Add support for retrieving negotiated SRTP profile

[jlaine]

The aiortc project uses pyOpenSSL to perform a DTLS handshake and export SRTP keying material. Currently, the only enable a single SRTP profile using ctx.set_tlsext_use_srtp(b"SRTP_AES128_CM_SHA1_80") but we would like to start enabling GCM profiles (aiortc/aiortc#1012).

Enabling multiple SRTP profiles means that when the handshake completes, we need to know which SRTP profile was selected, for which OpenSSL provides SSL_get_selected_srtp_profile:

https://www.openssl.org/docs/man3.0/man3/SSL_get_selected_srtp_profile.html

This function is present in cryptography's bindings, so I would like to add a Connection.get_selected_srtp_profile method to wrap it, and return bytes:

• empty if no SRTP profile was negotiated - or should this be None?
• the name of the SRTP profile if one was negotiated

Using bytes as the return value makes the API consistent with Context.set_tlsext_use_srtp.

I'm happy to submit a PR if this sounds acceptable.

[jlaine]

I think I borked the change log, I added my change at the bottom of what I assumed was the "latest" code but it looks as though I added it to an existing release!!

[alex]

Ooops, if you can send a cleanup PR moving it ot the right place, that'd be great.

[jlaine]

Ooops, if you can send a cleanup PR moving it ot the right place, that'd be great.

Sure, what's the usual phrasing for the "next release" section?

[alex]

It'll be 24.0.0 (UNRELEASED) because calver. See 5ba8ce1 for what the last bump looked like.