Question 4:

a. Assume we perform a known-plaintext attack against DES with one pair of plaintext and ciphertext. How many keys do we have to test in a worst-case scenario if we apply an exhaustive key search in a straightforward way? How many on average?

b. One important property which makes DES secure is that the S-boxes are nonlinear. In this problem we verify this property by computing the output of S1 for the input pairs x1 = 111111 and x2 = 000001. Question 5:

In DES, let the key K64 = 000000000000000F16

a. Find the 56-bit key K56.

b. Find the subkey k1 for the first round

c. Find the subkey k16 for the last round. Question 6:

In DES, Let k1 = 00000000090416 and R0 = 0000808016 be the inputs of the f-Function, find the output as follows:

a. Find the expansion E(R0).

b. Find E(R0) k1.

c. Find the substitution S(E(R0) k1)

d. Apply the permutation P(S(E(R0) k1)).

Question 7:

in DES, let the Message M = 000000000000000F16

a. Apply the initial permutation to this message.

b. Find L0.

c. Find R0. '

d. Find L1.