Re:Regression testing (Score 1)

Actually this is no proof. Each file could be hashed (to check for dedup before upload), encrypted with a file key (master key), and that file key wrapped with the private keys of users holding that file. So you only need to store the file encrypted by the file key, the hash, and the encrypted file key for each user with access to the file. Well this is how good file encryption works (like in EFS). So multiple users can each have access to a single file, with no key sharing, and no access for other users.