Research On Digital Image Encryption Algorithm Based On Improved Hyperchaotic System

With the continuous development of computer and communication technology,the expression of information is no longer simply text information,but also forms,voice,image and video and other multimedia information.Digital image has become one of the most important roles in these information transmission media because of its intuitive,image,large amount of information,strong correlation between pixels and strong post-processing ability.However,while enjoying the convenience life brought by digital images,people also face the risk of information being stolen,forged and used by unauthorized persons.Therefore,how to ensure the security and integrity of digital image information in the transmission process has become a research hotspot in the field of information security technology.Traditional information encryption algorithms only encrypt text information,while digital images are different from ordinary text data,which have the characteristics of large amount of information,high redundancy,strong correlation between adjacent pixels and high real-time.In addition,because the traditional encryption algorithm needs a lot of computer processing time,and the encryption efficiency is low,at the same time,there are some problems such as unsatisfactory encryption effect and low security performance,which can’t meet the current needs of digital image encryption.Based on the above problems,this thesis applies chaotic system to digital image encryption by utilizing the dynamic characteristics of chaotic system,such as quasi-randomicity,ergodicity,non-repeatability and long-term unpredictability.The trajectory of chaotic system has very complex dynamic characteristics and is very sensitive to initial conditions and control parameters.The time series of variables generated by chaotic system is pseudo-random in two dimensions,and there is no requirement for the size of plaintext image data when encrypted.In addition,the chaotic sequence generated is faster,which is more advantageous for encrypting large amount of image information.Firstly,based on the existing chaotic systems,two novel four-dimensional hyperchaotic systems with relatively simple structure are constructed,and the dynamic characteristics of the system are analyzed in detail.Then,based on these two hyperchaotic systems,a digital image encryption algorithm with plaintext-related and hierarchical keys is designed.The main work is as follows:(1)Two new four-dimensional hyperchaotic systems are constructed.Based on two different three-dimensional autonomous chaotic systems,two new four-dimensional hyperchaotic systems are constructed by modifying the non-linear term and feedback control.The dynamic characteristics of the system are studied and analyzed in terms of dissipation,phase diagram,time domain diagram,Lyapunov index(spectrum),bifurcation diagram and Poincaré section.The Lyapunov index caused by the change of system parameters is studied and elaborated in detail.Compared with traditional hyperchaotic systems,these two new hyperchaotic systems have simpler structure and less computational resources to generate chaotic sequences of the same length.They are very suitable for the field of digital image encryption.(2)A digital image encryption algorithm based on plaintext correlation and hierarchical key is proposed.Firstly,the first four-dimensional hyperchaotic system is set as the first-order chaotic system and its initial value is used as the system key in the hierarchical key encryption system.The second four-dimensional hyperchaotic system is considered as the second-order chaotic system.Secondly,the first-order chaotic system generates chaotic sequence by iteration and combines it with the original plaintext image to form the initial value key of the second-order chaotic system.Then,the chaotic sequence generated by the second-order chaotic system iteration is applied to the scrambling and diffusion operations of the original plaintext image pixels to obtain the final ciphertext image.In this thesis,image encryption evaluation criteria such as key space,key sensitivity,gray histogram,neighboring pixel correlation,differential attack,anti-noise attack and information entropy are used to analyze and verify the algorithm.The experimental results show that the encryption scheme is safe and reliable.Its encryption system has complex structure,large key space,strong sensitivity to initial key and better anti-attack ability.