Network Working Group P. Metzger
Request for Comments: 1828 Piermont
Category: Standards Track W. Simpson
Daydreamer
August 1995

IP Authentication using Keyed MD5

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.

Abstract

This document describes the use of keyed MD5 with the IP
Authentication Header.

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 ............................................. 3
REFERENCES ................................................... 3
AUTHOR'S ADDRESS ............................................. 4

1. Introduction

The Authentication Header (AH) [RFC-1826] provides integrity and
authentication for IP datagrams. This specification describes the AH
use of keys with Message Digest 5 (MD5) [RFC-1321].

All implementations that claim conformance or compliance with the
Authentication Header specification MUST implement this keyed MD5
mechanism.

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 128 bits MUST be supported by the
implementation, although any particular key may be shorter. Longer
keys are encouraged.

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 software speeds are adequate for commonly deployed LAN and WAN
links, but reportedly are too slow for newer link technologies [RFC-
1810].

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 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 form of the authenticated message is

key, keyfill, datagram, key, MD5fill

First, the variable length secret authentication key is filled to the
next 512-bit boundary, using the same pad with length technique
defined for MD5.

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 MD5 itself. The 128-bit MD5 digest is
calculated, and the result is inserted into the Authentication Data
field.

Discussion:
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 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.

At the time of writing of this document, it is known to be possible
to produce collisions in the compression function of MD5 [dBB93].
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 [vOW94] that it is possible to
build a machine for $10 Million that could find two chosen text

variants with a common MD5 hash value. However, it is unclear
whether this attack is applicable to a keyed MD5 transform.

This attack requires approximately 24 days. The same form of attack
is useful on any iterated n-bit hash function, and the time is
entirely due to the 128-bit length of the MD5 hash.

Although there is no 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.

Acknowledgements

This document was reviewed by the IP Security Working Group of the
Internet Engineering Task Force (IETF). Comments should be submitted
to the ipsec@ans.net mailing list.

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].

Steve Bellovin, Phil Karn, Charles Lynn, Dave Mihelcic, Hilarie
Orman, Jeffrey Schiller, Joe Touch, and David Wagner provided useful
critiques of earlier versions of this draft.

References

[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.

[dBB93] den Boer, B., and Bosselaers, A., "Collisions for the
Compression function of MD5", Advances in Cryptology --
Eurocrypt '93 Proceedings, Berlin: Springer-Verlag 1994

[KR95] Kaliski, B., and Robshaw, M., "Message authentication with
MD5", CryptoBytes (RSA Labs Technical Newsletter), vol.1
no.1, Spring 1995.

[RFC-1321]
Rivest, R., "The MD5 Message-Digest Algorithm", RFC1321,
MIT and RSA Data Security, Inc., April 1992.

[RFC-1446]
Galvin, J., and K. McCloghrie, "Security Protocols for
Version 2 of the Simple Network Management Protocol
(SNMPv2)", RFC1446, TIS, Hughes LAN Systems, April
1993.

[RFC-1700]
Reynolds, J., and J. Postel, "Assigned Numbers", STD 2,
RFC1700, USC/Information Sciences Institute, October 1994.

[RFC-1800]
Postel, J., "Internet Official Protocol Standards", STD 1,
RFC1800, USC/Information Sciences Institute, July 1995.

[RFC-1810]
Touch, J., "Report on MD5 Performance", RFC1810,
USC/Information Sciences Institute, June 1995.

[RFC-1825]
Atkinson, R., "Security Architecture for the Internet
Protocol", RFC1825, NRL, August 1995.

[RFC-1826]
Atkinson, R., "IP Authentication Header", RFC1826, NRL
August 1995.

[Schneier94]
Schneier, B., "Applied Cryptography", John Wiley & Sons, New
York, NY, 1994. ISBN 0-471-59756-2

[vOW94] van Oorschot, P. C., and Wiener, M. J., "Parallel Collision
Search with Applications to Hash Functions and Discrete
Logarithms", Proceedings of the 2nd ACM Conf. Computer and
Communications Security, Fairfax, VA, November 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