Framework Of Discrete Event Systems Research Articles

Synthesis of resilient fallback control system under cyber-attacks via supervisory control

Industrial control systems (ICS) require system design and operation under cyber-attacks. This study aims to design a fallback control system that can switch from normal control to fallback control and verify its superiority. The target system is a factory automation (FA) system consisting of a normal programmable logic controller (PLC) and a fallback PLC. In this system, we design a fallback logic that takes over control in the case of cyber-attacks. The design of this logic requires a system model that manages smooth state transitions between normal control and fallback control in an integrated manner under cyber-attacks. In response, we model a control program in the framework of discrete event systems (DESs) and apply supervisory control to derive a supervisor model that can manage the system in an integrated manner. To ensure the controllability of the FA system during cyber-attacks, we design a control specification that includes a detection function to enable rapid switching of the control state. As a result, we generate the fallback logic from the supervisor model with guaranteed controllability under cyber-attacks and implement it in a fallback PLC to verify the effectiveness of the proposed logic.

Information control in networked discrete event systems

How to control information exchange among different users is an important problem in networked systems with many users/agents. Generally speaking, there are several considerations in control of information exchange in a networked system, including (1) to ensure a friend user has sufficient information to perform its tasks, (2) to deprive an adversary user its information to perform its tasks, (3) to minimize information exchange among friend users so that the risk of information leaking is minimized, and (4) to maximize information broadcasted to all users to achieve maximum transparency. In this paper, we investigate the information control problems in the framework of discrete event systems. Based on the problem at hand, we divide users in a networked system into two or more groups. Users in the same group are consider as friends and users in a different group are consider as adversaries. Several information control problems are investigated and solved using a systematic and rigorous approach. Methods are developed to design controllers that send minimum information to its friends to help them to perform their tasks and broadcast maximum information without helping its adversaries.

Better Late than Never: On Epistemic Diagnosability of Discrete Event Systems

We investigate the diagnosability verification problem in the framework of discrete-event systems. Most of the existing works on this topic assume that faults are related to the internal behaviors of the system such as occurrences of particular events. In this work, motivated by information-flow security considerations, we model faults as some critical information leakages of the system to an intruder, which may have different observations from the system user. Specifically, we say that a fault occurs if the intruder knows that the system has passed by a secret state. We present a formal notion called epistemic diagnosability to capture whether or not the system user can always detect, based on its own observation, the critical information leakage to an intruder within a bounded delay. We show that this new notion subsumes the standard notion of event-based diagnosability. Furthermore, an effective algorithm is provided to verify this new notion.

Online prognosis of stochastic discrete event systems with guaranteed performance bound

In this paper, we consider the online prognosis of stochastic discrete event systems. Even though it is easy to predict the occurrence of faults in advance accurately for a prognosable discrete event system, it is much more difficult to do so for systems that are not prognosable. In this paper, we solve the online prognosis problem in a stochastic discrete event system framework. Our idea is to calculate the probability of the occurrence of faults in the future when a no-fault observable string is observed. We say an observable string is no-fault if no faults have occurred when it is observed. We then compare the probability with a pre-set threshold to decide whether to issue an alert or not. We develop a method to calculate the lower bound for possible thresholds, which has the fewest false alerts and no missed alerts of faults. For a given threshold, we use the rate of missed fault alerts and the rate of false fault alerts to evaluate the performance of fault prognosis. We formally define these two rates and develop algorithms to calculate them. These results can guide us in choosing a proper threshold to achieve the best trade-off between false fault alerts and missed alerts of faults.

Modelling and modular supervisory control for the AODV routing protocol

The model of the Ad-hoc on Demand Distance Vector (AODV) routing protocol, being favorable in several wireless networks (Vehicular Ad-Hoc Networks (VANET), ZigBee etc.) is presented for a network with parametric number of nodes in the Ramadge Wonham (RW) framework of discrete event systems (DES). For a generic network, consisting of n nodes, the first goal is to introduce two specification rules, per node. The first rule guarantees the hierarchy of the requests of the nodes and the routing paths. The second rule offers a first level security regarding requests’ replies. The two rules are expressed as two regular languages. Each regular language is realized by a supervisor automaton. The two supervisor automata of each node compose the local controllers of a distributed supervisory control architecture controlling the total network. The physical realizability of the local controlled automaton is proved. Also, the nonblocking property and the satisfactory marked behavior of the total controlled automaton are proved. The complexity of the proposed distributed architecture is computed. To illustrate the effectiveness of the present control scheme, a four-node network simulation of the controlled automaton of the protocol is presented.

