Digital Image Encryption Algorithms Based On Cellular Automata

The development of computer technology and multimedia technology makesdigital image a major medium for information communication. The confidentiality ofdigital images when they are transmitted via open channel becomes a focus of researchin area of information security. Cryptography, which is an important part of informationsecurity, is the main approach to ensure the confidentiality of digital images. Hugeamount of data, high correlation between neighbor pixels and high redundancy are thefeatures of digital image data. All these characteristics make traditional encryptiontechnology, which treats digital image as common text data, slow with low efficiency.There is need to design new specialized algorithms for digital image encryption.Existing technologies of image encryption include matrix transformation, encryptionwhich incorporated with compression, chaotic system, Cellular Automata (CA for short),and Neural Network etc. The goal of this dissertation is to analyze current encryptionschemes based on CA and design new secure and efficient algorithms based onproperties of CA.Following are the main contents and contributions accomplished in thisdissertation:①CA theory and background knowledge of image encryption are introduced.Besides, advantages of CA over other technologies of image encryption are brieflypresented. Applications of CA in cryptography and existing encryption algorithms basedon CA are introduced and analyzed. These applications in cryptography are presented inPart2from the respects of random number generators, Hash functions, and imageencryption algorithms. Two typical encryption algorithms based on CA are introducedin Part3, one of them is based on attractors of state transition graph of elementaryCellular Automata (ECA), and the other one is based on reversible toggle rules of CA.The merits and demerits of these two algorithms are analyzed in this dissertation.②An image encryption algorithm based on attractors of ECA is analyzed andimproved. The original algorithm in Part3has the defects of poor plain sensibility andvulnerability for chosen plaintext attack. The proposed algorithm, which combines theattractor encryption process and periodicity of two dimensional CA, overcomes theshortcomings of the original one. Simulation results and analysis of security show thatthe improved algorithm has a great performance of encryption with high sensibility to plaintext and key. Its structure is simple and can be implemented easily by software andhardware with high speed due to the parallelism of CA.③One image encryption algorithm based on two dimensional CA(2D CA for short)with toggle rules is proposed. Many current encryption algorithms based on2D CAtoggle rules have the defect of cipher expanding, and they are designed to encrypt textdata. The proposed algorithm divides cells of2D CA into two parts according to theircoordinates and updates one part every step. The toggle rule with parameter λ=0.5ensures the good performance of encryption. This algorithm overcomes the problem ofcipher expanding. Encryption and decryption processes are parallel with high speed.The results of stimulation show that the algorithm has a great performance and highsecurity level.