In message <200511261243.21694.mellon@xxxxxxxxx>, Ted Lemon writes: >Making a hash function interchangeable in DHCID makes the conflict detection >algorithm hugely more complicated, and possibly not workable at all. Think >about how that would work. > I confess that I don't see the problem. The updater would do a DNS query for DHCID RRs; it would be given all of the stored records. The updater would then use local policy -- that is, an ordered list of preferred hash functions -- until it found one that was in the response. That one would be used. If no locally-known hash functions are in the list, it should behave as if there were no DHCID records present for that name. DNSSEC could protect against downgrade attacks. (Speaking of which -- were I still AD, I'd ding this document for an inadequate Security Considerations section -- apart from the lack of discussion of brute force attacks, you should cite 3833 for DNS attacks and explain what the risks are if someone can crack the hash function by any means, including brute force or eavesdropping on the wire or (perhaps) a misbehaving updater.) If you don't agree, I'd strongly suggest using SHA-256 instead of MD5. Yes, it's more expensive, but I doubt that that's a major hit on overall system performance here. It would also be useful to include in the document some discussion of upgrade strategy -- how would we ever switch to a new hash function? That's non-trivial even for protocols designed for agility, as Eric Rescorla and I have shown. No matter how it's done, this one is among the very hardest, since DNS servers would have to supply DHCID records for several hashes for a number of years. --Steven M. Bellovin, http://www.cs.columbia.edu/~smb _______________________________________________ Ietf@xxxxxxxx https://www1.ietf.org/mailman/listinfo/ietf