Question: (Meet in the middle attack DES) Suppose E: K {0,1}n {0,1}n is a DES block cipher, we define EX as EX((k1,k2,k3), m) = k1 E(k2,

(Meet in the middle attack DES) Suppose E: K {0,1}n {0,1}n is a DES block cipher, we define EX as EX((k1,k2,k3), m) = k1 E(k2, mk3 ).

a) For EX: key-len = 64+56+64 = 184 bits, show that EX can be easily attacked in time 264+56 = 2120.

b) If we only add one more key for the DES, we define EX1 and EX2 as EX1(k1,k2) = k1 E(k2, m) EX2(k1,k2) = E(k2, mk1) Show that EX1 and EX2 do not improve the security of DES. (show the attack time)

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