Explain why the input differences to the first round of DES are chosen in specific form so that (L, R) and (L * , R* ) differ only in few positions.

Differential cryptanalysis is based on the so-called characteristics, that are essentially differences in plaintext pairs that have a high probability of causing certain differences in ciphertext pairs. a) Explain why the input differences to the first round of DES are chosen in specific form so that (L, R) and (L * , R* ) differ only in few positions. In the second round characteristic R0 1 is always chosen to be 0000000016. Why ? Careful motivation is needed. b) DESX was proposed by R. Rivest to protect DES against exhaustive key search. DESX uses one 64-bit secret key W to perform pre- and postwhitening of data and a 56-bit DES key K, and operates as follows, C = W ? EK(P ? W). Show that a similar construction, C = W ? EK(P) 10 without prewhitening is insecure and can be broken using an attack of complexity 256 .


 

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