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?