Network Working Group S. Bellovin Request for Comments: 4808 Columbia University Category: Informational March 2007 Key Change Strategies for TCP-MD5 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 IETF Trust (2007).
AbstractThe 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.
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. 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. RFC2119].
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. Section 3.1. This should be done at a moderately slow rate. 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.
designate the new key as preferred, and will use it for all of its transmissions. Note specifically that this will include retransmissions of any segments rejected because they used the old key. There is a problem if the unchanged machine is a "silent host" -- a host that has nothing to say, and hence does not transmit. The best way to avoid this is for an upgraded machine to try a variety of keys in the event of repeated unacknowledged packets, and to probe for new unused keys during silent periods, as discussed in Section 2.2. Alternatively, application-level KeepAlive messages may be used to ensure that neither end of the connection is completely silent. See, for example, Section 4.4 of [RFC4271] or Section 3.5.4 of [RFC3036]. Section 1, this is an interim strategy, intended to make TCP-MD5 operationally usable today. We do not suggest or recommend it as a long-term solution. In this section, we make some suggestions about the design of a future TCP authentication option. The first and most obvious change is to replace keyed MD5 with a stronger MAC [RFC4278]. Today, HMAC-SHA1 [RFC4634] is the preferred choice, though others such as UMAC [RFC4418] should be considered as well.
A new authentication option should contain some form of a Key ID field. Such an option would permit unambiguous identification of which key was used to create the MAC for a given segment, sparing the receiver the need to engage in the sort of heuristics described here. A Key ID is useful with both manual and automatic key management. (Note carefully that we do not prescribe any particular Key ID mechanism here. Rather, we are stating a requirement: there must be a simple, low-cost way to select a particular key, and it must be possible to rekey without tearing down long-lived connections.) Finally, an automated key management mechanism should be defined. The general reasoning for that is set forth in [RFC4107]; specific issues pertaining to BGP and TCP are given in [RFC3562]. RFC3562] are followed, this should not be a problem. New keys must be communicated securely. Specifically, new key messages must be kept confidential and must be properly authenticated. Having multiple keys makes CPU denial-of-service attacks easier. This suggests that keeping the overlap period reasonably short is a good idea. In addition, the Generalized TTL Security Mechanism [RFC3682], if applicable to the local topology, can help. Note that most of the time, only one key will exist; virtually all of the remaining time there will be only two keys in existence. RFC2385], and is currently listed by IANA.
[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. [RFC3036] Andersson, L., Doolan, P., Feldman, N., Fredette, A., and B. Thomas, "LDP Specification", RFC 3036, January 2001. [RFC3562] Leech, M., "Key Management Considerations for the TCP MD5 Signature Option", RFC 3562, July 2003. [RFC3682] Gill, V., Heasley, J., and D. Meyer, "The Generalized TTL Security Mechanism (GTSM)", RFC 3682, February 2004. [RFC4107] Bellovin, S. and R. Housley, "Guidelines for Cryptographic Key Management", BCP 107, RFC 4107, June 2005. [RFC4271] Rekhter, Y., Li, T., and S. Hares, "A Border Gateway Protocol 4 (BGP-4)", RFC 4271, January 2006. [RFC4278] Bellovin, S. and A. Zinin, "Standards Maturity Variance Regarding the TCP MD5 Signature Option (RFC 2385) and the BGP-4 Specification", RFC 4278, January 2006. [RFC4418] Krovetz, T., "UMAC: Message Authentication Code using Universal Hashing", RFC 4418, March 2006. [RFC4634] Eastlake, D. and T. Hansen, "US Secure Hash Algorithms (SHA and HMAC-SHA)", RFC 4634, August 2006.
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