Research On Block Image Encryption Algorithm Based On Compound Chaotic System

Digital images are widely used in network transmission due to their ability to carry large amounts of identifiable information.Image encryption,as an effective method of protecting image security,has become a hot research topic.Chaotic systems are often used in the field of image encryption because of their initial value sensitivity,stability,non-periodicity and ergodicity,which meet the cryptographic requirements for the characteristics of key sequences.This thesis is based on the construction of a well-performing hybrid chaotic system,combining various matrix transformation methods to design a chaotic image encryption algorithm based on the permutation and diffusion mechanisms,converting plaintext images into unrecognisable noise-like images,and evaluating the security performance of the proposed algorithm.The main work in this thesis covers the following areas:(1)Proposed bit-level block image encryption algorithm based on coupled Tent chaotic mapTo address the problems of narrow and discontinuous chaotic range of one-dimensional chaotic maps,this algorithm firstly takes Tent map as the base map and uses its special linear structure to couple Sine and Logistic maps to construct the coupled Tent chaotic map(CTCM),and verifies through evaluation tests that CTCM has better.Then,we use bit-shifting rules and chaotic sequences to reorganize and cyclically shift the plaintext pixels of the block scrambling,to achieve double bit scrambling and change the pixel values at the same time,and confuse the pixel positions by bit splicing and extraction methods to solve the problem that bit scrambling is limited to 8 bits.Finally,a different chaotic sequence from the encryption stage is introduced,and the pixel values of the intermediate ciphertext are changed by mod and XOR operations to achieve secondary diffusion of pixels and obtain an encrypted image.Experimental analysis shows that the algorithm is highly secure and sufficient to resist typical attacks such as exhaustive attacks.(2)Proposed block image encryption algorithm based on cosine-exponential chaotic mapTo address the problems of simple chaotic behaviour and easy prediction of chaotic trajectories in existing chaotic maps,this algorithm first modulates the cosine function with the introduction of Tent map by using the exponential term to construct a two-dimensional cosine-exponential chaotic map(2D-CECM)with better chaotic performance,and combines the SHA-256 function to generate a plaintext-dependent key to resist the chosen plaintext attack.Then,on the basis of bit-level transform and Latin square rule,the extended bit-level and double Latin square algorithms are designed,and combined with two-dimensional Josephus sequences,the block-internal scrambling is performed on the plaintexts that have completed inter-block pre-setting scrambling to achieve differential scrambling of sub-blocks.Finally,based on the Zig-Zag transform,the closed loop cross Zig-Zag transform algorithm is designed using a loop-like imitation Zig-Zag structure,and combined with chaotic sequences to achieve bidirectional nonlinear diffusion of the intermediate ciphertext to obtain the ciphertext image.The security analysis shows that the algorithm is highly sensitive to the key and can effectively resist typical attacks such as brute force attack and differential analysis attack.(3)Proposed block image encryption algorithm based on cascade modulation cosine chaotic systemTo address the problem that a single chaotic map cannot guarantee a high level of security for encrypted image,this algorithm provides a general chaotic framework,i.e.,a cascade modulation cosine chaotic system(CMCCS)that can generate several different chaotic maps simultaneously.By introducing existing chaotic maps as seed maps,using multiple non-linear function terms as seed functions,and performing cosine transformations after cascading to expand the dimensionality of the maps,this operation can effectively combine the chaotic properties of the seed maps to generate multiple chaotic maps with excellent performance at the same time.On this basis,a blocked image encryption algorithm incorporating the Latin square and U-shaped transforms is designed,which makes full use of the characteristics of CMCCS to achieve one-at-a-time encryption.Then,a double orthogonal Latin square scrambling algorithm is designed by combining multiple chaotic sequences to construct orthogonal Latin squares,which achieves double scrambling between and intra-block.Finally,based on the position transformation method of the Latin square index,a U-shaped transformation and a random index transformation are designed to achieve multiple non-linear diffusion intra and inter-block.Experimental analysis shows that the algorithm can effectively resist typical attacks such as noise analysis,statistical analysis and shear analysis with high security.Figure [47] Table [11] Reference [85]...