Q/ In network security answer this ?

Q/4 a) Alice (A) and Bob (B) want to secure their communication by using asymmetric encryption and nonce (n.). A nonce is an arbitrary number used only once in a cryptographic communication. It is often a pseudo-random number issued in an authentication protocol to ensure that old communications cannot be reused in replay attacks. Suppose a trusted server S that distributes public keys on behalf of A and B. Thus S holds Alice's public key K and Bob's public key KB. Note that S's public key, Ks, is well known. A and B initiate the secure communication by using the following protocol. Sender Receiver: Message A =S: A, B S-A(Ke, B}Ks AB:{na, A}Ke BSB.A S B: {KA, A}Ks BA:{na, ne}KA AB: (ne)ks [Description] [I'm A, and I'd like to get B's public keyl [Here is B's public key signed by me) [I'm A, and I've sent you a nonce only you can read] [I'm B, and I'd like to get A's public key] [Here is A's public key signed by me) [Here is my nonce and yours, proving I decrypted it] [Here is your nonce proving I decrypted it) However, this protocol has subtle vulnerabilities. Discuss one of the vulnerabilities, and how to fix the problem by changing the lines in the protocol. Q/4 a) Alice (A) and Bob (B) want to secure their communication by using asymmetric encryption and nonce (n.). A nonce is an arbitrary number used only once in a cryptographic communication. It is often a pseudo-random number issued in an authentication protocol to ensure that old communications cannot be reused in replay attacks. Suppose a trusted server S that distributes public keys on behalf of A and B. Thus S holds Alice's public key K and Bob's public key KB. Note that S's public key, Ks, is well known. A and B initiate the secure communication by using the following protocol. Sender Receiver: Message A =S: A, B S-A(Ke, B}Ks AB:{na, A}Ke BSB.A S B: {KA, A}Ks BA:{na, ne}KA AB: (ne)ks [Description] [I'm A, and I'd like to get B's public keyl [Here is B's public key signed by me) [I'm A, and I've sent you a nonce only you can read] [I'm B, and I'd like to get A's public key] [Here is A's public key signed by me) [Here is my nonce and yours, proving I decrypted it] [Here is your nonce proving I decrypted it) However, this protocol has subtle vulnerabilities. Discuss one of the vulnerabilities, and how to fix the problem by changing the lines in the protocol