Does sec_protocol_metadata_create_secret implement RFC 5705?

More information on how sec_protocol_metadata_create_secret_with_context and sec_protocol_metadata_create_secret work can be found in the TLS 1.3 specification, section 7.5, with references to RFC5705. I would recommending section 7.5 for more information on implementation.

I just did some experiments, with Network Framework on the client side, OpenSSL on the server side.

I compared the output from

Code Block 
sec_protocol_metadata_create_secret()

and

Code Block 
SSL_export_keying_material()

Outputs are the same in TLS 1.3 but different in TLS 1.2.

I guess sec_protocol_metadata_create_secret might uses the TLS 1.3 key deriving function even in TLS 1.2. But I can't prove it.
If that's true, the function is just useless unless your implementation is TLS 1.3 only.

I wish someone can tell me I am wrong...

If you find that you hit a wall here, I would recommend opening a TSI so I can look at your situation more in-depth. Make sure to reference this thread if you do end up opening a TSI.

Matt Eaton
DTS Engineering, CoreOS
meaton3@apple.com