Research On Speech-like Modulation Technology For Mobile End-to-end Secure Communication

With the popularization of mobile communication technology,various mobile voice services have provided great convenience to the public in many fields.However,there are hidden security hazards in voice communication that cannot be ignored,including eavesdropping and voice leakage.In particular,there are problems such as loopholes and backdoors in the computing environment of mobile terminals.After voice enters the mobile terminal,private information may be stolen by viruses and malware in the terminal.The existing encryption technologies which encrypt voice inside the mobile terminal cannot solve the problem of information stealing by viruses and malware.Therefore,it is necessary to encrypt the voice signal before it enters the mobile terminal,so that the voice signal always exists in the terminal and the voice channel in the form of ciphertext,so as to realize end-toend secure communication.However,due to the presence of the vocoder in the voice channel of mobile communication,the data stream formed by directly encrypting the voice data cannot be transmitted through the voice channel of mobile communication.Therefore,there is an urgent need to study a speech-like modulation method that can transform the encrypted data stream into a signal that can be transmitted through the vocoder.In this thesis,based on the analysis of the transmission characteristics of the mobile communication voice channel,several types of speech-like modulation and demodulation schemes are mainly studied,and their performance in terms of modulation rate,error performance and voice transmission quality are compared.On this basis,an end-to-end voice encryption and decryption secure communication prototype system is implemented.The main work of this thesis is as follows:1.The voice coding technology and the transmission characteristics of We Chat voice channel are analyzed,and a speech-like modulation and demodulation scheme based on multi-channel minimum shift keying（MSK）modulation is proposed.The digital filters are designed to separate multi-channel MSK signals at the receiving end,and the demodulation of MSK signals is realized based on the delayed phase coherence method.After testing under the actual We Chat voice channel,the data transmission rate of this scheme reaches 1.6kbps, and the bit error rate is 3.19%.2.Based on the research on the voice channel transmission model,a speech-like modulation and demodulation scheme based on speech parameters is proposed.This scheme maps the encrypted data stream to be transmitted to the formant and pitch period to synthesize speech-like signals.Three different mapping modulation schemes based on different parameters and corresponding demodulation methods are designed,and the parameter codebooks are designed with emphasis.Under the actual We Chat voice channel,the performance of these three schemes is tested.The two schemes with the best overall performance can achieve a transmission rate of 1.2kbps and a bit error rate of 2.08%,and a transmission rate of 1.15kbps and a bit error rate of 1.04%.Compared with the speech-like modulation and demodulation scheme based on MSK,this scheme has the characteristic of concentrated error bit distribution.3.An end-to-end voice encryption and decryption secure communication scheme is designed and implemented.The Codec2 codec is selected to compress the plaintext voice,and the Tsu Chung-Chi（ZUC）stream cipher algorithm is used to encrypt the compressed low-rate data stream.The end-to-end data frame synchronization scheme is designed using the linear frequency modulation signal as the frame synchronization head.The performance of the proposed scheme is tested on the built We Chat communication platform,which further verified the feasibility of the scheme.