Key Management Considerations for the TCP MD5 Signature Option

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.

      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
   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.

   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.

   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.

   [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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