B. The following is a key exchange protocol used by two clients,A and B, to obtain a symmetric key???, using a trusted server, S.Assume that A and B had previously obtained the symmetric keys ???and ??? securely with the server. Also assume that anyone cansecurely obtain a secret symmetric key with the server. I. A → S :A, B II. S → A : (???) ??? ,( ???) ??? III. A → B : (???) ??? , A Identify the attack that can be executed against this protocol,assuming that an attacker C can eavesdrop, block, or modify allmessages. The attacker also has access to old (expired) keys. C.The above protocol is now modified to become I. A → S : A, B II. S→ A : (???, B) ??? ,( ???, A) ??? III. A → B : (???, A) ???  Whydoes the new modification improve the security over the oldversion? Identify the new vulnerability that exists in the newversion D. The above protocol is now modified to become I. A → S :A, B, NA II. S → A : (???, B, NA,( ???, A) ??? ) ??? III. A → B :(???, A) ??? IV. B → A : (NB) ??? V. A → B : (NB − 1) ??? Identify the vulnerability in this new version. E. Modify theprotocol in part (D) to have a secure protocol.
