Kerberos is a computer-network authentication protocol that works on the basis of tickets to allow nodes communicating over a non-secure network to prove their identity to one another in a secure manner. Its designers aimed it primarily at a client/server model and it provides mutual authentication – both the user and the server verify each other’s identity. Kerberos protocol messages are protected against eavesdropping and replay attacks. Suppo

Kerberos is a computer-network authentication protocol that works on the basis of tickets to allow nodes communicating over a non-secure network to prove their identity to one another in a secure manner. Its designers aimed it primarily at a client/server model and it provides mutual authentication – both the user and the server verify each other’s identity. Kerberos protocol messages are protected against eavesdropping and replay attacks. Suppose that in a network, a Simplified Kerberos, as shown in the following figure, is used for servers to authenticate clients.   

Assume that you are a client of the network and has shared a secret key K_C with the Authentication Server (AS). The secret key K_C is the last 56 bits of H(your student ID and your full name), where H is SHA-1 hash function. You would like to get access to a server S which has shared a secret key sk (randomly chosen) with the Authentication Server (AS).  Assume that DES CBC mode (http://des.online-domain-tools.com) with IV=(0000000000000000)₁₆ is used for encryption and K_C,S, K_S are the last 56 bits of H(C,S) and H(S), resp.

(1) How do you request a ticket from AS for using the server S? Show the detail message.

(2) How do you send the ticket to the server S along with an authenticator to establish a shared secret? Show the detail message. 

(3) How do you and the server authenticate each other?  

(4) Is the simplified Kerberos secure against the eavesdropping attack? If not, how do you improve the security of the simplified Kerberos against the eavesdropping attack?

(5) Is the simplified Kerberos secure against the replaying attack? If not, how do you improve the security of the simplified Kerberos against replaying attack?