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