Analyses And New Designs Of Image Cryptosystem Based On Dispersed Chaotic System

Recent years, taking dispersed chaotic mapping as the basic transform layer, chaos cipherhas been widely used in the areas of image encryption and digital image watermarking, whilechaotic encryption has been one of research hotspot in the area of information security.In this thesis, we study the features of several chaotic encryptions’ transform layers. Thesecurity of three chaotic image encryption algorithms is analysed, and the attack algorithms andtheir efficiency analysis are given. With the research on the chaotic encryptions’ transform layerand chaotic image encryption algorithm, combined with the researching results of chaoticencryptions in existence, we design a dispersed chaotic image encryption algorithm based on twochaotic mapping. For the application circumstance, we design a digital image watermarkingalgorithm based on dispersed chaotic encryption. The main research of this thesis as followings:1. Research on the features of several chaotic encryptions’ transform layer. Firstly, weanalyse the monotonicity of chaotic map y tgx (1x)nwith the change of n. Secondly, as thedispersed transform layer of dispersed chaotic encryption algorithm, Cat-like mapping’s featuresare analysed, and the resolution is given. Finally, we research on the problem of mixed operationequation made of bitwise module2add operation and module2nadd operation, and we give theresolution of the problem in different conditions.2. For the PFTL image encryption algorithm, considering the weakness that the key used inconfusion has no relation to the key used in diffusion, we give the divide attack method ofdifferent circles by using some known-plaintext image, and the efficiency analysis is given. Theanalysis results show that the security of the encryption algorithm doesn’t reach the designtarget.3. Study the security of an image encryption algorithm based on Logistic mapping underknown-plaintext attack. Since chaotic sequences have the property “the lower bits of theinitialization do not significantly affect the output of the higher bits of the output state”, weobtain the index of coincidence degree between the test keys and the real key by experiments,base on which, we provide a divide attack method by using some known plaintext. The attackcan achieve successful rate0.815and computational complexity218for64-bit length key.4. For the LSW Algorithm, considering the weakness that the encryption algorithm onlycombines the grey value in the pixel with3-value sequence, we provide an attack method byusing some known plaintext. The analysis results show the attack algorithm can decrypt the LSWalgorithm absolutely.5. Design an image encryption algorithm based on three-dimension generalised Henonmapping and Cat mapping. The encryption algorithm solves problems of the shortage of key’s value space in the dispersing process of chaotic mapping and that keys of different transformlayers could be divided. Analysis and experiment results show that the algorithm has a strongsecurity and quick encrypting speed.6. Design a digital image watermarking algorithm based on a dispersed chaotic mappingsystem and wavelet transform. In the process of preprocessing watermark, the Logistic mappingproduces key to encrypt the the grey value in the pixel to insure the security of watermark. In theprocess of embedding watermark, we conduct three-level wavelet decomposition on image andembed watermark in the high frequency wavelet coefficients according to the size of waveletcoefficients. Analysis and experiment results show that the algorithm possesses relatively strongrobustness, and better security and transparency than ordinary algorithms.