Optical Image Encryption Research Based On Chaotic System

With the development of era,the transmission of information is more convenient and rapid than ever.And due to the intuition and visualization,the image information has been widely used in data communications both in the civil field and non-civil field.Thus the security of transmission of the images gets more and more important and attracts many researchers' interest.Owing to the huge amount of data,severe redundancy and the two-dimensional spatial distribution of the data storage structure,the traditional encryption method is very inefficient to encrypt the images.Therefore it is necessary to develop a more appropriate approach to address these problems.Here,We consider a new encryption method combined with chaotic mapping to improve the encryption efficiency for images due to the adequate similarity between chaos mapping and cryptosystem,for example,the topological transfer and aliasing properties of chaos are similar to the diffusion and confosion characteristics of cryptography;the sensitivity of chaos to parameters is corresponding to the sensitivity of cryptogram to cryptosystem;the chaotic map obtains an separated orbit exponentially through several iterations,while the encryption system encrypts the plaintext by scrambling and substituting pixels repeatedly.The main work of this thesis is as follows:Firstly,we present the basic concept and characteristics of cryptology and chaos.Then mathematical models for optical interference,Fresnel diffraction and Fourier transform of lens are expounded.Secondly,we put forward a bit-level digital image encryption method based on Logistic chaotic map.Each pixel value of the gray-scale image is represented by an eight-bit binary number so that the two-dimensional image can be converted into a one-dimensional digital string of 0 and 1 for the proceed process.According to the given initial value,the chaotic sequences are generated by the Logistic map and sorted by ascending order to acquire the position index.And then,we present a bit-level color digital image encryption algorithm based on Lorenz chaotic system.The three components R,G and B of the color digital image are encrypted respectively with the chaotic sequences of x,y and z generated by the Lorenz chaotic system,and then merged into one.The value of each pixel is expressed as an eight binary number and spreads to obtain the one-dimensional sequence that will be scrambled according to the chaotic sequence of ascending order.The validity and security of the algorithm are proved to be safe and effective by the simulation.Finally,we put forward a virtual optical imaging encryption system based on chaotic encryption combined with phase encoding.The random template is a combination of the random amplitude and random phase templates,both of which are generated by the chaotic sequences.Compared with the traditional stochastic template,it can reduce the volume of the key and enhance the efficiency of encryption.