Digest: Public API (draft)

[wysiwys]

This pull request begins implementing a public Digest API with multiplexing (as part of #1039).

• Oneshot API for all implementations (including Blake2s/Blake2b)
• Consistent Hasher struct initialization via DigestIncrementalBase::new()
• Reexport libcrux_traits::digest::Hasher structs in new libcrux-digest crate
  • Add crate documentation and README
• Multiplexed Hash in libcrux-digest

[franziskuskiefer]

I'm not sure what the usages are exactly because of the missing docs. We should have public APIs that can do something like

// oneshot, algorithm specific
let digest = Sha256::hash(b"input");

// oneshot, "multiplexed"
let digest = Digest::hash(Algorithm::Sha256, b"input");

// oneshot, "multiplexed" with caller provided memory
let mut digest = [0u8; 32];
Digest::hash(Algorithm::Blake2b, &mut digest, b"input");

// incremental
let mut hasher = Sha256::new();
hasher.update(b"input");
let digest = hasher.finalize();

// or with caller provided memory
hasher.finalize(&mut digest);

// Similar for the "multiplexed" version.

Without docs it's hard to tell if this is possible. But looking at the tests, this looks way more complex right now.

[franziskuskiefer]

Shall we add max input length here as well?

[franziskuskiefer]

Some doc comments would be nice 😊

[franziskuskiefer]

Maybe algorithm? algo sounds a little off.

[franziskuskiefer]

This seems like a lot of code for just calling a hash function.
I'd want to call something like Blake2sHash::hash(digest_mut, b"this is a test"). It's not clear why the extra two lines are necessary.