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.

[Xynnn007]

A common case, we use KwAes256 which is AesKw<aes::Aes256> where C is Aes256, defined almost like

        #[derive(Clone)]
        pub struct Aes256 {
            encrypt: Aes256Enc,
            decrypt: Aes256Dec,
        }
...
#[cfg(feature = "zeroize")]
impl zeroize::ZeroizeOnDrop for Aes256 {}
...

#[derive(Clone)]
        pub struct Aes256Enc {
            backend: Aes256BackEnc,
        }

impl Drop for Aes256Enc {
            #[inline]
            fn drop(&mut self) {
                #[cfg(feature = "zeroize")]
                unsafe {
                    zeroize::zeroize_flat_type(&mut self.backend)
                }
            }
        }

That means Aes256's Drop will call each components' drop - finally zeroize underlying data.

Thus we do not need to impl explicitly the Drop on top level. But need to impl the ZeroizeOnDrop trait when zeroize feature is on, or we cannot use Zeroizing::new(KwAes256) as the compiler will cry.

Not sure if I made anything wrong