Modify EncryptedFile model based on previously incorrect assumption.

Previously, I assumed that re-encrypting the same string a second time returns the exact same ciphertext/IV pair. This is wrong. Instead, it gives some randomized set of ciphertext and IV that, together, correspond to some unique plaintext. That does not mean that you can always get to the same ciphertext given plaintext and a secret key. Otherwise, there would be no point in having initialization vectors.