>to determine whether a specific known object (e.g. a known photo or video) was sent / stored by a user rather than decrypting all sent or stored objects?
It is standard practice to make it so that it is impossible to detect identical plaintext. What you are describing would count as a backdoor. So you might as well instead make it explicit and save any sort of brute forcing (rainbow tables?).
It is standard practice to make it so that it is impossible to detect identical plaintext. What you are describing would count as a backdoor. So you might as well instead make it explicit and save any sort of brute forcing (rainbow tables?).