Network Working Group S. Bellovin
Internet-Draft Columbia University
Expires: December 20, 2006 June 18, 2006
Key Change Strategies for TCP-MD5
draft-bellovin-keyroll2385-00.txt
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Abstract
The TCP-MD5 option is most commonly used to secure BGP sessions
between routers. However, changing the long-term key is difficult,
since the change needs to be synchronized between different
organizations. We describe single-ended strategies that will permit
(mostly) unsynchronized key changes.
1. Introduction
The TCP-MD5 option [RFC2385] is most commonly used to secure BGP
sessions between routers. However, changing the long-term key is
difficult, since the change needs to be synchronized between
different organizations. Worse yet, if the keys are out of sync, it
may break the connection between the two routers, rendering repair
attempts difficult.
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The proper solution involves some sort of key management protocol.
Apart from the complexity of such things, RFC 2385 was not written
with key changes in mind. In particular, there is no KeyID field in
the option, which means that even a key management protocol would run
into the same problem.
Fortunately, a heuristic permits key change despite this protocol
deficiency. The change can be installed unilaterally at one end of a
connection; it is fully compatible with the existing protocol.
1.1. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
2. The Algorithm
Separate algorithms are necessary for transmission and reception.
Reception is easier; we explain it first.
2.1. Reception
A receiver has a list of valid keys. Each key has a timestamp
associated with it. When a segment arrives, each key is tried in
turn. The segment is discarded if and only if it cannot be validated
by any key in the list.
In principle, there is no need to test keys in any order. For
performance reasons, though, a simple LRU strategy -- try the last
valid key first -- should work well. More complex mechanisms, such
as examining the TCP sequence number of an arriving segment to see
whether it fits in a "hole", are almost certainly unnecessary.
The newest key that has successfully validated a segment is marked as
the "preferred" key; see below.
Implicit in this scheme is the assumption that older keys will
eventually be unneeded and can be removed. Accordingly,
implementations SHOULD provide an indication of when a key was last
used successfully.
2.2. Transmission
Transmission is more complex, because the sender does not know which
keys can be accepted at the far end. Accordingly, the conservative
strategy is to delay using any new keys for a considerable amount of
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time, probably measured in days. This time interval is the amount of
asynchronicity the parties wish to permit; it is agreed-upon out of
band and configured manually.
Some automation is possible, however. If a key has been used
successfully to validate an incoming segment, clearly the other side
knows it. Accordingly, any key marked as "preferred" by the receiver
SHOULD be used for transmissions.
A sophisticated implementation could try alternate keys if the TCP
retransmission counter gets too high. (This is analogous to dead
gateway detection.) In particular, if a key change has just been
attempted but such segments are not acknowledged, it is reasonable to
fall back to the previous key and issue an alert of some sort.
Note that there is an ambiguity when an acknowledgment is received
for a segment transmitted with two different keys. The TCP Timestamp
option [RFC1323] can be used for disambiguation.
3. Security Considerations
In theory, accepting multiple keys simultaneously makes life easier
for an attacker. In practice, if the recommendations in [RFC3562]
are followed, this should not be a problem.
New keys must be communicated securely. In particular, new key
messages must be kept confidential and must be properly
authenticated.
4. References
[RFC1323] Jacobson, V., Braden, B., and D. Borman, "TCP Extensions
for High Performance", RFC 1323, May 1992.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2385] Heffernan, A., "Protection of BGP Sessions via the TCP MD5
Signature Option", RFC 2385, August 1998.
[RFC3562] Leech, M., "Key Management Considerations for the TCP MD5
Signature Option", RFC 3562, July 2003.
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Author's Address
Steven M. Bellovin
Columbia University
1214 Amsterdam Avenue
MC 0401
New York, NY 10027
US
Phone: +1 212 939 7149
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