Asymmetric Optical Image Encryption And Security Research

Nowadays,with the rapid development of the Network multi-media technology and the more convenient of the achievement and application of information,information security is facing severe challenges.Compared with the traditional information processing of computer image,optics has the inherent advantages of higher speed and parallel processing in processing 2D image.So the optical image encryption technology has a very attractive prospect and more and more researchers have devoted in the exploration of the optical image encryption technology.After many years of development,the research results of optical image encryption technology are very significant and lots of optical image encryption schemes are proposed.The method of double random phase encoding can utilize two random phase mask to make the plaintext become stationary white noise,but its inherent linearity is vulnerable to exploitation by attackers,thus leading to information leakage.The phase truncation Fourier transform encryption system has been widespread concern by academic researchers with its nonlinearity and asymmetry,and then different kinds of nonlinear encryption system are derived after that.This thesis discusses the security of nonlinear encryption system and further analyzes the characteristics of nonlinear encryption system.First,this paper reviews Fourier transform theory,introduces how Fourier transform make use of the lens in the optical path and how optical wave through lens modulate the phase.Double random phase technique has been widely used in the field of optical image encryption,so we make a brief introduction to it.The optical image encryption system based on phase truncated Fourier transform is a typical asymmetric encryption system,compared with the symmetric cryptographic system,some commonly used attract techniques cannot attract the original image information,so the security has been improved.This paper emphasizes the research on the security of the asymmetric encryption algorithm.And then,we analyze the security of the phase truncated Fourier transform asymmetric encryption system.The security of its encryption system still exist some potential problems under the attack of the iterative Fourier transform algorithm.The security of the cryptosystem would be improved by adding an amplitude mask in the Fourier plane.But it is also vulnerable under a newattack.The errors induced by different amplitude modulation in the attacks are analyzed in computer simulations.Results show that when the surrounding pixel values are enlarged or reduced more than 10 times,the relation between the pixels are broken,the plaintext information is difficult to be obtained.Second,we propose an advanced amplitude-phase retrieval algorithm attack the two-image encryption based on nonlinear amplitude-truncation and phase-truncation in Fourier domain.Making the public key and the ciphertext as constraint conditions and going to be a different iterative process can restore the two original images respectively.The result shows that the nonlinear double image encryption system has better security compared with single image encryption system and the restored image has higher error compared with the original image.However,an outstanding feature of nonlinear double image encryption system is the increasing of the key number,which increases risks in security key distribution and management.Suppose that a private key is intercepted by an attacker,the combination of the intercepted private key with the amplitude-phase recovery algorithm we proposed can improve the image identification.At last,an optical image encryption system based on unequal modulus decomposition and two-beam coherent superposition is proposed.Different form previous method decomposition or unit vector decomposition,the normal vector decomposition method applied to achieve encryption process is used here.In this method,the original image are Fourier transformed and the distribution of complex amplitude spectrum domain function,and then the complex amplitude distribution function is decomposed into two vectors with different amplitude and phase through the vector decomposition,and a random phase mask is finally joined to the two branches respectively,a inverse Fourier transform is performed to them respectively,its amplitude is taken as a ciphertext and its phase as the decryption key.This cryptosystem enhances the security of the system and silhouette problem is perfectly solved in this method.