Network Working Group M. Leech
Request for Comments: 3562 Nortel Networks
Category:Informational July 2003
Key Management Considerations for
the TCP MD5 Signature Option
Status of this Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright (C) The Internet Society (2003). All Rights Reserved.
The TCP MD5 Signature Option (RFC 2385), used predominantly by BGP,
has seen significant deployment in critical areas of Internet
infrastructure. The security of this option relies heavily on the
quality of the keying material used to compute the MD5 signature.
This document addresses the security requirements of that keying
The security of various cryptographic functions lies both in the
strength of the functions themselves against various forms of attack,
and also, perhaps more importantly, in the keying material that is
used with them. While theoretical attacks against the simple MAC
construction used in RFC 2385 are possible [MDXMAC], the number of
text-MAC pairs required to mount a forgery make it vastly more
probable that key-guessing is the main threat against RFC 2385.
We show a quantitative approach to determining the security
requirements of keys used with [RFC2385], which tends to suggest the
o Key lengths SHOULD be between 12 and 24 bytes, with larger keys
having effectively zero additional computational costs when
compared to shorter keys.
Leech Informational [Page 1]RFC 3562 Considerations for the TCP MD5 Signature Option July 2003
o Key sharing SHOULD be limited so that keys aren't shared among
multiple BGP peering arrangements.
o Keys SHOULD be changed at least every 90 days.
1.1. Requirements Keywords
The keywords "MUST", "MUST NOT", "REQUIRED", "SHOULD", "SHOULD NOT",
and "MAY" that appear in this document are to be interpreted as
described in [RFC2119].
2. Performance assumptions
The most recent performance study of MD5 that this author was able to
find was undertaken by J. Touch at ISI. The results of this study
were documented in [RFC1810]. The assumption is that Moores Law
applies to the data in the study, which at the time showed a
best-possible *software* performance for MD5 of 87Mbits/second.
Projecting this number forward to the ca 2002 timeframe of this
document, would suggest a number near 2.1Gbits/second.
For purposes of simplification, we will assume that our key-guessing
attacker will attack short packets only. A likely minimal packet is
an ACK, with no data. This leads to having to compute the MD5 over
about 40 bytes of data, along with some reasonable maximum number of
key bytes. MD5 effectively pads its input to 512-bit boundaries (64
bytes) (it's actually more complicated than that, but this
simplifying assumption will suffice for this analysis). That means
that a minimum MD5 "block" is 64 bytes, so for a ca 2002-scaled
software performance of 2.1Gbits/second, we get a single-CPU software
MD5 performance near 4.1e6 single-block MD5 operations per second.
These numbers are, of course, assuming that any key-guessing attacker
is resource-constrained to a single CPU. In reality, distributed
cryptographic key-guessing attacks have been remarkably successful in
the recent past.
It may be instructive to look at recent Internet worm infections, to
determine what the probable maximum number of hosts that could be
surreptitiously marshalled for a key-guessing attack against MD5.
CAIDA [CAIDA2001] has reported that the Code Red worm infected over
350,000 Internet hosts in the first 14 hours of operation. It seems
reasonable to assume that a worm whose "payload" is a mechanism for
quietly performing a key-guessing attack (perhaps using idle CPU
cycles of the infected host) could be at least as effective as Code
Red was. If one assumes that such a worm were engineered to be
maximally stealthy, then steady-state infection could conceivably
reach 1 million hosts or more. That changes our single-CPU
Leech Informational [Page 2]RFC 3562 Considerations for the TCP MD5 Signature Option July 2003
performance from 4.1e6 operations per second, to somewhere between