If hashes weren't illegal, cloud storage providers could probably save millions by deduplicating pirated content.
This way you can have deduplication of data, and only an attacker that knows the content of the file can decrypt the file, but since they already have the file, it does not matter.
1. You can tell that the user posses a known file. Such as a pirated movie. This is what this thread is about.
2. If the attacker knows the file except a sufficiently small secret part, they can learn that part.
https://tahoe-lafs.readthedocs.io/en/latest/convergence-secr...