* …and now I'm coming around to the idea that we don't need to do anything special to account for the "wrong" server behavior. Since RFC8446 already explicitly said that clients are allowed to not predict their most preferred groups, we can already reasonably infer that such servers actively believe that all their groups are comparable in security. It makes sense for some services to prioritize RTT reduction; others may prioritize group strength. There are a lot of services out there prioritizing weaker groups for CPU savings (e.g., this is one of the reasons why Curve 25519 is so popular).
* I... question whether taking that position is wise, given the ongoing postquantum transition, but so it goes Servers will have to be updated and reconfigured for PQC/hybrid support, at which time they will likely apply a different policy. * Hopefully your TLS server software, if it advertises pluggable cryptography with a PQ use case, and yet opted for a PQ-incompatible selection criteria, has clearly documented this so it isn't a surprise to you. ;-) Correct. * Between all that, we probably can reasonably say that's the server operator's responsibility? Yes. Cheers, Andrei From: TLS <tls-boun...@ietf.org> On Behalf Of David Benjamin Sent: Friday, March 8, 2024 3:25 PM To: Watson Ladd <watsonbl...@gmail.com> Cc: <tls@ietf.org> <tls@ietf.org> Subject: [EXTERNAL] Re: [TLS] Next steps for key share prediction On Thu, Mar 7, 2024 at 6:34 PM Watson Ladd <watsonbl...@gmail.com<mailto:watsonbl...@gmail.com>> wrote: On Thu, Mar 7, 2024 at 2:56 PM David Benjamin <david...@chromium.org<mailto:david...@chromium.org>> wrote: > > Hi all, > > With the excitement about, sometime in the far future, possibly transitioning > from a hybrid, or to a to-be-developed better PQ algorithm, I thought it > would be a good time to remind folks that, right now, we have no way to > effectively transition between PQ-sized KEMs at all. > > At IETF 118, we discussed draft-davidben-tls-key-share-prediction, which aims > to address this. For a refresher, here are some links: > https://davidben.github.io/tls-key-share-prediction/draft-davidben-tls-key-share-prediction.html > https://datatracker.ietf.org/meeting/118/materials/slides-118-tls-key-share-prediction-00 > (Apologies, I forgot to cut a draft-01 with some of the outstanding changes > in the GitHub, so the link above is probably better than draft-00.) > > If I recall, the outcome from IETF 118 was two-fold: > > First, we'd clarify in rfc8446bis that the "key_share first" selection > algorithm is not quite what you want. This was done in > https://github.com/tlswg/tls13-spec/pull/1331 > > Second, there was some discussion over whether what's in the draft is the > best way to resolve a hypothetical future transition, or if there was another > formulation. I followed up with folks briefly offline afterwards, but an > alternative never came to fruition. > > Since we don't have another solution yet, I'd suggest we move forward with > what's in the draft as a starting point. (Or if this email inspires folks to > come up with a better solution, even better! :-D) In particular, whatever the > rfc8446bis guidance is, there are still TLS implementations out there with > the problematic selection algorithm. Concretely, OpenSSL's selection > algorithm is incompatible with this kind of transition. See > https://github.com/openssl/openssl/issues/22203 Is that asking whether or not we want adoption? I want adoption. I suppose that would be the next step. :-) I think, last meeting, we were a little unclear what we wanted the document to be, so I was trying to take stock first. Though MT prompted me to ponder this a bit more in https://github.com/davidben/tls-key-share-prediction/issues/5, and now I'm coming around to the idea that we don't need to do anything special to account for the "wrong" server behavior. Since RFC8446 already explicitly said that clients are allowed to not predict their most preferred groups, we can already reasonably infer that such servers actively believe that all their groups are comparable in security. OpenSSL, at least, seems to be taking that position. I... question whether taking that position is wise, given the ongoing postquantum transition, but so it goes. Hopefully your TLS server software, if it advertises pluggable cryptography with a PQ use case, and yet opted for a PQ-incompatible selection criteria, has clearly documented this so it isn't a surprise to you. ;-) Between all that, we probably can reasonably say that's the server operator's responsibility? I'm going to take some time to draft a hopefully simpler version of the draft that only defines the DNS hint, and just includes some rough text warning about the implications. Maybe also some SHOULD level text to call out that servers should be sure their policy is what they want. Hopefully, in drafting that, it'll be clearer what the options are. If nothing else, I'm sure writing it will help me crystalize my own preferences! > Given that, I don't see a clear way to avoid some way to separate the old > behavior (which impacts the existing groups) from the new behavior. The draft > proposes to do it by keying on the codepoint, and doing our future selves a > favor by ensuring that the current generation of PQ codepoints are ready for > this. That's still the best solution I see right now for this situation. > > Thoughts? I think letting the DNS signal also be an indicator the server implements the correct behavior would be a good idea. I'm afraid DNS is typically unauthenticated. In most TLS deployments, we have to assume that the attacker has influence over DNS, which makes it unsuitable for such a signal. Of course, if we end up settling on not needing a signal, this is moot. David
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