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I want to make a double cipher consisting of two encryption method Enc1 and Enc2, and two corresponding decryption mechanisms Dec1 and Dec2. I want it so that:

(1) we can encrypt any message M1 to a ciphertext C1 using Enc1 and a key K1, and it can decrypt the ciphertext C1 with Dec1, using the same key to recover M1.

(2) There exist key K2, where Dec2(C1,K2) = M2 and Enc2(M2,K2) = C1, and M2 makes sense(intelligible).

In conclusion, I want to make a cipher designed for a scenario in which if someone were forced to decrypt the ciphertext, they could do so with a different key K2 to produce a message that makes sense but is not the original message.

What is the best cipher to use to accomplish this?

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    $\begingroup$ @kelalaka I think this is less about double encryption and related security gains, but rather about wanting a different security / correctness property than regular encryption. An example implementation of this would be TrueCrypt / VeraCrypt's hidden disk feature. $\endgroup$
    – SEJPM
    Mar 4, 2021 at 15:01
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    $\begingroup$ Also note that if an adversary knows about the ability to embed a second message, and they forced the fake password out of you, they might just continue and try to force a second key out of you, even if there's none (as they never "loose" anything when trying to force a second key out of you). $\endgroup$
    – SEJPM
    Mar 4, 2021 at 15:02
  • $\begingroup$ @SEJPM you are right, however Not easy to achieve the [Plausible deniability $\endgroup$
    – kelalaka
    Mar 4, 2021 at 15:07
  • $\begingroup$ how about a simple one-time pad? $\endgroup$
    – user253751
    Mar 4, 2021 at 15:20
  • $\begingroup$ @user253751 how do you keep the key? $\endgroup$
    – kelalaka
    Mar 4, 2021 at 15:38

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