Research On Digital Image Encryption Algorithm Based On Chaos

The security of digital image information has become a new topic along with therapid development of computer network and multimedia technology. Recently, imageinformation is increasingly popular on the Internet. While some image informationinvolves personal privacy, trade and even state secrets, which should be transmitted in asecret way, image encryption technology has drawn more and more attention. Chaos hassome excellent properties such as ergodicity, randomness, sensitivity to systemparameters/initial value and unpredictability, and has some corresponding relationshipswith cryptograph, which can provide some new ideas and references for imageencryption algorithms.This thesis firstly introduces the research background and significance of digitalimage encryption technology, briefly explains the basic theory of image encryption, andsummarizes the research status and basic idea of chaotic image encryption algorithms.Then the basic chaos theory including the development history, definition and mainproperties of chaos is introduced, and some familiar chaotic models are discussed. Themain work of this thesis is to study image encryption algorithms based on chaos. Withchaos scrambling using chaotic sequences generated by chaotic system, involvingencryption keys into the chaotic system, and combining with chaos synchronization asthe basic train of thought, two kinds of digital image encryption algorithms based onchaos are presented in this thesis.The first algorithm is a combination of the fractional Fourier transform and chaoticscrambling. The digital image is transformed from the spatial domain to the fractionalFourier transform domain. And the positions of image pixels are scrambled by chaoticsequence both in the time-domain and fractional Fourier transform domain. Thus bothpixel position scrambling and pixel value changing are well realized. In addition, theorder and its additivity of fractional Fourier transform provide more freedom for imageencryption schemes, and enlarge the key space. In this part the basic theory of fractionalFourier transform is introduced and a two-dimensional discretization algorithm offractional Fourier transform is presented.The second algorithm is combined with chaotic synchronization. The positions ofimage pixels are scrambled by chaotic sequence generated by the fractional-order Chenchaotic system. And then the disturbed image information is embedded into the chaotic signal and the encrypted image for transmission is obtained. In the receiver, the chaoticsignal is removed by the chaotic synchronization system. And the inversed process ofpixel scrambling is carried out and the original image is recovered. The fractional-orderchaotic dynamical systems have more complex dynamical behaviors than those ofinteger-order systems and can provide more freedom for image encryption schemes.The fractional-order Chen chaotic system and its synchronization system are introducedin this part. And the related experimental simulations are demonstrated.In the end, the experimental simulations of the digital image encryption algorithmsproposed are carried out. And the encryption and decryption effects are validated. Thesecurity of the algorithms is analyzed, including statistical characteristic analysis, keyspace and sensitivity analysis, and robustness analysis. Experimental results show thatthe proposed encryption algorithms are of good security.