Network Scheduling for Secure Cyber-Physical Systems

Existing design techniques for providing security guarantees against network-based attacks in cyber-physical systems (CPS) are based on continuous use of standard cryptographic tools to ensure data integrity. This creates an apparent conflict with common resource limitations in these systems, given that, for instance, lengthy message authentication codes (MAC) introduce significant overheads. We present a framework to ensure both timing guarantees for real-time network messages and Quality-of-Control (QoC) in the presence of network-based attacks. We exploit physical properties of controlled systems to relax constant integrity enforcement requirements, and show how the problem of feasibility testing of intermittently authenticated real-time messages can be cast as a mixed integer linear programming problem. Besides scheduling a set of real-time messages with predefined authentication rates obtained from QoC requirements, we show how to optimally increase the overall system QoC while ensuring that all real-time messages are schedulable. Finally, we introduce an efficient runtime bandwidth allocation method, based on opportunistic scheduling, in order to improve QoC. We evaluate our framework on a standard benchmark designed for CAN bus, and show how an infeasible message set with strong security guarantees can be scheduled if dynamics of controlled systems are taken into account along with real-time requirements.