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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?

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