3. Alice and Bob want to communicate over an insecure medium. Design, on the basis of a shared secret key K_AB, a protocol, which enables Alice and Bob to authenticate each other. Assume that the shared secret key K_AB is known in advance to both partners. b.) What are the disadvantages related to the use of a shared secret key?

4. Alice and Bob want to communicate over an insecure medium. Design, on the basis of a shared secret key KAB, a protocol, which enables Alice and Bob to authenticate each other. Assume that the shared secret key KAB is not known in advance to both partners