I am using differential cryptanalysis to attack the second key of a 4-variant DES and I am using Shamir and biham book as a reference . they say that we should choose two input Xor P3 and P4 that satisfy the following conditionconditions:
- the expansion of the right half of the input Xor does not equal zero for all the S-boxes using either P3 or P4 .
- the expansion of the right half of the input Xor P3 does not equal its corresponding of P4 , for every S-box.
what is the best way of choosing P3 and P4 so that I can get K2 correctly for every bit?