In contemporary interactive software system design, to maintain equilibrium between usability and security is a challenging task because strictly enforced security policies directly affect the usability of the software. As a solution to this problem, information theoretic measure of information leakage in interactive system design has been proposed in the present work. The present paper first models the software system as a coloured Petri net model and after that using information theory and Petri net algebra; it defines the leakage in the interacting system. Based on the leakage definition, the present paper further quantifies information leakage and tries to establish a relation between information leakage and interactive system design principles. The paper also hints to decide consensus on the equilibrium of security and usability.