In a three-message authentication protocol, Alice initiates contact with Bob. Bob is a stateless server, and thus it is inconvenient for him to remember the challenges sent to Alice. Assume that Alice and Bob share a key K_AliceBob and Bob also has a key K_Bob only known to himself. The protocol works as follows, where R1 and R2 are random numbers generated by Alice and Bob, respectively.

ALICEI'm Alice,R1Bob (cannot remember anything from a previous information)

AliceI'm Bob,K _Bob{R2},K_alice-Bob{R1+R2} Bob(Cannot Remember anything from previous information)

ALICEK_Bob{R2},K_Alice-Bob{R2}Bob (cannot remember anything from a previous information)

a.Does this protocol provide mutual authentication? If yes, why? If no, who cannot authenticate whom and why?

b.Could a third person, Trudy, impersonate Alice? Describe a possible attack scenario. If it is not possible, clearly state so and give reasons.

c.Could Trudy impersonate Bob? Describe a possible attack scenario. If it is not possible, clearly state so and give reasons.