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I am a newbie to crypto and just one quick question on the s-box and p-box of DES. I understood that the s-box is where the security comes from and NSA has somehow decided how it should be filled in.

But my question is that, can we really change the content of the s-box or p-box? Would that break the security somehow? Why or why not?

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S & P boxes are not random permutations. Changing them is not easy. The position of each element of an such a box is chosen following principles such as Strict Avalanche criterion, Bit independence criterion, non linearity, xor table distribution and maximum expected linear probability.

Not following these makes it prone to differential cryptanalysis.

See Biham & Shamir's work on the cryptanalysis of DES S-Boxes.

Also check out "Comparison of random s-box generation methods" by Dragan Lambić for a different perspective.

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    $\begingroup$ S-boxes can be random and a few SPN ciphers use random or key-dependent S-boxes, but the security properties are not ideal when that's done which is one big reason why they usually are not random. $\endgroup$ – forest Sep 10 at 7:27

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