Research On Multi-level Encryption Technology Of MPEG Video

With the rapid development of networks and multimedia digital technologies, the applications of multimedia information on the web become wider and wider, and information security issues have become increasingly prominent. From military affairs, political affairs and commercial affairs to individual privacy, it appears very important to keep security of multimedia information. Of course, the demands of security is different in different application situations of video informations. For example, video informations in fields of national defence and military, party and govenment demand higher security levels than those in fields of common commerce. For another examples, in fields of VOD, digital pay-TV, and video conference, different users demand different security levels of video informations.Video encryption algorithm based on multiple security levels becomes the hot topics in the research. Most of current video encryption schemes usually employ some single method which can provide only one security level but cannot meet different demands of security in all situations. By analysing and researching the video code and Cryptography, a video encryption scheme based on multiple security levels is proposed in this thesis.Firstly, this thesis introduces the backgroud and significance of research on video encryption, analyzes the status of video encryption, and presents all the methods of video encryption in rencent years and the development course of video encryption. By summarizing most of encryption methods and algorithms, the study of current encryption schemes based on multiple security levels and analysis of their advantages and disadvantages lay the foundation for further researches.Secondly, the basics of video compression encoding is presented and the encoding process of MPEG is studied in this thesis. In order to find the suitable key data for encryption, the thesis mainly discusses the basic structure and the format of frame, and analyzes the transform between spatial domain and frequency domain. Then, some theories of cryptology is discussed in detail and Stream cipher is analyzed.Basing on the research of Gray-code transform and characteristics of M-sequence, a video encryption scheme based on multiple security levels is proposed, in which XOR is processed between M-sequence cipher and key data in frequency domain, and scrambling is processed by Gray-code transform in spatial domain. This security-level consists of three levels. First Level: Encrypting signbits of DC and AC coefficients. Second Level: Encrypting signbits of DC and AC coefficients and signbits of MV. Third Level: Encrypting signbits of DC and AC coefficients and signbits of MV, and scrambling the macroblock in spatial domain before DCT.The security of the scheme can be enhanced level by level, applicable for the situations with different demands. Because the signbits of coefficients which are only encrypted is very small, the algorithm proposed in this thesis are faster and the encrypted data keep their original formats and sizes. In experiments, the scheme of encryption is proved to be more valuable in practice by better effects and simple implements.