I. Introduction
In recent years, security of cyber-physical systems has attracted a lot of interest within the control systems community [1], [2]. A large number of problems, previously considered in an attack-free setting, have been extended to account for cyber-attacks; examples include design of attack resilient estimators [3], detectors [4], and consensus protocols [5]. However, new problems such as cyber-attacks modeling [6], attack analysis [7], and development of security indices [8] have also emerged. The present paper is concerned with the latter.