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?