Maintaining secure and reliable distributed control systems

We consider the role of security in the maintenance of an automated system, controlled by a network of sensors and simple computing devices. Such systems are widely used in transportation, utilities, healthcare, and manufacturing. Devices in the network are subject to traditional failures that can lead to a larger system failure if not repaired. However, the devices are also subject to security breaches that can also lead to catastrophic system failure. These security breaches could result from either cyber attacks (such as viruses, hackers, or terrorists) or physical tampering. We formulate a stochastic model of the system to examine the repair policies for both real and suspected failures. We develop a linear programming-based model for optimizing repair priorities. We show that, given the state of the system, the optimal repair policy follows a unique threshold indicator (either work on the real failures or the suspected ones). We examine the behavior of the optimal policy under different failure rates and threat levels. Finally, we examine the robustness of our model to violations in the underlying assumptions and find the model remains useful over a range of operating assumptions.