  The coming decades may see the large scale deployment of networked
cyber-physical systems to address global needs in areas such as energy, water,
healthcare, and transportation. However, as recent events have shown, such
systems are vulnerable to cyber attacks. Being safety critical, their
disruption or misbehavior can cause economic losses or injuries and loss of
life. It is therefore important to secure such networked cyber-physical systems
against attacks. In the absence of credible security guarantees, there will be
resistance to the proliferation of cyber-physical systems, which are much
needed to meet global needs in critical infrastructures and services.
  This paper addresses the problem of secure control of networked
cyber-physical systems. This problem is different from the problem of securing
the communication network, since cyber-physical systems at their very essence
need sensors and actuators that interface with the physical plant, and
malicious agents may tamper with sensors or actuators, as recent attacks have
shown.
  We consider physical plants that are being controlled by multiple actuators
and sensors communicating over a network, where some sensors could be
"malicious," meaning that they may not report the measurements that they
observe. We address a general technique by which the actuators can detect the
actions of malicious sensors in the system, and disable closed-loop control
based on their information. This technique, called "watermarking," employs the
technique of actuators injecting private excitation into the system which will
reveal malicious tampering with signals. We show how such an active defense can
be used to secure networked systems of sensors and actuators.
