| Communication Based Train Control(CBTC)systems are the most widely used train control systems in urban rail transit in China.They are based on the communication network between train and ground equipment,and realize accurate train operation control through two-way communication between train and ground.Due to the use of commercial computers and communication technologies,the CBTC system faces huge information security risks.The attacker can use the vulnerability of network communication equipment to launch data tampering attack on CBTC system,which will affect the efficiency of trains and even cause safety incidents.Current research regarding Industrial Control System(ICS)security is not able to model the impact of the data tampering attack on train control system quantitatively,and current countermeasure schemes against data tampering attack are not designed for the operation mechanism of train control systems.Therefore,it is of great significance to study the information security defense strategy of the CBTC system for data tampering attack to ensure traffic safety and improve operation efficiency.This thesis studies the cross-layer defense strategy under data tampering attack in the information layer and physical layer of the CBTC system.In the information layer,in order to improve the security of data communication,the identity authentication function is added to the original communication protocol of the CBTC system.Aiming at the single-point failures of the existing centralized key management systems,a distributed key management system is designed and implemented by using the blockchain technology to perform distributed management of the keys required for identity authentication.In the physical layer,according to the dynamic characteristics of the train and the information security transfer law of the CBTC system,a train operating state transfer model that changes with the information security state of the system is established.The data tampering attack is detected by the state transition information of the system,and based on the information layer strategy,the Partially Observable Markov Decision Process(POMDP)mode is constructed to minimize the tracking error of train curves.Through the solution of the model,the optimal defense strategy of the train control system under different information security states is obtained.The main work of this thesis is as follows:(1)The impact of data tampering attacks on the CBTC system is analyzed.According to the working mechanism of Vehicle On-Board Controller(VOBC)and Zone Controller(ZC),four data tampering attacks existing between ZC and VOBC are studied.The influence of four attacks on CBTC system operation is analyzed;(2)The CBTC key management system based on blockchain is designed to improve the security of data communication at the information layer.The identity authentication function is used into the existing communication protocol of train control system,and the digital signature of the CBTC message is realized by asymmetric encryption.And the distributed key management system based on blockchain is used to to perform distributed management of the keys required for identity authentication;(3)The information security status of the CBTC system is detected,and the best defense strategy under different states is obtained.Kalman filter and chi-square test are used to detect the information security state of the CBTC system,then the state space,action space and state transition models of the CBTC system are constructed,and the curve tracking error of the train is used as the objective function to realize the construction of the POMDP model.By solving the model,the optimal defense strategy of the train under different information security states is obtained;(4)The distributed key management system based on blockchain and the CBTC system simulation platform are built.The function and performance of the blockchain-based key management system are tested,and the effectiveness of the proposed information security cross-layer strategy is verified by simulating data tampering attacks,and the feasibility of the strategy is evaluated.Experiments and simulation results show that the cross-layer defense method of the CBTC system can effectively reduce the impact of data tampering attack on train operation,and it is of great significance to improve the information security defense ability of the CBTC system.This thesis includes 62 figures,19 tables,99 references. |
