BTW, whatever algorithm you use (SHA-256 or even something much more complex) is not going to help -- it may make the work someone has to do a bit more involved, but it really doesn't make it impossible.
1. You always have a brute force attack. As you indicate, calculating the hash based on the mac address is always going to be a possible attack. And, 8000 is not 8-bits, but more like 13. But agreed that many of the Ethernet OIDs are unlikely to be of much interest. 2. If the attacker is on the same network as the client at some point, they can learn the identifier of the client (snoop DHCP and/or ARP traffic). Once they have that, it is possible to query DNS for DHCID RRs (query for the PTR, query for a DHCID RR for the name). Once you have that, you use the name and client's identity and run it through the algorithm and check for a match. If you looked up all 2^32 PTRs, you'd be able to locate the client at other sites that use DHCP and export the DHCID RR information. The number of PTRs you'd likely have to query is much smaller than 2^32s. You could target just some domains (network addresses) that you were most interested in or where you suspect the client connects to the network. Far reducing the number of queries needed. - Bernie > -----Original Message----- > From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] > On Behalf Of Steven M. Bellovin > Sent: Monday, November 28, 2005 11:49 AM > To: Ted Lemon > Cc: iesg@ietf.org; dhcwg@ietf.org; namedroppers@ops.ietf.org; > Pekka Savola; ietf@ietf.org > Subject: [dhcwg] Re: DHCID and the use of MD5 [Re: Last Call: > 'Resolution ofFQDN Conflicts among DHCP Clients' to Proposed > Standard] > > In message <[EMAIL PROTECTED]>, 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 > > > > _______________________________________________ > dhcwg mailing list > dhcwg@ietf.org > https://www1.ietf.org/mailman/listinfo/dhcwg > _______________________________________________ Ietf mailing list Ietf@ietf.org https://www1.ietf.org/mailman/listinfo/ietf