On Securing Cryptographic ICs against Scan-based Attacks: A Hamming Weight Distribution Perspective

Scan chain-based Design for Testability is the industry standard in use for testing manufacturing defects in the semiconductor industry to ensure the structural and functional correctness of chips. Fault coverage is significantly enhanced due to the higher observability and controllability of the internal latches. These ensuing benefits to testing, if misused, expose vulnerabilities that can be detrimental to the security aspects, especially in the context of crypto-chips that contain a secret key. Hence, it remains of paramount importance for a chip designer to secure crypto-chips against various scan attacks. A countermeasure is proposed in this article that preserves the secrecy of an embedded key in a cryptographic integrated circuit running an Advanced Encryption Standard (AES) implementation. A novel design involving a hardware unit is illustrated that circumvents differential scan attacks by essentially performing bit flips deterministically, using a pre-computed mask value. This helps secure the chip while retaining full testability. The controller logic directly depends on a mask determination algorithm that can defend against any scan attack with 𝒪 theoretical complexity. Security analysis of our proposed defense procedure is performed in the framework of Discrete Event Systems (DES). The sequential scan circuit of an AES cryptosystem is modeled as a DES using Finite State Automata. A security notion, Opacity , is used to quantify and formally verify the security aspects of our controlled system, which shows that the entropy of the secret key is preserved. A case study is performed that shows to mitigate state-of-the-art differential scan attacks successfully at a nominal extra overhead of 1.78%.

Complex Event Recognition From Discrete Sensor Data With a Discrete Event System Framework

Recognizing complex events revealed by sensor readings is an increasingly crucial task that serves as one of the foundations for system monitoring and decision-making. In this paper, we investigate the recognition problem for a class of complex events that can be represented by a sequence of discrete sensor outputs. We view the outputs of discrete sensors as sensor events. We use an automaton to describe all possible sequences of sensor events that can be generated in a given system. The complex events to be recognized is a set of sequences of sensor events and can be represented by the marked language of another automaton. For a given sensor event sequence, we introduce the notion of “partition” to stand for a possible complex event sequence. By constructing an augmented automaton that includes all possible partitions, we find a necessary and sufficient condition for the recognition problem to be solvable. We then find an algorithm to check the condition. Finally, an online complex event recognition procedure is proposed to determine the occurred complex events when the complex event recognition problem is solvable.

Observer design for labeled finite automata with inputs under stealthy actuators attacks

This paper addresses the problem of cyber-attacks in discrete-event systems framework. Labeled finite state automata with inputs derived from a particular class of Petri net, called Output Synchronized Petri nets, are used to model a given cyber-physical system along with the information that circulate between controllers and plant. Stealthy cyber-attacks that may alter the control symbols, i.e., the orders sent by the controllers to the actuators, are considered. The objective is to construct an observer that uses both input and output information to provide a state estimation of the system under such stealthy actuators attacks. This observer provides a refined state estimation related to both normal and attack conditions.

Mathematical modelling of aeronautical environment

On the basis of the study of air flight control, as well as the need to take into account the regional parameters of the airspace, the expediency and the need for mathematical modeling of the air navigation environment were identified. The article deals with the features of the model of air navigation environment, in which the base of the study adopted the basic information used to describe the plan for the execution of air flight. In the study, it was found that in navigation main feature is the geodesic of abstraction, however, such information was found, is not complete for the plan for air flight. The study emphasized the role of taking into account all the restrictions by which certain prohibitions on the use of specific volumes of airspace are imposed. Based on the results obtained, a graph of the geometric representation of the airspace with constraints was constructed. The results of the study allow us to conclude that the construction of a mathematical model of the air navigation environment allows us to achieve the following results: optimization of the distribution of the EAP loads by sectors, visualization of the air navigation situation in the region, the establishment of critical load directions, the collection of data on the load, the study of the factor effects on the regularity and safety of the aircraft movement. The mathematical model of the aeronautical situation was built with the help of a composition of hierarchical type. As a result of such construction of mathematical model its transformation with addition of new models is possible. Using the proposed mathematical model in the framework of discrete event systems can be used to simulate complex air navigation environment with a large number of aircraft. The use of the formalism presented in this study allows a clear distinction between the mechanism of information processing and the information itself in the process of modeling.

Assessment of Bisimulation Non-Interference in Discrete Event Systems Modelled With Bounded Petri Nets

Non-interference in discrete event systems deals with the possibility by an intruder to infer the occurrences of private and non observable events, the so called high-level ones, by interacting with the system at a user level, i.e., by observing the occurrence of the so called low-level ones. When bisimulation non-interference is considered, the security objective is not only to avoid the detection of high-level event occurrences, but also to avoid the detection of their non occurrences; i.e., the secret includes also the non occurrences of some events. This letter deals with such a more restrictive security property in the framework of discrete event systems modelled as Petri nets. A necessary and sufficient condition is given to assess bisimulation non-interference in bounded Petri nets. Such a condition requires the solution of integer linear programming optimization problems, whose solution can be used also to statically enforce bisimulation non-interference when this condition is not satisfied by the original system.

