A Review of Privacy Preserving Encrypted Control for Cyber-Physical Systems

Utilizing cloud computing and distributed computing has led to various advantages like enhanced performance, enabling outsourcing of complex computations, and higher scalability in a vast range of network control systems such as smart energy networks, smart buildings, and intelligent transportation. However, confidentiality breaches and manipulation of sensitive and private information, as well as lack of public trust in cloud-based decentralized and distributed approaches where agents are reluctant to share their information due to privacy concerns are among the emerging challenges in control of cyber-physical systems. This paper reviews the privacy-preserving encrypted control methods that address some of these challenges. In encrypted control methods, all the required computations are performed directly on the encrypted data, and thus, no intermediate decryption of private data is needed. In this way, the access of adversaries to the crucial information of the control system will be very restricted. Since implementing a complex cyberattack usually requires an in-depth knowledge of the system’s data, protecting the privacy of the system’s signals in the entire control loop considerably reduces the possibility of more complex cyberattacks. Therefore, in this paper, homomorphic encryption and secure multi-party computation methods are introduced as the basis for preserving the privacy of data and designing secure control approaches. Then, various control and optimization methods are reviewed. Shortcomings and challenges of existing results are discussed and the roadmap to further research in this emerging topic in control engineering is drawn.