Research On Vulnerability And Reliability Of Railway Time Synchronization Network

Railway time synchronization network is one of the important supporting networks of high-speed railway communication.It is mainly responsible for transmitting unified and standard time to various professional systems and equipment of railway.Whether the railway professional systems and equipment can obtain the correct time safely and reliably through the railway time synchronization network is a key issue affecting the safe operation of the railway system.Network Time Protocol is adopted as the core time synchronization protocol in railway time synchronization network.Network Time Protocol has been developed to the fourth edition,which guarantees the security of protocol sequence interaction by introducing Autokey mechanism.However,attacks against network time synchronization protocols have occurred frequently since 2013.These attacks have caused hundreds of millions of property losses.If the attacker attacks the railway time synchronization network by exploiting protocol's own vulnerability,the time disorder of the railway system and equipment will be caused,and the consequences will be disastrous.Therefore,whether the sequence of Network Time Protocol based on Autookey model can guarantee the safe transmission of time synchronization information is urgently needed.Railway time synchronization network is constructed with three-level tree structure.Dispatching center of China Railway Corporation is set as a first-level time node.Eighteen railway bureaus are set up as second-level time nodes.Railway stations,sections and locations are third-level time nodes.Railway time synchronization network is huge and complex,involving a large number of equipment,and the service life of each equipment is different.The quality of management and maintenance personnel is different.Failure of a device or function in the network does not mean failure of the whole railway time synchronization network.The theory of conventional two-state system reliability is no longer suitable for evaluating the reliability of railway time synchronization network.Therefore,it is urgent to find a suitable method for evaluating the reliability of railway time synchronization network.In this thesis,the formal verification method is introduced into the vulnerability research of railway time synchronization protocol,and the radar model and the cloud model are introduced into the reliability evaluation of railway time synchronization network system.The main contents and conclusions of this thesis include:(1)The theory of colored Petri nets is introduced into the vulnerability analysis of the railway time synchronization protocol authentication process.There are two authentication methods in protocol authentication: trusted certificate and private certificate.Under two authentication mechanisms,the model of the normal authentication and the authentication under the man-in-the-middle invasion are constructed respectively based on the colored Petri nets.The unsafe state of protocol authentication process during the middleman intrusion is analyzed,and the model state equation is established.The security of railway time synchronization protocol is analyzed by reversed state analysis.The implementation sequence of the man-in-the-middle attack protocol during authentication phase is obtained.The experimental results show that both trusted and private certificates are insecure in Network Time Protocol based on Autookey model.They all have vulnerabilities that can be exploited by middleman.In the trusted certificate authentication mode,due to the lack of client identity authentication,the key information of identity authentication is leaked.In the private certificate authentication mode,because the clients in the same group have the same private key,any client in the same group can masquerade as a server for other clients.(2)The stochastic Petri net theory is introduced into the vulnerability analysis of the time synchronization phase in railway time synchronization protocol.In order to determine the vulnerability factors affecting the time synchronization phase of the protocol and the impact of each factor on the vulnerability of the protocol.The vulnerability of the protocol time synchronization phase is analyzed through a stochastic Petri net.The interaction flow of protocol time synchronization phase is analyzed in detail,and the stochastic Petri net model of time synchronization protocol under man-in-the-middle attack in railway time synchronization network is established.The stochastic Petri net model is transformed into Markov chain.Through analyzing several vulnerability-related states,the relationship curves between the implementation rate of vulnerability-related states and steady-state probability of normal end state and abnormal end state are obtained.Finally,the factors that have impacts on vulnerability of railway time synchronization network protocol are determined.(3)Semi-Markov theory is introduced to quantify the security of railway time synchronization network protocol.In order to be able to identify the key factors affecting the security of the entire process of the protocol,this paper quantifies the entire process of protocol including authentication phase and time synchronization phase.By introducing semi-Markov process,the problem that the state dwell time of protocol is arbitrarily distributed under attack is solved.The state transition model of railway time synchronization protocol integrity verification process with network attack behavior is established.Through analyzing the relationship between the unsafe states of the protocol and the protocol related security indicators,the relationship curves between the safety of the relevant indicators and the transition probability and state dwell time of protocol unsafe state are obtained.The key factors influencing the security of railway time synchronization protocol are analyzed.(4)Aiming at the hierarchical structure of railway time synchronization network,a reliability evaluation method of complex hierarchical system based on weighted radar model and cloud model is proposed to evaluate the comprehensive reliability of railway time synchronization network.Firstly,the hierarchical structure of hardware equipment in railway time synchronization network is established in this thesis,and the corresponding relationship between system function evaluation index and hardware equipment is determined.Then,in order to obtain a more reasonable weight value of each evaluation index,the subjective and objective weights of the hardware device are calculated.The weight of the function evaluation index is determined by using the correspondence between the system function evaluation index and the hardware device.Finally,the cloud model is used to calculate the expectation of each functional indicator.A weighted radar model is constructed,which can reflect the actual operation state of railway time synchronization network more effectively.The comprehensive reliability of railway time synchronization network is evaluated by weighted radar model,and the weak links of system function are determined.