Audit and replace memcmp usage with s2n_constant_time_equals

[toidiu]

Security issue notifications

If you discover a potential security issue in s2n we ask that you notify
AWS Security via our vulnerability reporting page. Please do not create a public github issue.

Problem:

s2n codes should use s2n_constant_time_equals instead of memcmp.

Solution:

A description of the possible solution in terms of S2N architecture. Highlight and explain any potentially controversial design decisions taken.

• 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: Update PQ KEM branches to use constant time functions. #2590
• 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?

[boquan-fang]

#4709 has addressed this issue. There are three remaining memcmp s in the codebase. These were not removed because

• They were used for comparison (not just equal or not equal).
• Performance concerns about using s2n_constant_time_equals for large sizes of data.

For further discussion about these tradeoffs, see the linked PR.