(b) Consider the protocol shown below. It is used to sign messages without the use of Public/private keys.
Instead a trusted server S is used to sign messages. Where $K_{ss}$ is a secret key that is known to S only and is used
for signing and verifying signatures.
You are asked to answer the following:
(i) Does this protocol achieve its objectives? Explain or show attack example; and
(ii) Use BAN logic to prove your answer showing assumptions, proof obligations, and the proof.
(1) $A \to S: A, \{h(m)\}_{K_{as}}$
(2) $S \to A: \{A, h(m)\}_{K_{ss}}$
(3) $A \to B: \{m, \{A, h(m)\}_{K_{ss}}\}_{K_{ab}}$
(4) $B \to S: B, \{A, h(m)\}_{K_{ss}}$
(5) $S \to B: B, \{A, T_s, h(m)\}_{K_{bs}}$
