AES LightEngine Hazmat Visibility Module

[officialfrancismendoza]

Continuing off aes-lowmemory derivative crate work in (#98, #99). Every method is public API: Rust has no private trait methods, and encrypt_block applied to user data is ECB mode. The _blocks2 pair does not widen that hazard (it is the same operation on two blocks) so hiding the pair alone would be misleading. The question is how to present the trait as a unit.

[ounsworth]

Correct that rust has no private trait methods (because by definition, a trait is the public API of your struct or enum).

If you want to play with visibility (fn's that are either private to the struct (the default), or private to the crate (pub(crate)), then you need to declare those as inherent functions directly on the struct rather than on a trait.

A good example is the SHAKEInternal struct:
https://github.com/bcgit/bc-rust/blob/main/crypto/sha3/src/shake.rs#L44
You'll see that the inherent impl<..> SHAKEInternal contains both pub and private functions. And it also impls some traits (Suspendable and KDF, which are its public APIs).

Depending on what you're trying to do, the other trick you can do is to declare a trait within a module so that the entire trait is private.
An example where we already do that is pub(crate) trait MLKEMPrivateKeyInternalTrait, which we use within the crate to give a common API to MLKEMPrivateKey and MLKEMPrivateKeyExpanded, but it's a private trait so that users outside the library cannot impl it to create their own custom MLKEM private key holder.
Note that this trick is called Private Bounds, which the rust compiler complains about by default (cause I think more often than not, a private trait is a mistake), so you need to tell it that you're doing this on-purpose by setting this in the lib.rs:

#![allow(private_bounds)]

[dghgit]

The problem here is that will only silence the compiler warning, the BlockPermutation (actually I really hate that name, it would have been fine before ASCON, but now it just feels confusing, maybe it should be renamed ElectronicCodeBook) is required for any block cipher so it needs to be under core, but implemented in each cipher. The "Rusty" way of dealing with this is to use "hazmat" as it forces a user to at least acknowledge they're doing some dangerous, although not necessarily unsafe.