Assume that a and b share a long-term 256-bit aes key k. our key exchange protocol

Assume that a and b share a long-term 256-bit aes key k. our key exchange protocol has two goals: (1) let a and b agree on a new fresh session key k, to be used for a current session, and (2) perform mutual authentication. n, is a nonce and k, is a new session key generated by b. assume that k and nb are of the same bit-length and both are always chosen at random. ex( ) denotes aes encryption of everything inside ) with key k. hmack denotes sha2-based hmac of everything inside ) using key h protocol: 1. a -> b: ek(a, b, "hello") 2. b -> a: ek(ks, nb, hmack (a,b, "hello"))3. a -> b: hmack(ks,nb,1)what (if any) are the vulnerabilities of this protocol? and, how can you fix them? again, keep your answers limited to 8-10 lines.