Event-based cryptography for automation networks of cyber-physical systems using the stream cipher ChaCha20

In this paper, we consider Cyber-Physical Systems (CPS) in a Discrete-Event Systems (DES) framework, and consider cyber attacks in the automation network of the CPS, where a malicious agent eavesdrops communication channels with the objective to gather information about the system behavior. We propose a cryptographic scheme to be applied in an automation network which cipher events without altering the size or structure of the transmitted data. In addition, the proposed cryptographic scheme leads to small communication delays, which makes it suitable for application in automation networks. We call this scheme event-based cryptography, where an event is defined as any change in the binary signals transmitted in the network. In order to do so, we propose a method for the codification of events as event vectors, which is suitable for encryption. We also propose the use of a stream cipher called ChaCha20, which is known to have a high resistance to cryptanalysis. A simulated example is used to illustrate the application of the proposed event-based cryptography, and to make a comparison with the RSA cipher, a public-key cipher widely used in Information Technology.