Version 1 A,n,{npRA Alice Bob Question 1:Can we replace (n)by nUA? -Answer:No.Everyone knows PUa and can compute {n}PUA. Question 2:What is wrong with this authentication protocol? -Answer:No.An attacker can replay this message later to authenticate himself to Bob. -How to fix this problem? 55 Version 1  Question 1: Can we replace {n}PRA by {n}PUA ? ─ Answer: No. Everyone knows PUA and can compute {n}PUA .  Question 2: What is wrong with this authentication protocol? ─ Answer: No. An attacker can replay this message later to authenticate himself to Bob. ─ How to fix this problem? Alice Bob A, n, {n}PRA