Improve MAC validation

[dani-garcia]

Type of change

- [ ] Bug fix
- [ ] New feature development
- [x] Tech debt (refactoring, code cleanup, dependency upgrades, etc)
- [ ] Build/deploy pipeline (DevOps)
- [ ] Other

Objective

Change the name of the function validate_mac to the more appropiate generate_mac, and change the mac comparison to be constant time with the use of the subtle crate.

Note that hmac.finalize() returns a type CtOutput that supports constant time comparison by using the subtle crate internally, but CtOutput doesn't allow getting a reference to the internal array which would complicate the implementation of EncString, so I chose to use the subtle constant time comparison directly.

[bitwarden-bot]

Checkmarx One – Scan Summary & Details – 37febaa9-976f-4c4a-a74b-3d0eae4ddbdf

No New Or Fixed Issues Found

[Hinton]

Do you know how much of a performance improvement this actually is?

Could you also add some unit tests to generate_mac while at it?

[dani-garcia]

I don't think this will change much either way performance wise, if anything it's more of a tiny security improvement.

Ultimately the goal is more to keep in line with how the hmac crate handles the MAC comparisons, where they use a constant time equality check by default on their output types returned from calling hmac.finalize(). Sadly we can't just use those directly because we require access to the raw byte values for serialization of the EncStrings.