On Mon, Nov 13, 2017 at 9:56 AM, Kyle Rose <kr...@krose.org> wrote: > On Sat, Nov 11, 2017 at 11:12 AM, Eric Rescorla <e...@rtfm.com> wrote: > > ---------------------------------------------------------------------- > > DISCUSS: > > ---------------------------------------------------------------------- > > > > https://mozphab-ietf.devsvcdev.mozaws.net/D3970 > > > > 2^64 bytes in the underlying TCP datastream (which would cause the > > "offset" field to wrap) before re-keying. > > > > In TLS and other WGs, we have adopted the practice of > > salting the nonce with a secret per-connection value to avoid > > large-scale surveillance attacks. Why did you opt to use a weaker > > construction. See: > > https://tlswg.github.io/tls13-spec/draft-ietf-tls-tls13. > html#security-record-layer > > and https://eprint.iacr.org/2016/564. > > I think I need some clarification on the point here. From the relevant > passage in the TLS 1.3 draft: > > q( In order to prevent mass cryptanalysis when the same plaintext is > repeatedly encrypted by different users under the same key (as is > commonly the case for HTTP), the nonce is formed by mixing the > sequence number with a secret per-connection initialization vector > derived along with the traffic keys. See [BT16] for analysis of this > construction. ) > > 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. > FIN flag set, it MUST immediately send a frame (with empty > > application data if necessary) with "rekey = 1". > > > > I don't think that the algorithm in this section > > necessarily works properly, because you have to handle rekeys in > > sequence: > > > > Frame 1 [0:999] > > Frame 2 [1000:1999, rekey=1] > > Frame 3 [2000:2999, rekey=1] > > > > Now what happens if the frames are re-ordered so you get Frame 3 and > > then Frame 2. You will try to decrypt Frame 3 with generation 2 and > > Frame 2 with generation 3, neither of which will work (though you > > might be able to interpret the text loosely to have you try to decrypt > > Frame 2 with generation 2). Note that if you were to resequence before > > processing, this wouldn't happen. > > > > At minimum I think some clarification is needed here. > > 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? > Given that you are allowing P-256 and point reuse, you > > should be requiring point validation. See: > > https://tlswg.github.io/tls13-spec/draft-ietf-tls-tls13. > html#rfc.section.4.2.8.2 > > https://tlswg.github.io/tls13-spec/draft-ietf-tls-tls13. > html#elliptic-curve-diffie-hellman > > > > You should probably also be requiring Curve25519 output validation. > > Agreed. > > > You still seem to need to specify an MTI symmetric algorithm. > > Daniel noted this in his October 25 email, but you are right: I can't > seem to find it in the latest draft, either. Did that change get lost? > I am not sure. > ---------------------------------------------------------------------- > > COMMENT: > > ---------------------------------------------------------------------- > > > > > > The design of session resumption here essentially precludes doing > > tcpcrypt resumption across servers (as one does with TLS) because you > > need extremely tight control of ss[i] or you have catastrophic > > results. Was this a deliberate choice by the WG? > > 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). -Ekr
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