Synthesis of sensor deception attacks at the supervisory layer of Cyber–Physical Systems

We study the security of Cyber–Physical Systems (CPS) in the context of the supervisory control layer. Specifically, we propose a general model of a CPS attacker in the framework of discrete event systems and investigate the problem of synthesizing an attack strategy for a given feedback control system. Our model captures a class of deception attacks, where the attacker has the ability to hijack a subset of sensor readings and mislead the supervisor, with the goal of inducing the system into an undesirable state. We utilize a game-like discrete transition structure, called Insertion–Deletion Attack structure (IDA), to capture the interaction between the supervisor and the environment (which includes the system and the attacker). We show how to use IDAs to synthesize three different types of successful stealthy attacks, i.e., attacks that avoid detection from the supervisor and cause damage to the system.

Masked observation for majority‐based control of a democratic progress model in the framework of discrete event systems

AbstractThis paper addresses a simple democratic progress model represented by a discrete‐event system (DES) and the influence of information accessibility on the progressiveness of the model. Each agent makes a decision according to its selfish criterion, and the final decision for the progress or regression of the system is done by the majority rule based on individual decisions of agents. We first present the conditions for the closed‐loop system to be progressive or regressive under full observation where every agent can observe all the states. Then we focus on the masked observation problem, namely, whether the progressiveness of the closed‐loop behavior can be changed by restricting the information accessibility of agents that are characterized by their masks. Especially, we show that due to the elite class, the regressive closed‐loop system can never be transformed into progressive regardless of mask design, which conforms to the feature of modern democracy.

On detectability of labeled Petri nets and finite automata

Detectability is a basic property of dynamic systems: when it holds an observer can use the current and past values of the observed output signal produced by a system to reconstruct its current state. In this paper, we consider properties of this type in the framework of discrete-event systems modeled by labeled Petri nets and finite automata. We first study weak approximate detectability. This property implies that there exists an infinite observed output sequence of the system such that each prefix of the output sequence with length greater than a given value allows an observer to determine if the current state belongs to a given set. We prove that the problem of verifying this property is undecidable for labeled Petri nets, and PSPACE-complete for finite automata. We also consider one new concept called eventual strong detectability. The new property implies that for each possible infinite observed output sequence, there exists a value such that each prefix of the output sequence with length greater than that value allows reconstructing the current state. We prove that for labeled Petri nets, the problem of verifying eventual strong detectability is decidable and EXPSPACE-hard, where the decidability result holds under a mild promptness assumption. For finite automata, we give a polynomial-time verification algorithm for the property. In addition, we prove that strong detectability is strictly stronger than eventual strong detectability for labeled Petri nets and even for deterministic finite automata.

Developing a democratic progress model based on discrete event systems

In this paper, we model and analyze democratic progress and regression in the framework of discrete event systems. Since democratic progress strongly correlates with the growing number of people who meet their economic interests and political rights, it can be represented as an event-driven dynamical system evolved by local agents that make a decision to enable or disable the progress and regression events according to their private objectives. The final decision on enabling/disabling an event is made by the majority rule, which also complies with the feature of democracy. Based on majority-based supervisory control, we investigate the conditions for the democratic progress model to be progressive or regressive under full observation. Moreover, we present a scheme that can modify the characteristics of the model by designing masks for agents, i.e. the regressive system under full observation is changed to a progressive one under partial observation via masks, and vice versa.

Depth-first Search Approach for Language-based Opacity Verification Using Petri Nets

Abstract The rapidly growing use of modern computer-integrated systems involving critical assets underscores the need for developing measures to guarantee their security against malicious attacks. This paper deals with the problem of language-based opacity formulation and verification in the framework of discrete event systems modeled with partially observed Petri nets (POPNs). In this context, a secret behavior is defined as a sub-language that should be hidden from an intruder who partially observes the system’s evolution. A Petri net system is qualified as language opaque if no observation leads to an event sequence estimation that is entirely contained in a secret. In this paper, we develop a depth-first search algorithm that verifies the language opacity of a POPN system. More specifically, the proposed algorithm checks the existence of a non-secret transition sequence that is observably equivalent to a secret one by performing a depth-first search on a portion of the system’s execution. Complexity analysis and experimental results are presented to illustrate the efficiency of the proposed approach.

