Support the tls-exporter channel binding

[tlocke]

The tls-exporter channel binding is described in https://datatracker.ietf.org/doc/html/draft-ietf-kitten-tls-channel-bindings-for-tls13. This probably can't be implemented until Python allows access to Exported Keying Material, see https://bugs.python.org/issue37952

[Neustradamus]

It is official, it is here: RFC 9266: Channel Bindings for TLS 1.3:

• https://tools.ietf.org/html/rfc9266

[tlocke]

As far as I can see, in order to implement the tls-exporter channel binding we'd need to be able to get the EKM, which we can't do at the moment:

python/cpython#82133

[Neustradamus]

@tlocke: Linked to:

• gh-95341: Implement tls-exporter channel bindings and export key materials python/cpython#95366
• Add support for export_keying_material to SSL library python/cpython#82133
• ssl module incorrectly supports tls-unique channel binding for TLS 1.3 python/cpython#95341
• RFC 9266: Channel Bindings for TLS 1.3 support python/cpython#95350

[tlocke]

Thanks @Neustradamus, I've had a look at the links and I assume these are just updates on the situation, rather than anything we need to do with Scramp?