Can I use the cipher text ouput of only one DES rotation as plaintext to check validity? I have made a python code for one DES rotation. Can I use the resulting cipher text as a plain text for the same rotation with the same key? I think the answer is no, I can't do so. But I need to be sure, an explanation will be very helpful.
Yes, you can, but it's insecure. DES has a different key schedule for each round to avoid linearity and meet-in-the-middle attacks. Also you reduce the strenght of your cipher since not all encryption round embed the whole key. Be aware that you need to swap left part and right part of your data material after each round otherwise it's going to be terribly insecure.