I'm currently carrying out a differential attack on the FEAL-4 algorithm, however I have hit a bit of a wall.
Basically I attacked the final round first (K3) using the differential characteristic of 0x80800000 which yields a differential of 0x02000000 with a probability of 1. I have found four possibilities for the round 4 subkey. To advance the attack to the next round (round 3) and verify which key is the correct one, I need to choose a new differential characteristic (other than 0x80800000) to attack round 3.
I know the difference in the input to round 2 should be 0x02000000 so I can begin my attack on round 3, but how do I go about finding my differential characteristic so I can generate my plaintext ciphertext pairs?