Solutions_HW4_547 (1)

Then the variant of des des described above can be

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Unformatted text preview: . Exercise 5.14 Consider a variant of DES where the key-schedule is as follows: the left half of the master key is used to derive all the sub-keys in rounds 1-8, while the right half of the master key is used to derive all the sub-keys in rounds 9-16. Let k be the master key and k1 , k2 respectively the left half and the right half of k . Then the variant of DES, DES￿ , described above can be formalized as follows: k DES￿ (x) = DESk2 (DESk1 (x)), k where DESk1 and DESk2 are 8-rounds DES with a block length of 64 bits and k1 and k2 being their 28-bit master keys. On this modified scheme, we can find an attack that runs in time roughly 228 . This attack is called “meet-in-the-middle attack”. Suppose that the adversary knows a single plaintext/ciphertext pair (x, y ), where y = DESk2 (DESk1 (x)). Then the adversary can reason as follows: 1. First, he will set S to be equal to the ∅; 2. For each k1 ∈ {0, 1}28 , compute z := DESk1 (x) and store the pair (z, k1 ) in a list L 1 ; − 3. For each k2 ∈ {0, 1}28 , compute z := DESk21 (y ) and store the pair (z, k2 ) ￿ in a list L ; 4. Then sort L and L￿ by their first components;...
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This document was uploaded on 02/24/2014.

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