On Mon, Nov 13, 2017 at 10:09 AM, Eric Rescorla <e...@rtfm.com> wrote: > On Mon, Nov 13, 2017 at 9:56 AM, Kyle Rose <kr...@krose.org> wrote: >> What is the situation, commonly the case for HTTP, in which different >> >> users would be using the same key for encryption? > > > That's not the assumption. It's a time-space tradeoff. Say you have a > 128-bit > key. The attacker computes the value: > > Encrypt(K, "GET / HTTP/1.1") [or whatever] > > For a feasible subset of the key space (e.g., 2^80 or so). It then captures > a large fraction of the traffic on the Internet and looks for matches for > one > of these strings. If it has a match, it now immediately knows the key. Given > the amount of traffic on the Internet, this is just maybe barely feasible > with > 128-bit keys. However, with a randomized (secret) nonce, it becomes > infeasible.
Interesting. I don't have a strong opinion about this one, but if there are no (or cheap) tradeoffs, it seems reasonable to use this construction. >> I'm confused. The byte stream will be delivered in-order via TCP >> sequence semantics, so receiving frames (TLV within the TCP >> bytestream) out-of-order should not be possible. If a MitM reordered >> the frames, the receiver would either drop the frame or close the >> connection following a failure to authenticate upon decryption: >> >> q( In the latter case, the implementation MUST >> either drop the TCP segment(s) containing the frame or abort the >> connection; but if it aborts, the implementation MUST raise an error >> condition distinct from the end-of-file condition. ) > > > Maybe I'm misunderstanding tcpcrypt, so let me ask a preliminary > question: > > Say you have two TCP segments protected using tcpcrypt, N and N+1 > If the receiver receives N+1 before N, how does it behave? Tcpcrypt lives entirely within the ordered TCP data stream. If N+1 arrives before N, that payload will not be delivered to the tcpcrypt engine until after N arrives, in accordance with the in-order semantics of TCP payload delivery. >> The idea of doing something more ticket-like was discussed, but David >> pointed out (in the thread here: >> https://www.ietf.org/mail-archive/web/tcpinc/current/msg00919.html) >> that tickets might not meet the charter mandate for forward secrecy. I >> don't recall discussing the implications of mismanaging ss[i], but >> I'll defer to Daniel on that point. Same goes for the following: > > > I don't think the issue here is tickets, but rather the lack of a nonce. > The point is that (say) the implementation fails to properly delete > ss[i] after encrypting some data. Then if an attacker can force > you to reuse ss[i], it can potentially recover data. This would not > be the case if you mixed in a nonce from both sides (the way that > TLS and IKE do). I like the idea of added defense-in-depth against faulty implementations, but it's not clear to me what tradeoffs this involves re: (e.g.) SYN data on resumption, or on number of round trips, or on the latency-to-first-payload-byte. I'd like Daniel to chime in here. Kyle _______________________________________________ Tcpinc mailing list Tcpinc@ietf.org https://www.ietf.org/mailman/listinfo/tcpinc
