On Wed, Jun 5, 2024 at 6:35 AM Eric Rescorla <e...@rtfm.com> wrote:
> > > On Wed, Jun 5, 2024 at 6:19 AM Peter Gutmann <pgut...@cs.auckland.ac.nz> > wrote: > >> Nick Harper <i...@nharper.org> writes: >> >> >I see no requirement in section 9 nor in section 4.2.8 requiring MTI >> curves >> >be present in the key_share extension if that extension is non-empty. >> >> Just because it's possible to rules-lawyer your way around something >> doesn't >> make it valid (I also see nothing in the spec saying a TLS 1.3 >> implementation >> can't reformat your hard drive, for example, so presumably that's OK too). >> The point is that P256 is a MTI algorithm and Chrome doesn't provide any >> MTI >> keyex in its client hello, making it a noncompliant TLS 1.3 >> implementation. >> > > I don't believe this analysis is correct. You state: > > As Nick notes, RFC 8446 explicitly permits the extension to be empty, so > it clearly cannot be the case that mere failure to provide an MTI key_share > in CH makes an implementation noncompliant, contra your statement above. > > The only question at hand is whether the specification permits you to send > a non-empty key_shares field that excludes the MTI. However, the > specification > *also* permits you to send a subset of supported groups: > > the same order. However, the values MAY be a non-contiguous subset > of the "supported_groups" extension and MAY omit the most preferred > groups. Such a situation could arise if the most preferred groups > > I think the best reading of this text is that you are free to send *any* > subset of the supported groups, whether it includes the MTI or not. > > The requirement in S 9.1 is merely that the application "support > key exchange with secp256r1", which Chrome does: it's in "supported_groups" > and (presumably) works if the server sends an HRR. Given the above > more explicit text about "key_shares", I don't think it's reasonable to > infer that MTI requires more than this. > > This isn't to say anything about whether this is the best implementation > choice, > which is a distinct question from what the specification requires. > One more thing: we are finalizing RFC 8446-bis right now, so if there is WG consensus to require that clients offer all MTI curves in the key_shares of their initial CH, then that would be a straightforward text change. That might be a more productive discussion than debating whether Chrome is compliant with the specification as it currently stands. -Ekr > -Ekr > > >
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