Well, B makes no sense. A makes sense, but might have leaks, that depending on use case (and in particular, whether the attacker can execute chosen plaintext attacks like in SSL) may or may not weaken the system, but it still makes sense.

For a confusing example of a fairly leaky cryptosystem involving compression that works well in practice and has a proof of security that is aware of the leak, see Douceur et al.'s "convergent encryption".

http://research.microsoft.com/apps/mobile/publication.aspx?i...