[maaku]

TL;DR: AES_key = SHA-256(file)

This does introduce new avenues for attacks, however. You don't have to be able to decrypt to show that certain people have certain files.

Also, for files that contain just one piece of sensitive information and a the rest is predictable (i.e, the secret key file for a website back-end), you've effectively given up a hash of the secret which can then be brute-forced.

[SODaniel]

As long as you don't need to decrypt with user defined key to compare data, it is NOT zero-knowledge. Plain and simple.