I made a few notes on the pull request. Generally, I support the change, but I get the sense that it may aid the cryptographic properties if we keep the resumption_context and do not overload the resumption_psk as much.
I have a slight (i.e., unjustified) preference for doing ClientHello-with-block-of-zeros rather than prefix-of-ClientHello. (Is there a reason to require this extension to be the last one with block-of-zeros? Clearly there is for prefix-of-ClientHello.) -Ben On 09/06/2016 11:49 PM, Joseph Salowey wrote: > Hi Folks, > > The chairs want to make sure this gets some proper review. Please > respond with comments by Friday so we can make some progress on this > issue. > > Thanks, > > J&S > > On Tue, Sep 6, 2016 at 11:57 AM, David Benjamin <david...@chromium.org > <mailto:david...@chromium.org>> wrote: > > I think this is a good idea. It's kind of weird, but it avoids > giving the early Finished such a strange relationship with the > handshake transcript. Also a fan of doing away with multiple PSK > identities if we don't need it. > > As a bonus, this removes the need to route a "phase" parameter > into the traffic key calculation since we'll never derive more > than one epoch off of the same traffic secret. Combine that with > the two-ladder KeyUpdate and we no longer need any concatenation > or other label-munging at all. Simply use labels "key" and "iv" > and the record-layer just exposes a single UseTrafficSecret > function which saves the traffic secret (for KeyUpdate), derives > the traffic keys, and engages the new AEAD in one swoop without > mucking about with phases, traffic directions, whether we are > client or server, etc. > > David > > On Thu, Sep 1, 2016 at 6:19 PM Eric Rescorla <e...@rtfm.com > <mailto:e...@rtfm.com>> wrote: > > I should also mention that this makes the implementation a > fair bit simpler because: > > 1. You can make all the decisions on the server side > immediately upon receiving the ClientHello > without waiting for Finished. > 2. You don't need to derive early handshake traffic keys. > > From an implementor's perspective, this outweighs the messing > around with the ClientHello buffer. > -Ekr > > > On Thu, Sep 1, 2016 at 3:04 PM, Eric Rescorla <e...@rtfm.com > <mailto:e...@rtfm.com>> wrote: > > Folks, > > I have just posted a WIP PR for what I'm calling "Finished > Stuffing" > > https://github.com/tlswg/tls13-spec/pull/615 > > <https://urldefense.proofpoint.com/v2/url?u=https-3A__github.com_tlswg_tls13-2Dspec_pull_615&d=DQMFaQ&c=96ZbZZcaMF4w0F4jpN6LZg&r=sssDLkeEEBWNIXmTsdpw8TZ3tAJx-Job4p1unc7rOhM&m=IwfxQnHf5o1ACITZiQQEOKnbhWK40rJ7uWbCfhm0pSE&s=ukqGqua3EOfqImRb0EFPRRxMv7Hgom3t_5Ki4OvDG8M&e=> > > I would welcome comments on this direction and whether I > am missing > anything important. > > > OVERVIEW > This PR follows on a bunch of discussions we've had about > the redundancy > of Finished and resumption_ctx. This PR makes the > following changes: > > - Replace the 0-RTT Finished with an extension you send in the > ClientHello *whenever* you do PSK. > - Get rid of resumption context (because it is now replaced by > the ClientHello.hello_finished. > > > RATIONALE > The reasoning for this change is: > > - With ordinary PSK you don't get any assurance that the > other side > knows the PSK. > > - With 0-RTT you get some (subject to the usual anti-replay > guarantees) via the Finished message. > > - If we were to include the 0-RTT Finished message in the > handshake > transcript, then we wouldn't need the resumption context > because > the transcript would transitively include the PSK via > the Finished. > > So the natural thing to do would be to always send 0-RTT > Finished > but unfortunately: > > 1. You can't just send the 0-RTT Finished whenever you do > PSK because > that causes potential compat problems with mixed > 1.3/1.2 networks > (the same ones we have with 0-RTT, but at least that's > opt-in). > > 2. You also can't send the 0-RTT Finished with PSK because > you can > currently offer multiple PSK identities. > > The on-list discussion has suggested we could relax > condition #2 and > only have one identity. And we can fix condition #1 by > stuffing the > Finished in an extension (with some hacks to make this > easier). This > PR enacts that. > > > FAQS > - What gets included in the handshake transcript? > The whole ClientHello including the computed > hello_finished extension. > > - Isn't this a hassle to implement? > It turns out not to be. The basic reason is that at the > point where > the client sends the ClientHello and the server > processes, it doesn't > yet know which Hash will be chosen for HKDF and so NSS > (and I believe > other stacks) buffers the ClientHello in plaintext, so > hashing only > part of it is easy. I've done it in NSS and this part is > quite easy. > > > POTENTIAL VARIATIONS/TODOs > There are a number of possible variations we might want to > look at: > > 1. Moving obfuscated_ticket_age to its own extension (out of > early_data_indication). This provides additional > anti-replay > for the CH at the 0.5RTT sending point. I believe we should > make this change. > > 2. Tweaking the data to be hashed to just hash the ClientHello > prefix without the 0-filled verify_data. This is not > significantly > harder or easier to implement and basically depends on > whether > you prefer the invariant of "always hash complete > messages" or > "always hash valid pieces of transcript". See above for > notes > on buffering. > > 3. Allow multiple PSKs. Technically you could make this design > work with >1 PSK but stuffing multiple verify_data > values in > the ClientHello. E.g,, > > opaque FinishedValue<0..255>; > > struct { > FinishedValue finisheds<0..2^16-1>; > } HelloFinished; > > Based on the list discussion, it seems like nobody > wants >1 PSK, > so I think one is simpler; I just wanted to note that these > changes weren't totally coupled. > > 4. External context values. Several people have pointed > out that it > might be convenient to have an external context value > hashed > into the transcript. One way to do this would be to include > it under the Finished. That's not difficult if people > want to, > with the default being empty. > > 5. Hugo brought up on the list that we need to make very > clear that > the "hello_finished" is being used to bind the > handshakes and > that it depends on collision resistance. I have not > forgotten this > and text on that point would be appreciated. > > Comments welcome. > -Ekr > > > _______________________________________________ > TLS mailing list > TLS@ietf.org <mailto:TLS@ietf.org> > https://www.ietf.org/mailman/listinfo/tls > > <https://urldefense.proofpoint.com/v2/url?u=https-3A__www.ietf.org_mailman_listinfo_tls&d=DQMFaQ&c=96ZbZZcaMF4w0F4jpN6LZg&r=sssDLkeEEBWNIXmTsdpw8TZ3tAJx-Job4p1unc7rOhM&m=IwfxQnHf5o1ACITZiQQEOKnbhWK40rJ7uWbCfhm0pSE&s=wIZYtT4TzI4oljhi8_PX1pf95lodfWq4WmQBUXX5Q7g&e=> > > > _______________________________________________ > TLS mailing list > TLS@ietf.org <mailto:TLS@ietf.org> > https://www.ietf.org/mailman/listinfo/tls > > <https://urldefense.proofpoint.com/v2/url?u=https-3A__www.ietf.org_mailman_listinfo_tls&d=DQMFaQ&c=96ZbZZcaMF4w0F4jpN6LZg&r=sssDLkeEEBWNIXmTsdpw8TZ3tAJx-Job4p1unc7rOhM&m=IwfxQnHf5o1ACITZiQQEOKnbhWK40rJ7uWbCfhm0pSE&s=wIZYtT4TzI4oljhi8_PX1pf95lodfWq4WmQBUXX5Q7g&e=> > > > > > _______________________________________________ > TLS mailing list > TLS@ietf.org > https://www.ietf.org/mailman/listinfo/tls
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