Show that if someone, M, attact to the system, change the y_{a ,} y_b to y^q_{A and} y^q_B then he knows that the key K_AB is one of the +-1. If p=rq+1 with r is interger>2, then show all the possible values we can have for K_AB.

(INFORMATION: A and B agree that to the protocol Diffie-Hellman that primary p=2q+1 and one primary root g. Suppose that A sent y_A=g^X_A modp to the B and B sent to A y_{B =} g^X_B mod p.)