ret), while passing
the same tls_session context (this is how our automaton.py mostly works);
_or, if Scapy did not build any TLS message, it has to create a TLS context
and feed it with secrets retrieved by whatever technique. Note that the
knowing the private key of the server certificate will not be sufficient
if a PFS ciphersuite was used. However, if you got a master_secret somehow,
use it with tls_session.(w|r)cs.derive_keys() and leave the rest to Scapy.

When building a TLS message with raw_stateful, we expect the tls_session to
have the right parameters for ciphering. Else, .post_build() might fail.
r@