Clean up s2n_cipher struct

[lrstewart]

Problem:

is_available really shouldn't be returning a uint8_t:

s2n-tls/crypto/s2n_cipher.h

Line 85 in 405a888

uint8_t (*is_available)(void);

While we're fixing that, might as well make all the methods return S2N_RESULT ;)

Solution:

At least return a bool instead of a uint8_t. Maybe return S2N_RESULT and set a bool*-- I don't think is_available is called widely, so it shouldn't be too awkward to use that way.

All the ciphers implementing the structure will need to be updated.

• 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?