You could for example use ECB, OFB or CTR mode and actually update the file (some options are obviously a bad choice) but I thought of storing the file and the

You could for example use ECB, OFB or CTR mode and actually update the file (some options are obviously a bad choice) but I thought of storing the file and the diff together and only apply the diff after decryption so there is no need to mess with the encrypted data in the first place.