tzs's scheme proposes encrypting files with the hash of their contents (see http://news.ycombinator.com/item?id=2461713).
That's not semantically secure. Anyone can distinguish whether a particular plaintext produced a given ciphertext.
That's not semantically secure. Anyone can distinguish whether a particular plaintext produced a given ciphertext.
What are the practical implications? The only one I can think of is if someone were trying to prove in court that you had a particular file.
A related question I've been wondering about is whether a hash or key+ciphertext are considered proof of possession of the original file. In theory an infinite number of files have the same hash, and the ciphertext could have been produced using a different key (or it could just be random data). In practice it's extremely unlikely (for sufficiently secure hash and encryption functions). What are the laws'/courts' views on cryptography?
But that is the kind of thinking that lead to the whole issue with Dropbox in the first place. Its deceptively "secure" to any user who can't analyze the implementation implications themselves.