  Cyber-physical systems (CPS) are often defended in the same manner as
information technology (IT) systems -- by using perimeter security. Multiple
factors make such defenses insufficient for CPS. Resiliency shows potential in
overcoming these shortfalls. Techniques for achieving resilience exist;
however, methods and theory for evaluating resilience in CPS are lacking. We
argue that such methods and theory should assist stakeholders in deciding where
and how to apply design patterns for resilience. Such a problem potentially
involves tradeoffs between different objectives and criteria, and such
decisions need to be driven by traceable, defensible, repeatable engineering
evidence. Multi-criteria resiliency problems require a system-oriented approach
that evaluates systems in the presence of threats as well as potential design
solutions once vulnerabilities have been identified. We present a
systems-oriented view of cyber-physical security, termed Mission Aware, that is
based on a holistic understanding of mission goals, system dynamics, and risk.
