Apply Secret trait consistently across the library

[ounsworth]

Some of the earlier crates (SHA2 / SHA3) were done before I added the Secret trait, so circle back and add it, and impl all the traits that it requires.

At the same time, it would be good to study how other rust crypto libraries such as RustCrypto, Graviola, Kryoptic, handle this.

[ounsworth]

Tamish Dahiya has volunteered to work on this.

[tad-fr]

RustCrypto
Maintainer of the Zeroize crate and uses the API extensively across all repos.
As I understand, the idea is to use a "non-optimised" function to ensure proper zeroing behaviour. It's small enough for a quick read and is implemented here. This function is then wrapped in convenience methods that are exposed via the Zeroize trait.
It does provide some boilerplate for things like arrays and vectors of types that implement Zeroize but nothing too generic; I suppose it would be quite hard to generalize. There is also a derive macro which calls zeroize on all members.
Implementing drops via the Zeroize does provide access to the aforementioned "non-optimised" function which seems like a good approach. The bubbling of the Zeroize as a consequence also seems desirable.

The following also use zeroize copying the RustCrypto pattern:
Rustls : https://github.com/rustls/rustls
dalek-cryptography : https://github.com/dalek-cryptography
rust-hpke: https://github.com/rozbb/rust-hpke

libcrux
In the process of moving to zeroize: pull req

Graviola
Just implements Drop where appropriate: no marker traits or generic implementations.
Custom infrastructure to zeroize. Implementation.
Implements memset(ptr, 0, len) equivalent in assembly to avoid optimisation and calls it when required. x86 here.

ring
Does not zeroize at all yet.

[npajkovsky]

The Zeroize is a good choice to start with. We can harden BC-Rust with supply-chain security
tools like cargo vet or cargo crev in case something goes sideways.