10. Consider the following security protocol that authenticates two parties A and B, using a trusted server S. KA is a secret key known by A and S, KB is the secret key known by B and S, N1 and N2 are nonces, generated by A and B, respectively. E[(M),K] denotes the encryption of message M with key K. || denotes concatenation of the message. Answer the following questions:

Message 1: A-> B: A || B || E[(A, B, N1), KA] Purpose of the message: What A knows and why: What B knows and why: What S knows and why:

Message 2: B -> S: A|| E[(A, B, N1), KA] || B || E[(A, B, N2), KB] Purpose of the message: What A knows and why: What B knows and why: What S knows and why:

Message 3: S -> B: E[(K-session, N2), KB] Purpose of the message: What A knows and why: What B knows and why: What S knows and why:

Message 4: S -> A: E[(K-session, N1), KA] Purpose of the message: What A knows and why: What B knows and why: What S knows and why:

Show how Mary may compromise the above protocol by blocking the first message from reaching Bob and replacing the 2nd message with the following new message 2. Complete the protocol. Message 1: A B: A || B || E[(A, B, N1), KA] ( Message 1 is Blocked by Mary, Bob never receives it.) New Message 2: M S: A|| E[(A, B, N1), KA] || M || E[(A, M, N2), KM] Message 3: Message 4: