------------------------------------------------------------------------------ Mohamed G. Gouda Network Protocol Security CS 395T Midterm 2 (1 hour and 45 minutes) ------------------------------------------------------------------------------ 1. [8 points] Modify the explicit sequencing antireplay protocol such that it uses bounded sequence numbers. In the modified protocol, each sent data message is of the form data(x,s,m), where x is a message text, s is a sequence number in the range 0 .. k-1, and m is the message digest. Each of the sending process p and the receiving process q updates its shared key sk, from sk to MD.(sk), when its current sequence number wraps around and becomes 0 again. Assume that the sent data messages can be lost, with a specified degree dl, but never reordered, i.e. the degree of reorder dr is 0. Specify processes p and q in this protocol. State the condition under which your specified protocol is guaranteed to detect fresh messages. ------------------------------------------------------------------------------- 2. [7 points] Design a protocol that allows a client C to authenticate a server S using an authentication center ac. The protocol also allows server S to authenticate client C using both a user name and a password of C. This protocol is assumed to satisfy the following three conditions. 1) Client C has two user names C1, C2, and has an individual key I.C1 that C shares with the authentication center ac. 2) The authentication center ac knows the user name C1 of client C and shares with C the individual key I.C1. The authentication center ac also knows server S and shares with S an individual key I.S. 3) Server S knows the user name C2 of C and the password P of C. It also has the individual key I.S that S shares with the authentication center ac. The first two steps of this protocol are as follows. C -> ac : C1, S ac -> C : I.C1> Specify the remaining steps of this protocol. -------------------------------------------------------------------------------