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Computer Security and Cryptography by Alan G. Konheim

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9.10 DIFFERENTIAL CRYPTANALYSIS

Suppose two plaintexts are enciphered by S-box S[0] with the same key.

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We conclude that

image

and write this last relationship as

image

where

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How much of the 6-bit key is revealed by corresponding pairs of plain- and ciphertext (xi, yi) (i = 1, 2) enciphered by S-box S[0] with the same unknown key? That is, how many solutions are there to

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given

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Define

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A pair (z1, z2) in imagex, Δy) determines a possible unknown key by setting

image

If the size of imagex, Δy) is much smaller ...

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