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?

[Neustradamus]

In more @wingel, we can add other Python contributors who can help to have a solution about the missing tls-exporter support in Python: @ronf, @chrisburr, @planetf1, @matiuszka, @paulmon, @remilapeyre, @Fidget-Spinner, @Dobatymo, @RanKKI, @1st1.

The same problem exists for xmpppy (@amotl, @normanr), Slixmpp (formerly named SleekXMPP) and other Python libs and other Python based projects:

• SCRAM-SHA-1-(PLUS) + SCRAM-SHA-256(-PLUS) + SCRAM-SHA-512(-PLUS) supports xmpppy/xmpppy#32
• https://codeberg.org/poezio/slixmpp/issues/3498

@jchampio has done a comment on the initial @wingel PR:

• Add support for export_keying_material to SSL library python/cpython#82133

Little details, to know easily:

• tls-unique for TLS =< 1.2 (RFC5929)
• tls-server-end-point =< 1.2 + 1.3 (RFC5929)
• tls-exporter for TLS = 1.3 (RFC9266)

Note that @tlocke has added tls-unique and tls-server-end-point support in scramp.

Comment are very welcome here.

[tlocke]

Hello all, Scramp has moved over to Codeberg, so this issue is now at https://codeberg.org/tlocke/scramp/issues/9