Consider the following authentication protocol given in common syntax:
A, B, S : Principal
Kab : Key
Na : Nonce
k : (Principal, Principal) -> key
1. A->S: {A,B,Na}k(A,S)
2. S->A: {A,B,Kab,Na,Ns,{A,B,Kab,Na,Ns}k(B,S)}k(A,S)
3. A->B: {A,B,Ns}Kab,{A,B,Kab,Na,Ns}k(B,S)
4. B->A: {Na}Kab
We shall call this protocol PROTOCOL_THREE.

Describe the message exchange of PROTOCOL_THREE.