QUESTION 2 a) What are timeliness flaws in cryptographic protocols? How can timestamps be used in cryptographic protocols to address timeliness flaws? [3 marks] b) Give a critical analysis of the Needham Schroeder symmetric key protocol reproduced below. Your analysis should include a disjoint sessions attack, based on the intruder compromising a session key Kab, that allows the intruder to impersonate A and replay an old session key. [12 marks] (1) A S : A, B, Na (2) S A : {Na, B,Kab, {Kab, A}Kbs}Kas (3) A B : {Kab, A}Kbs (4) B A : {Nb}Kab (5) A B : {dec(Nb)}Kab Note that the dec operator in step (5) decrements its operand, thus x = dec(x). c) Show that including timestamps in the messages generated by S in the Needham Schroeder symmetric key protocol prevents the attack that you constructed in part b). [5 marks]