`BlockSizes` up to only 256 bytes makes it impossible to implement `BlockSizeUser` for wide-block ciphers

[NumberFour8]

The BlockSizes trait is implemented only up to 256 bytes.

traits/crypto-common/src/lib.rs

Line 89 in 593a0ea

impl<T: Unsigned> BlockSizes for T where Self: IsLess<U256, Output = True> + NonZero {}

It therefore makes it impossible for a wide-block cipher (with a fixed block length) to implement BlockSizeUser.

Would it perhaps make sense to lift this up (at least 2k ?), or even make it unlimited.

[newpavlov]

We were previously relying on this constraint in the block-buffer crate to store cursor position as u8. It looks like we removed it in favor of runtime checks during buffer construction (the main motivation was to remove crypto-common dependency from block-buffer), but forgot to lift the restriction in crypto-common.

I guess we could remove it in a future breaking release, but I am not sure if we want to do it in the near future.

Do you plan to implement an algorithm which requires such block sizes?

[NumberFour8]

We were previously relying on this constraint in the block-buffer crate to store cursor position as u8. It looks like we removed it in favor of runtime checks during buffer construction (the main motivation was to remove crypto-common dependency from block-buffer), but forgot to lift the restriction in crypto-common.

I guess we could remove it in a future breaking release, but I am not sure if we want to do it in the near future.

Do you plan to implement an algorithm which requires such block sizes?

I'm currently implementing Lioness wide-block cipher, with block sizes up to 1536 bytes. I guess until it is removed, I would need to stay with the older version of traits, or remove the BlockSizeUser there.

[NumberFour8]

Would the limitation removal be actually breaking?

[newpavlov]

I think yes. Downstream code may rely on the promise in the docs:

Trait implemented for supported block sizes, i.e. for types from U1 to U255.

We would need to review all our uses of BlockSize and probably remove the trait completely in favor of ArraySize.