EVP hashing broken the HMAC SAW proof

[lrstewart]

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Problem:

#5121 broke the HMAC SAW proof by removing legacy hashing and replacing it with EVP hashing. The HMAC SAW proof is very dependent on the exact representation of Openssl legacy hash structs like SHA256_CTX: https://github.com/aws/s2n-tls/blob/main/tests/saw/HMAC/spec/HMAC.saw#L64-L104

Solution:

Either update the proof or also move away from custom HMACs

• 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?