Mitigation of Classes of Attacks using a Probabilistic Discrete Event System Framework

Abstract Cyber-attack models and their respective mitigation strategies have been recently studied in the discrete event systems setting. Previous work focuses on whether unsafe behaviour can be prevented using supervisory control theory. When unsafe behaviour cannot be prevented with certainty, mitigation strategies are limited. This paper proposes the use of a probabilistic discrete event system (PDES) framework to incorporate a likelihood measure for unsafe behaviour in the attack models presented in Carvalho et al. (2018). The least-unsafe (LU) supervisor problem is introduced to minimize this unsafe likelihood measure and improve existing attack mitigation techniques. The LU supervisor problem under full observability is solved by reformulating it into an MDP problem, and a computable algorithm is developed. Lastly, the implementation of LU supervisors is discussed and illustrated with an example.

Communication-Free Multi-Agent Coordination in an Unknown Environment

Abstract We propose a communication-free control protocol for the coordination of multi-agent systems of autonomous vehicles. We consider a maze solving problem in which two agents must search an unknown maze as fast and as securely as possible. We formulate this problem using a discrete-event systems framework and present a coordination strategy for two agents to search a simple maze that involves no communication, thereby eliminating any cyber-security threats. Our strategy involves two agents having reverse priority decision-making policies, while searching the maze with a depth-first search control policy. Under our definition of inefficiency, which is a function of the number of dead end states that are searched by both agents, we show that our method achieves the minimum inefficiency cost, and thus performs as well as any communication-based solution.

Discrete event system framework for fault diagnosis with measurement inconsistency: case study of rogue DHCP attack

Fault detection and diagnosis &#x0028 FDD &#x0029 facilitates reliable operation of systems. Various approaches have been proposed for FDD like Analytical redundancy &#x0028 AR &#x0029, Principal component analysis &#x0028 PCA &#x0029, Discrete event system &#x0028 DES &#x0029 model etc., in the literature. Performance of FDD schemes greatly depends on accuracy of the sensors which measure the system parameters. Due to various reasons like faults, communication errors etc., sensors may occasionally miss or report erroneous values of some system parameters to FDD engine, resulting in measurement inconsistency of these parameters. Schemes like AR, PCA etc., have mechanisms to handle measurement inconsistency, however, they are computationally heavy. DES based FDD techniques are widely used because of computational simplicity, but they cannot handle measurement inconsistency efficiently. Existing DES based schemes do not use Measurement inconsistent &#x0028 MI &#x0029 parameters for FDD. These parameters are not permanently unmeasurable or erroneous, so ignoring them may lead to weak diagnosis. To address this issue, we propose a Measurement inconsistent discrete event system &#x0028 MIDES &#x0029 framework, which uses MI parameters for FDD at the instances they are measured by the sensors. Otherwise, when they are unmeasurable or erroneously reported, the MIDES invokes an estimator diagnoser that predicts the state&#x0028 s &#x0029 the system is expected to be in, using the subsequent parameters measured by the other sensors. The efficacy of the proposed method is illustrated using a pump-valve system. In addition, an MIDES based intrusion detection system has been developed for detection of rogue dynamic host configuration protocol &#x0028 DHCP&#x0029 server attack by mapping the attack to a fault in the DES framework.

Achieving a global objective with competing networked agents in the framework of discrete event systems

ABSTRACT This paper formulates a new control problem of achieving the global objective of a discrete event system with networked control agents that pursue only their own objectives and compete with each other. In this framework, each agent makes a decision to meet its own private objective with a given mask during state observation of the system and such a decision can be changed by the influence of other agents connected through a directed network. Agents with an initial decision of enabling or disabling of an event do not change their decision, but those agents reserving their decision in an undetermined way can change their decision to either enabling or disabling depending on the influence from other agents. We derive a condition for convergence of the decision of all agents. Based on the individual decision of each agent, the final decision for enabling or disabling of an event is made by majority rule. We present the network-controllability of a global objective with respect to a given mask set as a main condition with which the networked agents can meet the global objective while they pursue only their own private objectives.

A Game-Theoretic Decentralized Model Predictive Control of Thermal Appliances in Discrete-Event Systems Framework

This paper presents a decentralized model predictive control (MPC) scheme for thermal appliances coordination control in smart buildings. The general system structure consists of a set of local MPC controllers and a game-theoretic supervisory control constructed in the framework of discrete-event systems (DES). In this hierarchical control scheme, a set of local controllers work independently to maintain the thermal comfort level in different zones, and a centralized supervisory control is used to coordinate the local controllers according to the power capacity and the current performance. Global optimality is ensured by satisfying the Nash equilibrium at the coordination layer. The validity of the proposed method is assessed by a simulation experiment including two case studies. The results show that the developed control scheme can achieve a significant reduction of the peak power consumption while providing an adequate temperature regulation performance if the system is $\mathcal{P}$ -observable.