  The integration of cyber technologies (computing and communication) with the
physical world gives rise to complex systems referred to as Cyber Physical
Systems (CPS), for example, manufacturing, transportation, smart grid, and
water treatment. Many of those systems are part of the critical infrastructure
and need to perform safely, reliably, and securely in real-time. CPS security
is challenging as compared to the conventional IT systems. An adversary can
compromise the system in both the cyber and the physical domains. However, the
unique set of technologies and processes being used in a CPS also bring up
opportunities for defense. CPS security has been approached in several ways due
to the complex interaction of physical and cyber components. In this work, a
comprehensive study is taken to summarize the challenges and the proposed
solutions for securing CPS from a Physics-based perspective.
