Research On Image Encryption Integrated With Compression Coding

Ever since Internet came into every home, it completely changed the way ofpeople’s life. Multimedia online videos, images, sounds enrich the way ofhuman-computer interaction drastically. Entertainment, business, finance, news, sports,military, and many more areas need to use digital images whose security involves thecommercial secret, sensitive information, military secrets, etc. Hence, the informationof images must be stored and transmitted in security. Image is a special data structure,which has its own unique syntax features. Compared with the ordinary text information,the volume of image data is also much larger, and there is more redundant informationin image. Using traditional encryption algorithms to encrypt image directly will causethe problem of the slow encryption speed or low encryption efficiency, which cannotmeet the requirements of real-time transmission of Internet. Therefore, it’s necessary todesign the encryption algorithm according to the characteristics of the image data. Atpresent, there are two mainstream image encryption methods: selective encryption, andjoint image compression and encryption. The instability and the sensitivity to initialvalue of chaos system are similar to cryptosystem, which makes the chaos system getmore and more focused. In existing literatures, a large numbers of encryptionalgorithm based on chaotic theory are proposed, and at the same time many scholarscontinue the detailed security analysis of the algorithms. Therefore, it’s necessary tocontinue studying image encryption on these basics.Based on the existing image encryption methods, this thesis will focus on themost widely used image compression standard, the JPEG standard, to design the novelalgorithms for color images. The main contributions of this thesis are as follows:(1) We carry out a systematic study of the basic theory of image compressioncoding, including the information theory basis, the procedure of compression and somecommon methods of compression. And then we research joint photographic expertsgroup (JPEG) still image compression standard deeply, and describe the basic codingalgorithm steps.(2) We study the basic Chaos theory and some relevant research achievements ofimage encryption and its current situation. Some crack methods to image encryptionand some evaluation methods are introduced, and the application of Chaos in imageencryption is summarized. (3) We propose a color JPEG image encryption algorithm based on self-adaptiveimage encryption frame. The method of self-adaptive image encryption and selectiveencryption on frequency domain are introduced into the proposed algorithm. Part ofDCT coefficients are chosen to perform self-adaptive encryption. And then the signsymbols of DC coefficients will be encrypted. The experimental results show that theencryption effect and the security of the proposed algorithm are good.(4) We propose a joint compression and encryption scheme for color JPEG image.By comprehensively choosing the selective encryption and joint compression andencryption, an encryption algorithm combining with spatial domain and frequentdomain for color JPEG image is proposed. The blocks are first diffused on spatialdomain, and then the edge-detection method is utilized to find out the significantblocks containing abundant details. After encrypting all the DC coefficients, part of ACcoefficients in significant blocks are chosen to be encrypted. Finally the informationabout marking the significance of blocks is embedded into AC coefficients fortransmission. Theoretical analyses and experiment results show that the proposedalgorithm is compatible with JPEG format. The cipher image is with good visualquality and uniform color distribution. The algorithm is with key-sensitivity, huge keyspace and good security.In the end of this thesis, the conclusions and drawbacks of this thesis are given.And the future direction is also discussed.