Cyber Physical Systems (CPS), are embedded systems composed of computing elements as well as physical processes. In the past, CPS were proprietary and not connected to the cyber space. With the advent of networked control systems to enable better operations and monitoring of the physical processes, these systems are increasingly becoming part of cyberspace. Connection to the cyber space enables effective management of public infrastructures such as public transportation, smart grid and water treatment facilities. However, these advantages come with new security challenges. Attacks on CPS may lead to performance degradation to complete shutdown, or even equipment damage depending on the knowledge, goals and resources of the attacker. Most work in area of CPS security focuses on the cyber part and attempts to ensure secure exchange of information across controllers, sensors, and actuators.