Validate early TLS1.3 handshake types in s2n_conn_set_tls13_handshake_type

[lrstewart]

Problem:

In TLS1.3, some handshake flags are set before SERVER_HELLO, so are set before s2n_conn_set_tls13_handshake_type. This is potentially a concern for the proofs, which now have to assume those flags are set in a reasonable way because they are set outside of the logic we prove.

See #3031 (comment)

Solution:

We could check in s2n_conn_set_tls13_handshake_type that the early handshake flags are set properly. For example, fail if EARLY_CLIENT_CCS is set but MIDDLEBOX_COMPAT isn't. That would bring the missing handshake type logic back into the scope of the proofs.

• Does this change what S2N sends over the wire? If yes, explain.
• Does this change any public APIs? If yes, explain.
• Which versions of TLS will this impact?

Requirements / Acceptance Criteria:

What must a solution address in order to solve the problem? How do we know the solution is complete?

• RFC links: Links to relevant RFC(s)
• Related Issues: Link any relevant issues
• Will the Usage Guide or other documentation need to be updated?
• Testing: How will this change be tested? Call out new integration tests, functional tests, or particularly interesting/important unit tests.
  • Will this change trigger SAW changes? Changes to the state machine, the s2n_handshake_io code that controls state transitions, the DRBG, or the corking/uncorking logic could trigger SAW failures.
  • Should this change be fuzz tested? Will it handle untrusted input? Create a separate issue to track the fuzzing work.

Out of scope:

Is there anything the solution will intentionally NOT address?