Research On And Implementation Of Chaos-based Digital Image Encryption System

With the rapid development of computer and communication technology,more and more digital image information transmit over public networks such as Internet and wireless networks,which shows a great convenience of the information accessing and sharing.Meanwhile,the security of digital information has been widely concerned.Compared to word information,digital image information own some special characteristics such as bulk size,high redundancy and high correlation between adjacent pixels,conventional encryption algorithms,such as Triple-DES,IDEA,AES are not suitable for image encryption,especially in aspect of encryption speed,which can not satisfy the demand of real-time network transmission.Recently,chaos-based image encryption technology has been widely investigated.There are many basic features of chaos system such as extremely sensitive to initial condition and parameter,ergodicity and pseudo randomness;therefore a secure image encryption system can be built based on chaos system.Moreover,the chaos-based encryption system also has the advantages of ease implementation of software or hardware and fast encryption/decryption speed,which is very suitable for ciphering images with large size.This thesis firstly analyses the advantages and flaws of existence chaos-based image encryption system,and then make the main achievements as follows:Most existence chaos encryption algorithms exist drawbacks such as the periodic of chaotic sequence,fixed point and restriction to image shape.To solve these problems this thesis proposes an image algorithm with high security and efficiency based on a modified HPP model and multi-scroll chaotic system.The modified HPP model used in this system has general applicability to different precision.This novel confusion based on bit level can also introduce diffusion effect in this stage which improves the efficiency in execution.What is more,most existence algorithms using low dimension chaotic systems whose key space is small and security is low.To solve it this algorithm introduces Multi-scroll chaotic system in the diffusion stage.The multi-scroll chaotic system has more variables and its system dynamic behavior is more complicated compared to low-dimensional chaotic system which introduces larger key space and stronger unpredictability to this system.In the traditional "permutation-diffusion" architecture,the diffusion stage usually involves high computational complexity.To address this problem,this thesis proposes a bidirectional parallel diffusion arithmetic based on chaotic system.The double thread collision mechanism makes it obtain the ideal diffusion effect with only 1 round operation time of traditional method,which highly improves the execution efficiency on the premise of security.The cryptography evaluation system of the existing image encryption system lacks of a unified quantitative evaluation standard.This thesis derivation NPCR and UACI statistical distribution models and histogram distribution models through mathematical modeling of the ideal cipher image.Further,theoretical values are used to form statistical hypothesis NPCR\UACI tests and chi-square test is applied to measure the fitting standards between actual cipher and ideal cipher text under a certain significant level.Moreover this thesis adopts a new image randomness measure using entropy over local image blocks.The novel local entropy measure overcomes several weaknesses of the conventional global entropy measure including low efficiency,unfair randomness comparisons between images of different sizes,and possible inaccurate scores for synthesized images.Meanwhile,it demonstrates that the security level of proposed cryptosystems can effectively resist most commonly used attacks,including brute force attack,known/chosen plaintext attack,ciphertext only attack,differential attack and various statistical attack through key sensitivity analysis,key space analysis,correlation between adjacent pixels and the proposed histogram analysis,local information entropy measure,as well as NPCR/UACI randomness test.Overall,the proposed image cryptosystems are very suitable for real-time secure image communication applications.