A Complex Network-based Approach To Cyber Security Assessment Of Urban Rail Transit Control Systems

The wide application of Communication-based Train Control(CBTC)technology has greatly improved the operational efficiency of urban rail transit.With improving the real-time and accuracy of train control system,utilization of wireless communications,general protocols and computer platform brings increase of the openness of CBTC systems,which can introduce potential cyber security risks as well.Although the inherent safety mechanism of CBTC systems restricts the dissemination and evolution of cyber security risks to a certain extent,the lack of dedicated cyber security protection mechanisms brings great hidden dangers to the stable and efficient operation of urban rail transit.Comprehensive and dynamic quantitative security assessment is of great significance for accurately describing the impact of cyber security risks on CBTC systems,and provides a key technical basis for the construction of accurate and effective cyber security protection architecture.Starting with the hierarchical structure of the CBTC system,this thesis firstly quantitatively analyzes the vulnerability of computer nodes,subsystems and the whole system.And then,based on the Cyber Physical System(CPS)characteristics of CBTC systems,the cyber security risk assessment model of CBTC systems is built through combining computer node status and communication link indicators in cyber domain and train running status indicators in physical domain.The main innovations of the thesis are as follows:1)An attack tree-based approach for vulnerability assessment of CBTC systems.We construct a CBTC attack tree model based on the hierarchical structure characteristics of urban rail transit systems,and the vulnerability analysis of equipment is carried out considering devices'security status,computer system vulnerabilities,access control and communication security.The vulnerability indexes are proposed to measure the likelihood of devices,attack scenarios and the system being maliciously invaded.2)A two-dimensional structure entropy-based approach to security risk assessment for CBTC Systems.Based on the characteristics of deep integration of cyber domain and physical domain of urban rail transit systems,we build a complex network model of cyber domain and introduce two-dimensional structure entropy to measure the network topology performance.From the perspective of operation service quality,the train performance index is proposed to measure the characteristics of system's physical domain.In view of the time-varying characteristics of the network attack process and the changes of the system state,a dynamic security risk assessment model of CBTC systems is proposed by integrating the performance indicators of information domain and the physical domain.3)A resilience-based approach to quantification on comprehensive operation service quality of CBTC systems.According to three stages of security event including pre-event,during the event and post-event,a resilience triangle model is built to measure the fault absorption,adaptability and recovery capabilities of the CBTC system,which can provide a comprehensive evaluation and dynamic assessment under different cyber attacks.