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Background

Bob's goal: Receive data E = E(D) (encryption of D) from Alice that he knows for sure is encrypted and that he can't possibly decrypt (without brute force). This gives his data backup company peace of mind - no need to worry about what sorts of data he stores (copyright issues, etc.). When Bob sends E back to Alice, she can decrypt it with a key known only to her to recover D.

Solution/question

  • Alice has key A, known only to her.
  • Bob has nonce N that he sends to Alice, so Alice knows A and N.

Is it possible for Alice to encrypt her data D with both keys A and N and send E(D) to Bob, such that Bob can verify that N (plus some other data, in this case A, unknown to him) was used for the encryption?

In this case the problem is solved (as far as I can see), because now he can simply destroy the nonce N, and now he knows for sure he only has encrypted data in his possession that only Alice could possibly decrypt.

Other solutions?

Are there other ways? TLDR: The goal is zero-knowledge data storage with the added peace of mind of knowing you're only storing unrecoverable encrypted data. Is there a way around using a temporary nonce?

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    $\begingroup$ Someone could upload infringing materials to the storage service, and share the login/decryption keys in forums to allow other people to download and decrypt it. I don't think there is a way around this unless there is some kind of penalty if the login/decryption keys are leaked. $\endgroup$
    – knaccc
    Dec 12, 2022 at 12:01

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