5. Consider the following key establishment protocol. Alice selects a random number x and computes x9, where g is a number that is known to the public. Bob selects a random number y and computes y9. Alice and Bob send x9 and y9 to each other. The shared key is computed as (xy)g. Describe an attack against such key establishment protocol. 2. A server uses a Challenge-Response protocol to authenticate users. The server stores the legitimate users' public key. Each time when a user wants to login, the server generates a random challenge and sends it to the user. The user then generates a digital signature on his/her workstation and sends the digital signature to the server. The server authenticates the user by verifying his/her signature on the random message. Describe an attack against the server