cipher: add encrypt/decrypt-only block cipher traits

[tarcieri]

Closes #349

Adds BlockEncrypt and BlockDecrypt traits which can be used in cases where e.g. only the encryption portion of a block cipher is used, as in CTR mode.

This PR does not otherwise attempt to add things like a blanket impl.

[tarcieri]

Note: I used the new suggested method names here: *_block, *_par_blocks, *_blocks.

Perhaps it would make more sense to leave them the same for consistency for now, then change them all in the next breaking release.

[tarcieri]

After a few attempts at eliminating the redundancy, I think it might make more sense to transform the BlockCipher trait (via a breaking change) into a marker trait for the case where a type impls both Encrypt and Decrypt with compatible BlockSize/ParBlocks.

[newpavlov]

Aren't Encrypt/Decrypt too generic? It could be worth to use something like BlockEncrypt instead of block::Encrypt. Such naming would also allow us to simplify the crate structure by removing stream and block modules.

[tarcieri]

It could be worth to use something like BlockEncrypt instead of block::Encrypt. Such naming would also allow us to simplify the crate structure by removing stream and block modules.

That's a potential alternative, yes. I think it makes sense for the code to still be factored into two modules, but we could re-export everything at the toplevel rather than using modules.

[tarcieri]

In 5096a41 I renamed the traits to BlockEncrypt and BlockDecrypt, and added a BlockSizeMarker trait for carrying the BlockSize associated type/constant.

[newpavlov]

I think it makes sense for the code to still be factored into two modules

I agree, I meant how API will look on outside.

[tarcieri]

@newpavlov does this look ok now, or would you rather hold off until cipher v0.3?

[newpavlov]

I think it will be better to release these changes in v0.3.

[tarcieri]

@newpavlov I've gone ahead and made the discussed breaking changes (targeting cipher v0.3) in 2c61ca2, including splitting BlockCipherMut into BlockEncryptMut and BlockDecryptMut which require the given type impls BlockCipher.

This had the added benefit of eliminating the need for a stream::FromBlockCipherMut trait.