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. -Ekr
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