Develop trait for tweakable block ciphers

[newpavlov]

Recently added Threefish is a tweakable block cipher, thus we need an appropriate trait for it.

Currently I am thinking of using this:

pub trait TweakableBlockCipher {
    type BlockSize: ArrayLength<u8>;
    type TweakSize: ArrayLength<u8>;

    fn encrypt(&self, block: &mut Block<Self::BlockSize>,  tweak: &Block<Self::TweakSize>);

    fn decrypt(&self, block: &mut Block<Self::BlockSize>, tweak: &Block<Self::TweakSize>);
}

In addition to it I think tweakable ciphers should auto implement BlockCipher using zero tweak.

[eternaleye]

One thing worth noting is that tweak changes aren't necessarily free (some constructions even use tweak-dependent keys for the underlying block cipher), and an interface that can amortize this may be beneficial.

EDIT: Optimally Secure Tweakable Blockciphers may be of interest; it shows tweakable block cipher constructions that can break the birthday bound, though they make use of tweak-dependent keys.

EDIT 2: Even in LRW2, the construction works as Ẽ(k, t, m) = E(k, m ^ h(t)) ^ h(t) - precomputing h(t) may have nontrivial benefit.

[WildCryptoFox]

For HCTR the TBC trait would need to recognize Variable Input Lengths and variable tweaks (depending on the AXU it is instantiated with).

I don't think GenericArray can currently capture variable lengths.

[newpavlov]

I will close it in favor of RustCrypto/traits#177.