i.e. if
   it is not included, the header will have the SPI directly followed by
   the Authentication Data).  Without this field it is possible to
   attack a system by retransmitting packets.

   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.  The
   implementation may accept of out of order packets. The number of
   packets to accept out of order is an implementation detail. If a "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.

   [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 computation of the 160-bit SHA digest is described
   in [FIPS-180-1].  The digest 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.

   To compute HMAC-SHA over the data 'text', the following is calculated:

       SHA (K XOR epad, SHA (K XOR ipad, text))

   where 'K' denotes the secret key shared by the parties, and 'epad', 'ipad'
   denotes a fixed string for internal and external padding respectively. The
   two strings are:

       ipad = the byte 0x36 repeated 64 times,
       epad = the byte 0x5C repeated 64 times.

   The calculation of the authentication data consists of the following steps:

   (1) append zeros to the end of K to create a 64 byte string (e.g., if K is
       of length 20 bytes it will be appended with 44 zero bytes 0x00)
   (2) XOR (bitwise exclusive-OR) the 64 byte string computed in step (1) with
       ipad
   (3) concatenate to the 64 byte string resulting from step (2) the data
       stream 'text'
   (4) apply SHA to the stream generated in step (3)
   (5) XOR the 64 byte string computed in step (1) with epad
   (6) concatenate to the 64 byte string resulting from step (5) the SHA result
       of step (4)
   (7) apply SHA to the stream generated in step (6)
   (8) Pad to 64-bit boundary if necessary for word alignment



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   A similar computation is described in more detail, along with example
   code and performance improvements, in [HMAC-MD5].
3. Security Considerations

   The security provided by this transform is based on the strength of
   SHA and the associated secret key.  At this time there are no known
   cryptographic attacks against SHA [SCHNEIER]. The 160-bit digest
   makes SHA more resistant to brute force attacks than MD4 and MD5
   which produce a 128-bit digest.

Acknowledgments

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.

References


   [RFC-1825]    R. Atkinson, "Security Architecture for the Internet Protocol",
                 RFC-1825, August 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.
   [HMAC-MD5]    H. Krawczyk, M. Bellare, R. Canetti, "HMAC-MD5: Keyed-MD5
                 for Message Authentication", Internet Draft, March, 1996.
   [FIPS-180-1]  NIST, FIPS PUB 180-1: Secure Hash Standard, April 1995.
   [SCHNEIER]    B. Schneier, "Applied Cryptography Protocols, Algorithms, and
                 Source Code in C", John Wiley & Sons, Inc. 1994.
   [ESP-DES-MD5] J. Hughes, "Combined DES-CBC, MD5, and Replay Prevention
                 Security Transform", Internet Draft, April, 1996.

Contacts

   Shu-jen Chang
   NIST
   Building 820, Room 456
   Gaithersburg, MD 20899

   shu-jen.chang@nist.gov

   Robert Glenn
   NIST
   Building 820, Room 455
   Gaithersburg, MD 20899




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   rob.glenn@nist.gov


















































Chang, Glenn                                                    [Page 7]