proposal: crypto/tls: Export list of Supported TLS Client Extensions

[fishboy25uk]

The clientHelloMsg unmarshal function (in handshake_messages.go) currently exports client information such as supported cipher suites and curves but does not export a list of supported extensions in the form of a uint16 slice. Client extension information is already parsed so this would be a simple mod. Modification of the ClientHelloInfo struct (common.go) and the clientHelloInfo functions (handshake_server.go) would also be required to export this info in the GetConfigForClient callback.

A use case for this would be to allow for TLS server client fingerprinting on the server side using JA3 (https://engineering.salesforce.com/tls-fingerprinting-with-ja3-and-ja3s-247362855967).

[FiloSottile]

We don't surface values to the application when there is no other use case than fingerprinting. Not because fingerprinting is not a valid use case, but because it asymptotically tends to require access to everything, polluting the API.

Instead, I usually recommend making a net.Conn wrapper that reads the ClientHello off the wire and makes all of the details available as needed.

[rsc]

Adding to proposal minutes, seems headed for likely decline.

[rsc]

Based on the discussion above, this seems like a likely decline.

[rsc]

No change in consensus, so declined.