  This paper considers the development of information flow analyses to support
resilient design and active detection of adversaries in cyber physical systems
(CPS). The area of CPS security, though well studied, suffers from
fragmentation. In this paper, we consider control systems as an abstraction of
CPS. Here, we extend the notion of information flow analysis, a well
established set of methods developed in software security, to obtain a unified
framework that captures and extends system theoretic results in control system
security. In particular, we propose the Kullback Liebler (KL) divergence as a
causal measure of information flow, which quantifies the effect of adversarial
inputs on sensor outputs. We show that the proposed measure characterizes the
resilience of control systems to specific attack strategies by relating the KL
divergence to optimal detection techniques. We then relate information flows to
stealthy attack scenarios where an adversary can bypass detection. Finally,
this article examines active detection mechanisms where a defender
intelligently manipulates control inputs or the system itself in order to
elicit information flows from an attacker's malicious behavior. In all previous
cases, we demonstrate an ability to investigate and extend existing results by
utilizing the proposed information flow analyses.
