Research On Image Optical Encryption In The Fractional Fourier-domain

With the rapid development of multi-media technique,it is of importance that the information with certain relations is encrypted simultaneously with high-efficiency and high-security.Multiple-image optical encryption can offer the possibility of high-speed parallel,larger capacity and more key degrees of freedom.Multiple-image optical encryption has been applied widely in the field of military,government,commercial and medical care.In a multiple-image encryption system,complex mode of multiple images and transform algorithms directly affect the security of the system,the efficient of the encryption and the quality of decrypted images.The main content of this thesis is double image optical encryption based on fractional Fourier transform,one branch of the multiple-image encryption.Combining scrambling technique with chaos theory,some encryption algorithms improved are proposed in this thesis.The main contents are as follows:First of all,a novel technique for double image encryption based on fractional Fourier transform is proposed.Both of the original images by the amplitude-phase modulation are encoded into a complex function,and the synthesized input signal is then encrypted into stationary white noise by utilizing random phase encoding in fractional Fourier domain.The physical structure of encryption system is uncomplicated with the utilization of only single random phase mask,which will help the practical use of the encryption system.Then,a novel double image encryption method is proposed by utilizing double pixel scrambling technique and fractional Fourier random domain encoding.The security level of amplitude-based image can be enhanced by the private key of pixel scrambling operation.The problem is solved that the key space of the amplitude-based image is lower than that of the phase-based image in the amplitude-phase modulation.Second,a new method of image encryption is presented by utilizing fractional Fourier transform and chaotic pixel scrambling technique.Chaotic system is used to generate pseudo-random sequence instead of random phase mask.The parameters of chaotic system serve as keys to make up for the obvious drawback,that utilizing random phase mask as a key leads to the difficulty of storage,transmission and restructure.Finally,we apply the new method we refer to last passage to multiple pictures.The simulation results show that the proposed algorithms have better capability for robustness,noise immunity and security.The schemes have simplified operation,less computation and have great prospects of application.