Abstract
Recent studies on cyber-physical systems as the next generation engineered systems highlight the cybersecurity risks and their effects on system efficiency and safety. Cy-bersecurity relies on many systems properties to build a trust relationship between its components. Examples of these system properties are availability, reliability, resilience, privacy, intrusion detection/prevention, confidentiality, to name a few. The computa-tional and physical elements of a cyber-physical system are interacting with each other. Both aspects may open doors for the attackers to cause catastrophic consequences. Therefore, the goal of the proposed dissertation is to focus on an important require-ment, which is the secure time synchronization. Many clock synchronization protocols based on message passing, e.g., the Network Time Protocol (NTP), assume symmetric network delays to estimate the one-way packet transmission time as half of the round-trip time. As a result, asymmetric network delays caused by either network congestion or malicious packet delays can cause significant synchronization errors. This disser-tation exploits sinusoidal voltage signals of an alternating current (ac) power grid to limit the impact of the asymmetric network delays on these clock synchronization pro-tocols. Our extensive measurements show that the voltage signals at geographically distributed locations in a city are highly synchronized. Leveraging calibrated voltage phases, we develop a new clock synchronization protocol, which we call Grid Time Protocol (GTP), that allows direct measurement of one-way packet transmission times between its slave and master nodes, subject to an analytic condition that can be easily verified in practice. The direct measurements render GTP resilient against asymmetric network delays under this condition. A prototype implementation of GTP maintains sub-ms synchronization accuracy for two nodes tens of kilometers apart in Singapore and Hangzhou, China, respectively, in the presence of malicious packet delays. Simu-lations driven by real network delay measurements between Singapore and Hangzhou under both normal and congested network conditions also show the synchronization accuracy improvement by GTP. We believe that GTP is suitable for grid-connected dis-tributed systems that are currently served by NTP but desire higher resilience against unfavorable network dynamics and packet delay attacks.