Abstract: With the development and prosperity of Internet technology,the dissemination and application of digital images are more and more extensive,and the security of digital images is also paid more and more attention.In image encryption algorithm,scrambling-diffusion encryption algorithm is widely used,because it conforms to the characteristics of two-dimensional distribution of image data.However,the common scrambling-diffusion encryption algorithm has some problems,such as low security and low encryption efficiency.Thereofre,fractional order Chen hyperchaos is used to scramble in frequency domain,then hyperchaotic S-box is designed to replace it.Finally,bi-directional exclusive or cyclic left-shift diffusion is used to achieve a complete encryption process combining frequency domain with spatial domain,scrambling,substitution and diffusion.The algorithm has large key space,high key sensitivity,uniform statistical histogram of ciphertext,low correlation between adjacent pixels of ciphertext,high security and strong resistance to differential attack,and r is close to ideal value.The algorithm can achieve the same security level as the previous image encryption algorithm only through three iterations,and the encryption efficiency is significantly improved.

Key words: Bidirectional diffusion, Fractional order Chen hyperchaos, Frequency domain scrambling, Hyperchaotic S-box substitution, Integer Haar wavelet