1. Fix the first plain text X and the corresponding cipher text Y .
2. Assuming the second plain text X1 obtain the corresponding cipher text Y 1.
3. Compute A and B values for all possible K1 values using (12) and (13).
4. Compute C and D values for all possible K4 values using (14) and (15).
5. Assume a K3 value for all possible (A,B) values according to (16).
6. Assume a K3 value for all possible (C,D) values according to (17).
7. If K3 on step 5 does not match K3 on step 6, return to step 2.
8. Obtain the round subkey K4 from the values of K1, K2 and K3.