Is it possible to compare two ciphertexts (two different encryption algorithms are used to create ciphertexts) to identify redundancy?

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    $\begingroup$ Not if either is secure. If you're interested in deduplication under the same cipher, look into Deterministic Authenticated Encryption (DAE). $\endgroup$ – cypherfox Mar 10 '18 at 5:56
  • $\begingroup$ It depends if you asking about such identification in a proper encryption scheme (answered by Gilles) or if you wonder if a scheme can be broken in such way that allow this (look at ECB mode and famous penguin: blog.filippo.io/the-ecb-penguin ). $\endgroup$ – v6ak Mar 10 '18 at 12:45

Not if the encryption algorithms work. And not even if it's the same encryption algorithm. This is a consequence of one of the properties required for an encryption algorithm to be considered secure: ciphertext indistinguishability under chosen ciphertext attack. If an adversary can send ciphertext 1 for decryption, and from that learns something about ciphertext 2 (such as a similarity in their corresponding plaintext), then this property is violated.

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