a) What is the effect of a dropped ciphertext block ( if the transmitted cipher text c1, c2, c3...... is received as c1, c3....) when using the CBC, OFB and CTR MODES OF OPERATION
b) let F be a pseudorandom function.
Show that the following MAC for messages of length 2n is insecure: The shared key is arandom k ∈ {0, 1}n. To authenticate a message m1||m2 where |m1| = |m2| = n, compute thetag fk(m1)||fk(fk(m2)).