Optical Image Steganography Based On Ptychography And Visual Cryptography

In recent years,with the rapid development of the Internet and information technology,the problem of illegal acquisition,dissemination,tampering and distribution of digital content has become increasingly serious.As an important part of cyberspace security,how to protect information from being illegally stolen,tampered with and destroyed during network transmission has become a key issue related to national security and economic development.Information hiding,also known as watermarking or steganography,is a technique that invisibly alters the carrier signal to embed hidden information(watermark signal).The traditional information hiding method is to directly embed the secret image into the host image in the space domain or frequency domain,that is,the attacker can find some information of the secret image from the host image and identify it.This results in slow processing,low efficiency,and most importantly,insufficient security.Relatively speaking,optical information security technology has the advantages of parallelism,fast processing and transmission speed,and large capacity,and the keys provided by various parameters such as phase,amplitude,wavelength,and polarization state involved in the optical system are even greater.Strengthen the security of information,can meet people’s growing demand for information security.In real-life applications,after the secret information is embedded into the host image,the generated secret image is transmitted through open network channels,such as Facebook,Twitter,We Chat and other social platforms.The secret image usually suffers from lossy operations such as compression and scaling.Noise may also be introduced in the process of transmission,resulting in serious image degradation and information loss.Therefore,the image information hiding technology adapted to the open network channel should not only pursue the indistinguishability of the encrypted image and the natural image,but also ensure the complete and correct extraction of the embedded information after suffering the lossy image operation that may exist in the network channel.This thesis designs two optical image steganography systems that integrate stack imaging and visual cryptography,and realizes an optical information processing method with high security,strong robustness,simple operation and easy implementation.The main work and research contents of the thesis are as follows:(1)A stack diffraction imaging technique with a theoretically infinite field of view is introduced.This technique utilizes the recorded overlapping diffraction patterns,and the phase information of the sample can be reconstructed by corresponding iterative algorithms.Here,we use the stacked diffraction imaging technique to measure the surface topography of the phase optical element to verify the large imaging field of view and high recognition accuracy of this technique.(2)A large-capacity lossless image steganography system combining stack diffraction imaging and visual cryptography is proposed.After the secret information is decomposed by the visual cryptography scheme,it is loaded into the diffractive optical path through the spatial light modulator for block diffraction to realize secondary encryption.We demonstrate the feasibility of the system by optical experiments.(3)An image steganography system that fuses stacked diffractive imaging and diffractive speckle is proposed.Different from the traditional coding method of visual cipher,here we use the speckle-based visual cipher coding scheme to directly encode the speckle pattern obtained by diffracting the scatterer,without processing specific optical components,greatly reducing the information Steganographic costs.We then combine this algorithm with diffractive optical paths to construct a highly secure and easy-to-implement optical image steganography system.