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 Notice
Copyright (C) The Internet Society (2003). All Rights Reserved.
Abstract
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
material.
1. Introduction
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
following:
o Key lengths SHOULD be between 12 and 24 bytes, with larger keys
having effectively zero additional computational costs when
compared to shorter keys.
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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
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performance from 4.1e6 operations per second, to somewhere between
1.0e11 and 1.0e13 MD5 operations per second.
In 1997, John Gilmore, and the Electronic Frontier Foundation [EFF98]
developed a special-purpose machine, for an investment of
approximately USD$250,000. This machine was able to mount a
key-guessing attack against DES, and compute a key in under 1 week.
Given Moores Law, the same investment today would yield a machine
that could do the same work approximately 8 times faster. It seems
reasonable to assume that a similar hardware approach could be
brought to bear on key-guessing attacks against MD5, for similar key
lengths to DES, with somewhat-reduced performance (MD5 performance in
hardware may be as much as 2-3 times slower than DES).
3. Key Lifetimes
Operational experience with RFC 2385 would suggest that keys used
with this option may have lifetimes on the order of months. It would
seem prudent, then, to choose a minimum key length that guarantees
that key-guessing runtimes are some small multiple of the key-change
interval under best-case (for the attacker) practical attack
performance assumptions.
The keys used with RFC 2385 are intended only to provide
authentication, and not confidentiality. Consequently, the ability
of an attacker to determine the key used for old traffic (traffic
emitted before a key-change event) is not considered a threat.
3. Key Entropy
If we make an assumption that key-change intervals are 90 days, and
that the reasonable upper-bound for software-based attack performance
is 1.0e13 MD5 operations per second, then the minimum required key
entropy is approximately 68 bits. It is reasonable to round this
number up to at least 80 bits, or 10 bytes. If one assumes that
hardware-based attacks are likely, using an EFF-like development
process, but with small-country-sized budgets, then the minimum key
size steps up considerably to around 83 bits, or 11 bytes. Since 11
is such an ugly number, rounding up to 12 bytes is reasonable.
In order to achieve this much entropy with an English-language key,
one needs to remember that English has an entropy of approximately
1.3 bits per character. Other human languages are similar. This
means that a key derived from a human language would need to be
approximately 61 bytes long to produce 80 bits of entropy, and 73
bytes to produce 96 bits of entropy.
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A more reasonable approach would be to use the techniques described
in [RFC1750] to produce a high quality random key of 96 bits or more.
It has previously been noted that an attacker will tend to choose
short packets to mount an attack on, since that increases the
key-guessing performance for the attacker. It has also been noted
that MD5 operations are effectively computed in blocks of 64 bytes.
Given that the shortest packet an attacker could reasonably use would
consist of 40 bytes of IP+TCP header data, with no payload, the
remaining 24 bytes of the MD5 block can reasonably be used for keying
material without added CPU cost for routers, but substantially
increase the burden on the attacker. While this practice will tend
to increase the CPU burden for ordinary short BGP packets, since it
will tend to cause the MD5 calculations to overflow into a second MD5
block, it isn't currently seen to be a significant extra burden to
BGP routing machinery.
The most reasonable practice, then, would be to choose the largest
possible key length smaller than 25 bytes that is operationally
reasonable, but at least 12 bytes.
Some implementations restrict the key to a string of ASCII
characters, much like simple passwords, usually of 8 bytes or less.
The very real risk is that such keys are quite vulnerable to
key-guessing attacks, as outlined above. The worst-case scenario
would occur when the ASCII key/password is a human-language word, or
pseudo-word. Such keys/passwords contain, at most, 12 bits of
entropy. In such cases, dictionary driven attacks can yield results
in a fraction of the time that a brute-force approach would take.
Such implementations SHOULD permit users to enter a direct binary key
using the command line interface. One possible implementation would
be to establish a convention that an ASCII key beginning with the
prefix "0x" be interpreted as a string of bytes represented in
hexadecimal. Ideally, such byte strings will have been derived from
a random source, as outlined in [RFC1750]. Implementations SHOULD
NOT limit the length of the key unnecessarily, and SHOULD allow keys
of at least 16 bytes, to allow for the inevitable threat from Moores
Law.
4. Key management practices
In current operational use, TCP MD5 Signature keys [RFC2385] may be
shared among significant numbers of systems. Conventional wisdom in
cryptography and security is that such sharing increases the
probability of accidental or deliberate exposure of keys. The more
frequently such keying material is handled, the more likely it is to
be accidentally exposed to unauthorized parties.
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Since it is possible for anyone in possession of a key to forge
packets as if they originated with any of the other keyholders, the
most reasonable security practice would be to limit keys to use
between exactly two parties. Current implementations may make this
difficult, but it is the most secure approach when key lifetimes are
long. Reducing key lifetimes can partially mitigate widescale
key-sharing, by limiting the window of opportunity for a "rogue"
keyholder.
Keying material is extremely sensitive data, and as such, should be
handled with reasonable caution. When keys are transported
electronically, including when configuring network elements like
routers, secure handling techniques MUST be used. Use of protocols
such as S/MIME [RFC2633], TLS [RFC2246], Secure Shell (SSH) SHOULD be
used where appropriate, to protect the transport of the key.
5. Security Considerations
This document is entirely about security requirements for keying
material used with RFC 2385.
No new security exposures are created by this document.
6. Acknowledgements
Steve Bellovin, Ran Atkinson, and Randy Bush provided valuable
commentary in the development of this document.
7. References
[RFC1771] Rekhter, Y. and T. Li, "A Border Gateway Protocol 4
(BGP-4)", RFC 1771, March 1995.
[RFC1810] Touch, J., "Report on MD5 Performance", RFC 1810, June
1995.
[RFC2385] Heffernan, A., "Protection of BGP Sessions via the TCP
MD5 Signature Option", RFC 2385, August 1998.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[MDXMAC] Van Oorschot, P. and B. Preneel, "MDx-MAC and Building
Fast MACs from Hash Functions". Proceedings Crypto '95,
Springer-Verlag LNCS, August 1995.
[RFC1750] Eastlake, D., Crocker, S. and J. Schiller, "Randomness
Recommendations for Security", RFC 1750, December 1994.
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[EFF98] "Cracking DES: Secrets of Encryption Research, Wiretap
Politics, and Chip Design". Electronic Frontier
Foundation, 1998.
[RFC2633] Ramsdell, B., "S/MIME Version 3 Message Specification",
RFC 2633, June 1999.
[RFC2246] Dierks, T. and C. Allen, "The TLS Protocol Version 1.0",
RFC 2246, January 1999.
[CAIDA2001] "CAIDA Analysis of Code Red"
http://www.caida.org/analysis/security/code-red/
8. Author's Address
Marcus D. Leech
Nortel Networks
P.O. Box 3511, Station C
Ottawa, ON
Canada, K1Y 4H7
Phone: +1 613-763-9145
EMail: mleech@nortelnetworks.com
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9. Full Copyright Statement
Copyright (C) The Internet Society (2003). All Rights Reserved.
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Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
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