Network Working Group P Metzger
Internet Draft W A Simpson
expires in six months March 1995
IP Authentication using Keyed MD5
draft-ietf-ipsec-ah-md5-02.txt |
Status of this Memo
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Abstract
This document describes the use of keyed MD5 with the IP
Authentication Header.
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1. Introduction
The Authentication Header (AH) [A-AH] provides integrity and
authentication for IP datagrams.
This specification describes the AH use of Message Digest 5 (MD5)
[RFC-1321].
All implementations that claim conformance or compliance with the
Authentication Header specification MUST implement this MD5
mechanism.
Implementors should consult the most recent version of the IAB
Standards [RFC-1610] for further guidance on the status of this
document.
This document assumes that the reader is familiar with the related
document "Security Architecture for the Internet Protocol" [A-SA],
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 pseudo-random number, not a guessable string of
any sort.
1.2. Data Size
MD5's 128-bit output is naturally 64-bit aligned. Typically, there
is no further padding of the Authentication Data field.
1.3. Performance
MD5 reportedly has a throughput of about 60 Mbps on a fast 64-bit
RISC processor with slightly tuned MD5 code [Touch94].
Nota Bene: This is possibly too slow. Suggestions are sought on
alternative authentication algorithms that have significantly
faster throughput, are not patent-encumbered, and still retain
adequate cryptographic strength.
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2. Calculation
The 128-bit digest is calculated as described in [RFC-1321]. The
specification of MD5 includes a portable 'C' programming language
description of the MD5 algorithm.
The invariant fields of the entire IP datagram are hashed first. The
variable length secret authentication key is concatenated with
(immediately followed by) this initial 128-bit digest, and the
combination is hashed again. This final 128-bit digest is inserted
into the Authentication Data field.
The MD5 algorithm requires a particular format of padding after the *
end of the authenticated data. This padding is not sent over the
link. *
Security Considerations
Users need to understand that the quality of the security provided by
this specification depends completely on the strength of the MD5 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.
Among other considerations, applications may wish to take care not to
select weak keys, although the odds of picking one at random are low
[Schneier94, p 233].
At the time of writing of this document, it is known to be possible
to produce collisions in the compression function of MD5 [BB93].
There is not yet a known method to exploit these collisions to attack
MD5 in practice, but this fact is disturbing to some authors
[Schneier94].
It has also recently been determined [OW94] that it is possible to
build a machine for $10 Million that could find messages that hash to
an arbitrary given MD5 hash. This attack requires approximately 24
days. Although this is not a substantial weakness for most IP
security applications, it should be recognized that current
technology is catching up to the 128-bit hash length used by MD5.
Applications requiring extremely high levels of security may wish to
move in the near future to algorithms with longer hash lengths.
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Acknowledgements
Some of the text of this specification was derived from work by
Randall Atkinson for the SIP, SIPP, and IPv6 Working Groups.
The basic concept and use of MD5 is derived in large part from the
work done for SNMPv2 [RFC-1446].
Burt Kaliski suggested the two step keyed-MD5 technique.
Steve Bellovin, Steve Deering, Frank Kastenholz, Charles Lynn, and
Dave Mihelcic provided useful critiques of earlier versions of this
draft.
References
[A-SA] Randall Atkinson, "Security Architecture for the Internet
Protocol", work in progress.
[A-AH] Randall Atkinson, "IP Authentication Header", work in
progress.
[BB93] B. den Boer and A. Bosselaers, "Collisions for the
Compression function of MD5", Advances in Cryptology --
Eurocrypt '93 Proceedings, Berlin: Springer-Verlag 1994
[CN94] Carroll, J.M., and Nudiati, S., "On Weak Keys and Weak Data:
Foiling the Two Nemeses", Cryptologia, Vol. 18 No. 23 pp.
253-280, July 1994.
[RFC-1321]
Ronald Rivest, "The MD5 Message-Digest Algorithm", RFC-1321,
DDN Network Information Center, April 1992.
[RFC-1446]
Galvin, J., and McCloghrie, K., "Security Protocols for
Version 2 of the Simple Network Management Protocol
(SNMPv2)", RFC-1446, DDN Network Information Center, April
1993.
[RFC-1610]
Postel, J., "Internet Official Protocol Standards", STD 1,
RFC 1610, USC/Information Sciences Institute, July 1994.
[RFC-1700]
Reynolds, J., and Postel, J., "Assigned Numbers", STD 2, RFC
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1700, USC/Information Sciences Institute, October 1994.
[OW94] Paul C. van Oorschot & Michael J. Wiener, "Parallel
Collision Search with Application to Hash Functions and
Discrete Logarithms", Proceedings of the 2nd ACM Conf.
Computer and Communications Security, Fairfax, VA, Nov 3-5
1994.
[Schneier94]
Schneier, B., "Applied Cryptography", John Wiley & Sons, New
York, NY, 1994. ISBN 0-471-59756-2
[Touch94]
Touch, J., "Report on MD5 Performance", work in progress,
December 1994.
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
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Table of Contents
1. Introduction .......................................... 1
1.1 Keys ............................................ 1
1.2 Data Size ....................................... 1
1.3 Performance ..................................... 1
2. Calculation ........................................... 2
SECURITY CONSIDERATIONS ...................................... 2
ACKNOWLEDGEMENTS ............................................. 2
REFERENCES ................................................... 3
AUTHOR'S ADDRESS ............................................. 4
