Folks, I have just posted a WIP PR for what I'm calling "Finished Stuffing"
https://github.com/tlswg/tls13-spec/pull/615 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
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