The Research On Digital Image Encryption Based On Hybrid Chaos

Digital images have been more and more widely used due to their features such as vivid and visual impact,strong visual impact,etc.But their security problems during transmission have also become increasingly prominent.Image encryption is the key technology to solve such problems.Because of the large amount of information and high redundancy in the image,traditional encryption methods such as AES and DES applied to characters and numbers cannot work well.On account of its natural similarity with cryptography,such as the extreme sensitivity to initial values,ergodicity,aperiodicity,pseudo randomness,and very high iterative rate,chaotic systems make encryption more secure than conventional encryption in the image domain.The method is more excellent and has become a new direction to solve the problem of image encryption.However,the limited accuracy of the computer will also reduce the randomness of the chaotic sequence.In order to make up for this defect,the idea of composite encryption was adopted in this thesis to design a more secure and efficient chaotic encryption algorithm.Firstly,for single chaotic systems,the problems of small key space and low security are existed.Therefore,an image encryption algorithm based on one-dimensional chaos and three-dimensional chaos was adopted.At the beginning of the algorithm,two group Logistic chaotic sequences for pixel permutation are generated by iteration and the initial values of the Bao system are also generated.Then the chaotic sequences used for pixel substitution and diffusion are jointly generated by the plaintext pixel values and the Bao system.Finally,encryption and decryption formulas are used to achieve plaintext encryption and decryption.The compound chaos encryption algorithm improves the randomness of the chaotic sequence,and it also improves the problem that the single chaotic system has a small key space and low security,making the performance index better.Secondly,in order to take advantage of the more complex dynamics features of hyper-chaotic and ensure the encryption rate,an image encryption algorithm based on two-dimensional hyper-chaos and three-dimensional chaos is designed.The kawakami hyper-chaotic system is used for permutation and the permutation method adopts the way that the current pixel is only exchanged with the pixel after its position which result in that only one round of permutation can produce better results;the way of substitution with the three-dimensional Lu system is also improved which makes itself related with the plaintext pixel value,the pixel value of the shuffled image and the current ciphertext.Furthermore,the parameter setting is more random and complex.Simulation results show that the algorithm has excellent anti-attack capability and speed.Finally,decipherable experiments and the real-time encryption and decryption experiment were performed.Decipherable experiments were tested on two computers,one used to encrypt and the other used to decrypt.The results show that the ciphertext could be decrypted correctly when the encryption and decryption keys are exactly the same,and the mean square error of the image fidelity analysis remains below 10-5.The second experiment is real-time encryption and decryption Experiment.It can be proceeded well which verified that the algorithm is efficient and promote its application in real life.