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Fully Homomorphic Encryption lets us operate over encrypted data. Is there something analogous that let us operate over plaintexts directly, without knowing the circuit?

For example, let's say that I store AES of the user's password, and I want to be able to change the user's password to password +1 at my will without interacting with him. The user would be able to send me a circuit that decrypts the AES, adds +1 to the password, and re-encrypts it. The difference from FHE here is that I would be able to see the result AES(password+1) in plaintext, where in FHE I'd have the FHE(AES(password+1))

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  • $\begingroup$ This talks about circuits Representing a function as FHE circuit. No, you can design an FHE protocol that, you will have still the AES encrypted password+1. It is quite costly, though. With a true FHE that has semantical security, you can execute almost all circuits with some huge cotst. $\endgroup$
    – kelalaka
    Commented Jan 31, 2022 at 13:45
  • $\begingroup$ In your case, why does the user has the AES key of the password? $\endgroup$
    – kelalaka
    Commented Jan 31, 2022 at 13:47
  • $\begingroup$ And Can we proxy-re-encrypt using homomorphic encryption schemes? $\endgroup$
    – kelalaka
    Commented Jan 31, 2022 at 14:42
  • $\begingroup$ Not sure if I totally understand your answer but maybe you want to check out the malleability property, however malleable ciphers aren't semantically secure. $\endgroup$
    – tur11ng
    Commented Jan 31, 2022 at 18:27
  • $\begingroup$ Obfuscation allows someone to run an unkown circuit on a known input and get the answer in clear. You can search for indistinguishability obfuscation, for example. $\endgroup$ Commented Feb 1, 2022 at 12:19

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