Two parties Alice and Bob wish to exchange a session key Kab with the help of a trusted party Cathy. Initially, Alice has a shared key Kac with Cathy. Likewise, Bob has a shared key Kbc with Cathy. Th

Two parties Alice and Bob wish to exchange a session key Kab with the help of a trusted party Cathy. Initially, Alice has a shared key Kac with Cathy. Likewise, Bob has a shared key Kbc with Cathy. The following protocol is used to exchange the session key Kab.

First, Alice generates a new session key Kab. She then encrypts this key and a timestamp Ta using Kac. She sends the encrypted message to Bob. Bob, in turn, encrypts the received message and a new timestamp Tb using Kbc. He then sends the encrypted message to Cathy. Cathy, in turn, double-decrypts the message (i.e. decrypts the message twice) and extracts the session key Kab. Then, she encrypts the session key Kab using Kbc and sends the message to Bob.

a.      Show graphically how the protocol works. Use arrows and numbers to complete the following diagram. All symbols used in your answer must be justified and explained clearly.

b.      Use the Alice-Bob notation to describe the protocol. Use A for Alice, B for Bob and C for Cathy.