Hybrid Encryption for State Estimation Against Stealthy Attacks

Abstract

The “crypticity” is characterized as confidentiality property that characterizes whether system information can be inferred by an attacker. When the system is uncryptic, transmission data are easily intercepted and compromised by attackers, further endangering system stability. The existing computational security in the computer field only ensures that attackers cannot recover the plaintext from the ciphertext and key in polynomial time. However, in control systems, due to system vulnerability, the attacker can directly utilize the ciphertext data to launch stealthy attacks rather than decrypting. To meet the security requirement for state estimation, this paper formulates a new crypticity-enhancing framework. A hybrid encryption strategy with low computational complexity is proposed. Specifically, symmetric encryption algorithms encrypt transmitted innovations of state estimation, while asymmetric encryption algorithms encrypt keys of symmetric encryption. A sufficient condition for partial crypticity and a necessary and sufficient condition for complete crypticity of the hybrid encryption algorithm are derived. A simulation example is presented to sustain the theoretical results.