[kwantam]

The example you give is similar to but not quite the same as "lzAES". The distinction is that in your example, the application is deciding whether to compress or not---the input/output behavior of the cipher doesn't include the compression step, so the cipher it self doesn't suffer from the problem I mentioned in my first note.

But it's still possible for an application to use a cipher incorrectly. In particular, an application-level decision about whether to compress some data before encrypting can have an effect on the application's security. In the case you mention it seems unlikely to be a problem (but that's an application-level question, so it could be).

As an example where it seems like the application-level decision to compress or not matters a lot, imagine an application that sends an encrypted password to a server. If the application compresses the password first, an attacker could learn which values are not my password via guess-and-check-length. (Of course, even without compression the attacker can learn something about the length of my password just by looking at the length of the ciphertext---so probably this is a case where the application should first pad the message to some fixed length before encrypting. But in any case it almost certainly shouldn't compress the password!)