Correct kx group selection #1784
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djc
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Feb 12, 2024
Codecov ReportAttention:
Additional details and impacted files@@ Coverage Diff @@
## main #1784 +/- ##
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+ Coverage 95.92% 96.02% +0.09%
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Files 81 81
Lines 18803 18678 -125
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- Hits 18037 17935 -102
+ Misses 766 743 -23 ☔ View full report in Codecov by Sentry. |
cpu
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This looks good to me.
After skimming draft-davidben-tls-key-share-prediction-00 I expected there might be client changes needed here or in #1785 to track the notion of a prediction-safe supported group, but it looks like neither branch considers that. Am I overlooking something?
Sorry, yes, that isn't very clear. The only part I am fixing at this point is this paragraph:
I'm cherry-picking just that part, because our old way of selecting a group wasn't supported by RFC8446 (an RFC we definitely want to support as correctly as possible!), specifically this text:
(emphasis mine) I think if draft-davidben-tls-key-share-prediction-00 is adopted by the WG and becomes a real thing, then we should reflect the "prediction safe" concept (eg, |
Makes sense to me 👍 |
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Feb 13, 2024
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Feb 14, 2024
In 3355e06 we generalised the error type here, but we didn't get rid of code that discarded the information-less error.
By ignoring everything not precisely expected, these ran the risk of incorrectly passing. eg, `assert_server_requests_retry_and_echoes_session_id` would pass if the server sent a `ServerHello`.
Prior to this, we preferred to avoid a `HelloRetryRequest` when any supported `KeyShare` was supplied. But as [1] describes, this means a client which sends a `KeyShare` for a less-preferred group would end up using that, rather than a more-preferred group supported by both peers. [1]: https://www.ietf.org/archive/id/draft-davidben-tls-key-share-prediction-00.html#name-downgrades
This is complex because the choice of usable cipher suites depends on selected protocol version, and the set of mutually supported key exchange groups. Then, the usable set of key exchange groups depends on the actually-selected cipher suite.
Test the behaviour of `ServerConfig::ignore_client_order` at the public API level.
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See https://datatracker.ietf.org/doc/draft-davidben-tls-key-share-prediction/ for background.
The first three commits are "the friends we made along the way".
fixes #1512