4. (20 points) PRP-security of 2-round Feistel structure. In this exercise, we consider a two-round Feistel structure, like in Figure 2 again. Let the keys K1, K2 be 32 bits long, and let for both keys K1, K2, the map FK;: {0, 1}32 + {0,1}32 be indistinguishable from a random function. 10 pt (a) Based only on exhaustive key search, what is an upper bound on the security strength of this cipher? 3 pt (b) Show that the 2-round Feistel structure is not PRP-secure. (c) Give the probability that you guess that you are talking to a 2-round Feist el structure when you are 3 pt actually talking to a random permutation. (d) Given your attack to show the 2-round Feistel structure is not PRP-secure, what is the new upper 4 pt bound on the security strength? 4. (20 points) PRP-security of 2-round Feistel structure. In this exercise, we consider a two-round Feistel structure, like in Figure 2 again. Let the keys K1, K2 be 32 bits long, and let for both keys K1, K2, the map FK;: {0, 1}32 + {0,1}32 be indistinguishable from a random function. 10 pt (a) Based only on exhaustive key search, what is an upper bound on the security strength of this cipher? 3 pt (b) Show that the 2-round Feistel structure is not PRP-secure. (c) Give the probability that you guess that you are talking to a 2-round Feist el structure when you are 3 pt actually talking to a random permutation. (d) Given your attack to show the 2-round Feistel structure is not PRP-secure, what is the new upper 4 pt bound on the security strength