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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?

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  • $\begingroup$ Since you are reading it, do they state/hint that every key bit can be obtained? This seems optimistic. $\endgroup$ – kodlu Mar 26 '19 at 20:29
  • $\begingroup$ I am implementing all the attack on the 4-variant of DES , it worked well for K4 and K3 and I did get every bit of them using different one-round characteristics with probability=1 but for K2 and K1 we can't use any of these characteristic since the right half of their input Xor is zero ,that's why we only using the plaintext xors with no characteristic $\endgroup$ – siba36 Mar 26 '19 at 22:19
  • $\begingroup$ I've tried multiple xor inputs each one of them return different right bits , but none of them get the full K2 correctly $\endgroup$ – siba36 Mar 26 '19 at 22:23
  • $\begingroup$ to answer your question they did mention that they found K2 and K1 using those input xors but they didn't say which values they used and they didn't mention if they get the full K2 and K1 or some bits of them , but since in the previous round they do get all bits correctly , I've concluded that they did get all bits of K2 and K1 $\endgroup$ – siba36 Mar 26 '19 at 22:30

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