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Suppose Alice has encrypted some plaintext $P$ to Bob. Bob wishes to re-encrypt this information to Charlie via proxy, in such a manner that:

  1. The proxy cannot obtain $P$
  2. The proxy can prove that Bob has provided a valid re-encryption key, meaning that Charlie will be able to obtain $P$ by decrypting with his private key
  3. The proxy can prove that the ciphertext originated from Alice

Is such a scheme possible?

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  • $\begingroup$ Q3 implies that there is some kind of sender authentication, which is not typical in public-key encryption $\endgroup$ – cygnusv Mar 30 '18 at 4:33
  • $\begingroup$ For Q2: Proxy can forward the ciphertext directly to Charlie. Why is this not suitable? $\endgroup$ – zetaprime Nov 1 '18 at 6:20
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PRE schemes guarantee Q1, if they are even remotely worthwhile. Alice could add a signature over her public signing key and the ciphertext to accomplish Q3. I haven't seen any works that guarantee a re-encryption key from B to C is valid.

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