Studies On Anti-Interference Techniques Of Network Covert Timing Channels

Network covert timing channels involve that the senders modulate information into the packet sending time such that the receivers can extract the information from the packet arriving time.Some accidents such as network jitters,packet losses and disorders often occur to the packets of carrier traffic.The transmissions of covert messages are interfered by these accidents inevitably,which can lead to bit errors or erasures at the receiver side.At present,designing of covert timing channels with anti-interference and resistance detection capabilities is one of the focus and advanced subjects in this field.This thesis conducts studies on the subject from the embedding algorithm of information and channel coding.The main work and contributions are as follows:Firstly,the impact of network jitters on the embedding and extracting of secret messages transferred through CDF-based covert timing channels is analyzed.On this basis,a detailed analysis of bit error performance for CDF-based covert timing channels is presented and the sufficient and necessary condition of correct decoding is given and proven.A general calculation method of theoretical bit error rate is derived.Moreover,an improved scheme with variable modulation parameters for CDF-based covert timing channels is proposed to reduce bit error rate.Results of simulation experiments indicate that the proposed scheme is comparable to prior schemes.Secondly,the impact of packet losses on the embedding and extracting of secret messages transferred through covert timing channels is analyzed.The covert timing channel algorithm based on Reed-Solomon codes is proposed to resist the interference of packet losses.Secret messages are coding by Reed-Solomon encoder before embedding into IPDs.The interleaving technique is exploited to handle the bursts of packet losses and spread out the possible consecutive errors in time.On the receiver,the arriving inter-packet delays preprocessing algorithm is proposed to solve the bit erasure problems caused by packet losses,then,then possible errors are corrected by RS decoder.The effectiveness of the proposed scheme is validated by the results of simulation experiments.Thirdly,an adaptive transmission scheme is proposed for covert timing channels with time-varied interferences to utilize the limited resources of carrier channels more efficiently.The impact of disorder packets to covert timing channels is analyzed.Some channel quality indicators including the strength of network jitters and generalized packet loss rate are defined to measure the interference strength.The decision methods of switching thresholds for channel quality indicators and the switch rule for different modulation and coding schemes are presented.Simulation experiments are conducted to validate the effectiveness of the proposed scheme.Fourthly,a novel scheme named over multiple-input/multiple-output(MIMO)covert timing channels that exploit multiple packet flows between the source and destination hosts is proposed.The application scenarios of covert timing channels are extended from a single packet flow to multiple packet flows.Space-time trellis codes are adopted to encode secret message data before they are embedded into inter-packet delays(IPDs).The receiver recovers secret messages from arriving IPDs by using a decoding mechanism based on Viterbi algorithm.The design criterion of the proposed scheme is derived based on the analysis of pairwise error probability.Fifthly,the multicast covert timing channels method with unequal error protection is proposed to provide different error protection for secret message data and enable receivers of covert timing channels to retrieve more important data more reliably.The proposed solution models the multicast carrier channels as erasure channels and presents a coding algorithm based on expanding window fountain(EWF)codes.The more important data of secret message can be transferred with higher priority and lower bit error rates relatively.In the end,the research work is summarized and the further research direction is pointed out.