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rfc:rfc1852

Network Working Group P. Metzger Request for Comments: 1852 Piermont Category: Experimental W. Simpson

                                                            Daydreamer
                                                        September 1995
                 IP Authentication using Keyed SHA

Status of this Memo

 This document defines an Experimental Protocol for the Internet
 community.  This does not specify an Internet standard of any kind.
 Discussion and suggestions for improvement are requested.
 Distribution of this memo is unlimited.

Abstract

 This document describes the use of keyed SHA with the IP
 Authentication Header.

Table of Contents

   1.     Introduction ..........................................    2
      1.1       Keys ............................................    2
      1.2       Data Size .......................................    2
      1.3       Performance .....................................    2
   2.     Calculation ...........................................    3
   SECURITY CONSIDERATIONS ......................................    4
   ACKNOWLEDGEMENTS .............................................    4
   REFERENCES ...................................................    5
   AUTHOR'S ADDRESS .............................................    6

Metzger & Simpson Experimental [Page 1] RFC 1852 AH SHA September 1995

1. Introduction

 The Authentication Header (AH) [RFC-1826] provides integrity and
 authentication for IP datagrams.  This specification describes the AH
 use of keys with the Secure Hash Algorithm (SHA) [FIPS-180-1].
    It should be noted that this document specifies a newer version of
    the SHA than that described in [FIPS-180], which was flawed.  The
    older version is not interoperable with the newer version.
 This document assumes that the reader is familiar with the related
 document "Security Architecture for the Internet Protocol" [RFC-
 1825], which defines the overall security plan for IP, and provides
 important background for this specification.

1.1. Keys

 The secret authentication key shared between the communicating
 parties SHOULD be a cryptographically strong random number, not a
 guessable string of any sort.
 The shared key is not constrained by this transform to any particular
 size.  Lengths of up to 160 bits MUST be supported by the
 implementation, although any particular key may be shorter.  Longer
 keys are encouraged.

1.2. Data Size

 SHA's 160-bit output is naturally 32-bit aligned.  However, many
 implementations require 64-bit alignment of the following headers, in
 which case an additional 32 bits of padding is added, either before
 or after the SHA output.
 The size and position of this padding are negotiated as part of the
 key management.  Padding bits are filled with unspecified
 implementation dependent (random) values, which are ignored on
 receipt.

1.3. Performance

 Preliminary results indicate that SHA is 62% as fast as MD5, and 80%
 as fast as DES hashing.  That is,

Metzger & Simpson Experimental [Page 2] RFC 1852 AH SHA September 1995

                         SHA < DES < MD5
 Nota Bene:
    Suggestions are sought on alternative authentication algorithms
    that have significantly faster throughput, are not patent-
    encumbered, and still retain adequate cryptographic strength.

2. Calculation

 The 160-bit digest is calculated as described in [FIPS-180-1].  At
 the time of writing, a portable C language implementation of SHA is
 available via FTP from ftp://rand.org/pub/jim/sha.tar.gz.
 The form of the authenticated message is
          key, keyfill, datagram, key, SHAfill
 First, the variable length secret authentication key is filled to the
 next 512-bit boundary, using the same pad with length technique
 defined for SHA.
 Then, the filled key is concatenated with (immediately followed by)
 the invariant fields of the entire IP datagram (variant fields are
 zeroed), concatenated with (immediately followed by) the original
 variable length key again.
 A trailing pad with length to the next 512-bit boundary for the
 entire message is added by SHA itself.  The 160-bit SHA digest is
 calculated, and the result is inserted into the Authentication Data
 field.
 Discussion:
    The leading copy of the key is padded in order to facilitate
    copying of the key at machine boundaries without requiring re-
    alignment of the following datagram.  The padding technique
    includes a length which protects arbitrary length keys.  Filling
    to the SHA block size also allows the key to be prehashed to avoid
    the physical copy in some implementations.
    The trailing copy of the key is not necessary to protect against
    appending attacks, as the IP datagram already includes a total
    length field.  It reintroduces mixing of the entire key, providing
    minimal protection for very long and very short datagrams, and
    marginal robustness against possible attacks on the IP length
    field itself.

Metzger & Simpson Experimental [Page 3] RFC 1852 AH SHA September 1995

    When the implementation adds the keys and padding in place before
    and after the IP datagram, care must be taken that the keys and/or
    padding are not sent over the link by the link driver.

Security Considerations

 Users need to understand that the quality of the security provided by
 this specification depends completely on the strength of the SHA hash
 function, the correctness of that algorithm's implementation, the
 security of the key management mechanism and its implementation, the
 strength of the key [CN94], and upon the correctness of the
 implementations in all of the participating nodes.
 The SHA algorithm was originally derived from the MD4 algorithm
 [RFC-1320].  A flaw was apparently found in the original
 specification of SHA [FIPS-180], and this document specifies the use
 of a corrected version [FIPS-180-1].
 At the time of writing of this document, there are no known flaws in
 the SHA algorithm.  That is, there are no known attacks on SHA or any
 of its components that are better than brute force, and the 160-bit
 hash output by SHA is substantially more resistant to brute force
 attacks than the 128-bit hash size of MD4 and MD5.
 However, as the flaw in the original SHA algorithm shows,
 cryptographers are fallible, and there may be substantial
 deficiencies yet to be discovered in the algorithm.

Acknowledgements

 Some of the text of this specification was derived from work by
 Randall Atkinson for the SIP, SIPP, and IPv6 Working Groups.
 Preliminary performance analysis was provided by Joe Touch.
 Comments should be submitted to the ipsec@ans.net mailing list.

Metzger & Simpson Experimental [Page 4] RFC 1852 AH SHA September 1995

References

 [CN94]   John M. Carroll & Sri Nudiati, "On Weak Keys and Weak Data:
          Foiling the Two Nemeses", Cryptologia, Vol. 18 No. 23 pp.
          253-280, July 1994.
 [FIPS-180]
          "Secure Hash Standard", Computer Systems Laboratory,
          National Institute of Standards and Technology, U.S.
          Department Of Commerce, May 1993.
          Also known as: 58 Fed Reg 27712 (1993).
 [FIPS-180-1]
          "Secure Hash Standard", National Institute of Standards and
          Technology, U.S. Department Of Commerce, April 1995.
          Also known as: 59 Fed Reg 35317 (1994).
 [RFC-1320]
          Ronald Rivest, "The MD4 Message-Digest Algorithm", RFC-1320,
          April 1992.
 [RFC-1700]
          Reynolds, J., and J. Postel, "Assigned Numbers", STD 2, RFC
          1700, USC/Information Sciences Institute, October 1994.
 [RFC-1825]
          Atkinson, R., "Security Architecture for the Internet
          Protocol", RFC-1825, Naval Research Laboratory, July 1995.
 [RFC-1826]
          Atkinson, R., "IP Authentication Header", RFC-1826, Naval
          Research Laboratory, July 1995.

Metzger & Simpson Experimental [Page 5] RFC 1852 AH SHA September 1995

Author's Address

 Questions about this memo can also be directed to:
    Perry Metzger
    Piermont Information Systems Inc.
    160 Cabrini Blvd., Suite #2
    New York, NY  10033
    perry@piermont.com
    William Allen Simpson
    Daydreamer
    Computer Systems Consulting Services
    1384 Fontaine
    Madison Heights, Michigan  48071
    Bill.Simpson@um.cc.umich.edu
        bsimpson@MorningStar.com

Metzger & Simpson Experimental [Page 6]

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