A cyber-physical system (CPS) is a tight coupling of computational resources, network communication, and physical processes. They are composed of a set of networked components, including sensors, actuators, control processing units, and communication agents that instrument the physical world to make "smarter." However, cyber components are also the source of new, unprecedented vulnerabilities to malicious attacks. In order to protect a CPS from attacks, three security levels of protection, detection, and identification are considered. In this chapter, we will discuss the identification level, i.e., secure state estimation and attack reconstruction of CPS with corrupted states and measurements. Considering different attack plans that may assault the states, sensors, or both of them, different online attack reconstruction approaches are discussed. Fixed-gain and adaptive-gain finite-time convergent observation algorithms, specifically sliding mode observers, are applied to online reconstruction of sensor and state attacks. Next, the corrupted measurements and states are to be cleaned up online in order to stop the attack propagation to the CPS via the control signal. The proposed methodologies are applied to an electric power network, whose states and sensors are under attack. Simulation results illustrate the efficacy of the proposed observers.Keywords: cyber-physical systems, sensor attack, state attack, sliding mode observers Recent real-world cyber-attacks, including multiple power blackouts in Brazil [3], and the Stuxnet attack [4] in 2010, showed the importance of providing security to CPSs. Identification and modeling process as [5,6] which are based on data can be seriously affected by corrupted data. As a result, information security techniques [7] may be not sufficient for protecting systems from sophisticated cyberattacks. It is suggested in [8] that information security mechanisms have to be complemented by specially designed resilient control systems. Controlling CPS with sensors and actuators, who are hijacked/corrupted remotely or physically by the attackers, is a challenge. The use of novel control/observation algorithms is proposed in this chapter for recovering CPS performance online if an attacker penetrates the information security mechanisms.Cyber security of CPS must provide three main security goals: availability, confidentiality, and integrity [7]. This means that the CPS is to be accessible and usable upon demand, the information has to be kept secret from unauthorized users, and the trustworthiness of data has to be guaranteed. Lack of availability, confidentiality, and integrity yields denial of service, disclosure, and deception, respectively. A specific kind of deception attack called a replay attack has been investigated when the system model is unknown to the attackers but they have access to the all sensors [9,10]. Replay attacks are carried out by "hijacking" the sensors, recording the readings for a certain time, and repeating such readings while injecting them together with an exo...
The problem of resilient control of linear cyber-physical systems with cyber-attacked sensor measurements and actuator commands is studied in this article. Online reconstruction of unknown cyber-attacks is accomplished by an adaptive sliding mode observer with a novel injection term in the form of a dynamically extended equivalent control. Then, the attacked/corrupted sensor's measurements and the actuators are proposed to be cleaned up using the reconstructed attacks. The performance of the system prior to the attacks is shown to be retained after a transient response required for the attack reconstruction/estimation. The efficacy of the proposed algorithms is validated on an electrical power network. K E Y W O R D Scyber-physical systems, electric power networks, sliding mode observer INTRODUCTIONA collection of units that bridge the cyber-world of computing and communications with the physical world are called cyber-physical systems (CPSs). 1 A variety of CPSs exist in critical infrastructures, including electric power networks, water resources, oil and gas distribution networks, medical devices (pacemakers, insulin pumps, etc.), chemical process industries, transportation and vehicles, and distributed robotics. [1][2][3] In CPSs, cyber components make the physical devices "smarter" and allow building intelligent systems that far exceed the capabilities of the simple embedded system components. However, the cyber components of a CPS are susceptible to unprecedented vulnerabilities in terms of malicious cyber-attacks named "attacks" throughout the article.Note that in CPSs the cyber and physical worlds are so integrated, sometimes it is not clear whether the functional properties are due to the cyber or physical components, or both. 1 Availability, confidentiality, and integrity are three major security features that should be provided to a CPS, and the lack of them leads to denial of service, disclosure, and deception attacks. 4 The focus of this work is on reconstructing deception attacks on sensors and actuator commands. Using the attacked/corrupted measurements for feedback control of the CPS allows the propagation of sensor attacks within the CPS causing CPS performance degradation-up to the loss of stability.Recent events have shown attackers using increasingly sophisticated attacks against industrial control systems, and numerous countries have acknowledged that cyber-attacks have targeted their critical infrastructures. 5,6 A specific kind of deception attack, called a replay attack, has been investigated, when the system model is unknown to the attackers but they have access to the all sensors. 7 Replay attacks are carried out by "hijacking" the sensors, recording the readings for a certain time, and repeating such readings while injecting them as exogenous signal into the CPS's sensors. In the case when the system's dynamic model is known to the attacker, another kind of deception attack, called a covert attack, has
In this work, linear (linearized) cyber-physical systems with output feedback control, whose sensors are experiencing faults or are under cyber-attack, are studied. Two different cases are investigated. First, when all sensors are attacked, then, when some sensors are protected from the attacks. Finite time convergent observers, specifically the sliding mode ones, including the observers with gain adaptation, are employed for on-line reconstruction of the cyber-attacks. The corrupted measured outputs are "cleaned" from cyber-attacks, and feedback control that uses the "cleaned" outputs is shown to provide elevated cyber-physical system performance close to the one without attack. Finally, the proposed methodology is applied to an electric power system under cyber-attack. Simulation results illustrate the efficacy of the proposed observers.
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