On Wednesday, 5 June 2024 15:38:57 CEST, Eric Rescorla wrote:
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.
I definitely don't agree with such a requirement.
--
Regards,
Hubert Kario
Principal Quality Engineer, RHEL Crypto team
Web: www.cz.redhat.com
Red Hat Czech s.r.o., Purkyňova 115, 612 00, Brno, Czech Republic
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