Study Of Optical Information Security Techniques Based On Iterative Phase Retrieval Algorithm And Phase-shifting Interferometry

With the rapid development of Internet and modern communication techniques, the study of information security and anti-counterfeiting applications has been faced with more and more serious challenges in recent years. Since Javidi and Réfrgier proposed the method of double random-phase encoding technique in the 4f setup in 1995, many researchers have been attracted to the field of optical information security. Several novel optical image encryption and watermarking architectures have been proposed and extended from Fourier domain to other domains, such as fractional Fourier domain, Fresnel domain, etc. Combined with the principle of cryptography, optical information security mainly adopts the principles and techniques of optical information processing to realize information encryption or hiding. Compared with the traditional information security fully based on cryptography and computer and electrical systems, optical information security techniques have obvious advantages such as the ability of high-speed parallel processing, large capacity and high degree of freedom (e.g. amplitude, phase, wavelength, polarization) for encryption and decryption.This dissertation gives a systematic review of the developments of optical information system, introduces some most-frequently used techniques of optical image encryption or watermarking such as the double random phase encoding technique and iterative phase retrieval algorithms. On basis of a comprehensive analysis in the field of optical information security and other related areas, the aim of this dissertation is to propose some novel information security schemes based on information optics theories, such as iterative multiple-phase retrieval algorithm, phase-shifting interferometry, etc. The main initiative contributions in this dissertation include following aspects:(1) A novel information security system based on multiple-phase retrieval by an iterative Fresnel transform algorithm is proposed. In this method a series of phase masks cascaded in free space are employed and the phase distributions of all the masks are adjusted simultaneously in each iteration. It can achieve faster convergence and better quality of recovered image compared with iterative double phase retrieval and the similar approach in spatial frequency domain with the same number of phase masks, and can provide a higher degree of freedom in key space with more geometrical parameters as supplementary keys. The feasibility of this method and its robustness against occlusion and additional noise attacks are verified by computer simulations.(2) A novel hierarchical image encryption system based on cascaded multiple-phase retrieval by an iterative Fresnel-transform algorithm is proposed. In this method a series of phase masks cascaded in free space are used to retrieve different target hidden images in their corresponding hierarchical levels. It can realize different level of accessibility to the secret data for different authority level with the same system, and the higher level authority can get access to more hidden images. Furthermore, this approach can achieve high quality of the recovered images without any recognizable cross-talk noise or distortion. Besides the phase keys and the geometrical parameters keys, their correct sequential orders also play a very important role in the hierarchical image encryption system. The feasibility of this method and its unique features are verified and analyzed by computer simulations. This system can be used in hierarchical security verification and local admittance authentication systems.(3) A novel digital method of cross-talk free double image encryption and watermarking by constructing a complex field with one nonnegative image as its amplitude and the other as its phase and then encrypting this complex field by means of digital optics such as phase-shifting interferometry (PSI) with double random phase encoding is proposed. Since no direct addition of the information of two hidden images is utilized in the encryption process, each image can be perfectly retrieved without cross talk caused by the existence of the other. The feasibility of this method and its robustness against occlusion and additional noise of watermarked images with different weighting factors are verified by computer simulations.(4) A novel single-channel color image watermarking with digital optics means based on phase-shifting interferometry and neighbor pixel value subtraction algorithm in the discrete-cosine-transform (DCT) domain is proposed. The converted two-dimensional indexed image matrix from an original color image is encrypted to four interferograms by PSI and double random-phase encoding technique, then the interferograms are embedded in one chosen channel of an enlarged color host image in the DCT domain, and the hidden color image can be retrieved by DCT, the improved neighbor pixel value subtraction algorithm, inverse encryption process, and color image formats conversion. The feasibility of this method and its robustness against some types of distortion and attacks from the superposed image with different weighting factors are verified and analyzed by computer simulations.(5) A novel algorithm of two-step generalized phase-shifting interferometry with an arbitrary phase shift in (0,π) is proposed, and the unknown phase shift can be extracted blindly by a statistic average algorithm. Both digital simulations and optical experiments are carried out to verify the proposed two-step generalized PSI. In optical experiments, we have investigated the feasibility of this method for both the indirect object (on a spatial light modulator) and the direct input object, and made some efforts, such as increasing the effective size of recording CCD and decreasing the distance between the object plane and the recording plane, to improve the quality of reconstructed image. As a result, a good resolution higher than those reported recently has been obtained.(6) The two-step generalized PSI is applied for the first time to optical information security system based on double random-phase encoding in Fresnel domain, and the hidden information can be encrypted into only two interferograms. Therefore, this approach can greatly cut down the communication load and considerably improve the efficiency of data transmission. A series of digital simulations are made to verify the feasibility and robustness of this system against occlusion and noise attacks for both the amplitude-image encryption and the pure-phase image encryption, and the different characters and advantages in both cases have been compared in detail.Arranging from the encryption and watermarking of single image to multiple images, and from grayscale images to color images, these research works provide a systematic study in this field and have been published in a series of international journals.