aes-kw: add optional zeroize feature for wrappers

[Xynnn007]

Close #79

[baloo]

ZeroizeOnDrop is only a marker trait. Here you marked the objects as "pinky promise, this will be zerorized" but not actually zeroized.

I think you wanted to #[derive(Zeroize)] instead, or more precisely: #[cfg_attr(feature = "zeroize", derive(zeroize::ZeroizeOnDrop))] which would implement zeroize and mark the objects.

[newpavlov]

I believe that the implementation is correct. The types just wrap C and the impl says "pinky promise, this will be zeroized, assuming that the cipher's pinky's promise is true".

[baloo]

That's true, but I'd rather have the explicit impl Drop { self.field.zeroize() } to be refactor resistant though (in case there is an additional field in the downstream struct) as it should be essentially free.

[newpavlov]

It's not possible. The block cipher implementations intentionally do not implement Zeroize (but do ZeroizeOnDrop and Drop with zeroizing impl) since "zeroized" state may not be a valid block cipher state and explicitly zeroizing block cipher is likely to be a mistake either way.

Meanwhile, #[derive(ZeroizeOnDrop)] will result in the exactly same impl, but at the cost of pulling the proc macro dependency.