Using the format of the "game" to establish if a cipher is IND-CPA secure:
- The adversary submits two distinct $M_0$, $M_1$ plain-texts to the challenger.
- The challenger selects one of them at random and encrypts it with the symmetric key and gives cipher-text C.
- The adversary has to guess which message was encrypted based on C with a probability greater than 1/2 for the cipher-text to be distinguishable.
If the challenger is using an encryption algorithm only based on Feistel structures, would it be possible to always correctly identify, which of the messages the challenger encrypted?