Research On Image Encryption Techniques Based On Double Random Phase Encoding And Optical Interference

With the rapid development of computer technology and the widespread application of Internet, the safety and secrecy of the information is more and more important. Optical information processing, which has been developing over forty years, is a very important part of information science. Recently, the information security techniques based on optical theory and methods have received increasing attention because of the potential for new technological applications in telecommunications. The optical technique has many advantages over the traditional information security technology, such as multi-dimension, large-capacity and the natural ability of parallel data processing, and so on. As the image, which is simple, lively characteristic, is one of the main carriers of information, how to prevent unauthorized user from modifying, copying, transmitting and printing images has become a very important issue. Thus, the exploration and development of optical image encryption technique has great academic and application values.The thesis focuses on optical information encoding techniques that are based on the double random phase encoding and optical interference and has accomplished several works:(1) A specific attack based on a two-step iterative amplitude retrieval approach is proposed to break the asymmetric cryptosystem that is based on the phase-truncated Fourier transforms (PTFTs);(2) An improvement over the asymmetric cryptosystem is made by adding an amplitude mask the Fourier plane of the encryption scheme;(3) A double-image encryption technique that based on an asymmetric algorithm is proposed;(4) A new method for image encryption based on optical interference and analytical algorithm is proposed. The target image is hidden into three POMs and the silhouette problem existed in the previously proposed method with two POMs, can be resolved;(5) A simple and effective method for image encoding based on optical coherent superposition and basic vector operations is proposed;(6) An optoelectronic encryption system based on coherent superposition principle and digital holography is designed for multiple-image encryption.The whole thesis is organized as follows: In Chapter1, a brief introduction to optical information processing including its research progress in optical encryption and image process is given. We also briefly introduce the research methods and basic theories used in optical image encoding.In Chapter2, we analyze the security of a recently proposed asymmetric cryptosystem that is based on the phase-truncated Fourier transforms. Since almost all reported optical encryption techniques belong to the category of symmetric cryptosystems, from the perspective of cryptology, the proposed PTFT-based asymmetric cryptosystem has great practical significance. However, if the encryption keys are used as two public keys and applied to encode different plaintexts, the cryptosystem will be placed into a more exposed and vulnerable position. We propose a specific attack method to break the cryptosystem in this chapter. This specific attack, which is based on a two-step iterative amplitude retrieval approach and works by using the public keys and ciphertexts, would allow an attacker to reveal the encrypted information and the decryption keys that generated in the encryption procedure. An improvement over the asymmetric cryptosystem may be taken by relocating the amplitude values in the output plane. In order to achieve this, an amplitude mask is added in the Fourier plane of the encryption scheme. Some numerical simulations are presented to prove the good performance of the proposed cryptosystems.In Chapter3, we propose a double-image encryption technique that is based on an asymmetric algorithm, in which the encryption process is different from the decryption and the encryption keys are also different from the decryption keys. The encryption process is performed digitally while the decryption process can be implemented optically. The main purpose of the nonlinear operations in the encryption is to achieve an asymmetry property and a high level of robustness against attacks while retaining the linearity of decryption scheme is to provide a convenient decryption for authorized users. By using the classical double random phase encoding system, the primary images can be collected by an intensity detector that located at the output plane. Additionally, it should be pointed out that the cryptography can also be safely applied in single-image encryption.In Chapter4, we analyze the earlier proposed interference-based encryption method, which is quite simple and does not need iterative encoding. The method with two phase-only masks (POMs) has been found recently that the encryption method has security problems and cannot be directly applied to image encryption due to the inherent silhouette problem. A new method for image encryption based on optical interference and analytical algorithm is proposed in this chapter, which can be directly used for image encryption. The information of target image is hidden into three POMs and the silhouette problem existed in the method with two POMs, can be resolved during the generation procedure of POMs based on interference principle. Simulation results are presented to verify the validity of the proposed approach.In Chapter5, we propose a simple and effective method for image encoding based on optical coherent superposition and basic vector operations. The original image can be directly separated into two phase masks. One is a random phase mask (RPM) and the other is a modulation of the RPM by the original image. The mathematical calculation for obtaining the two POMs is quite simple and direct resulting from the simple principle of optical coherent superposition. The arbitrarily selected RPM can be treated as the encrypted result while the POM can be taken as the key for decryption. With this technique, the same encrypted result can be obtained for images with the same size while the keys for decryption are different. Furthermore, a new method to realize double-image self-encoding and hidden is proposed. Two original images are self-encoded in the manner that one of the two images is directly separated into two POMs and used as keys for encryption. For the sake of linearity removal by phase truncation, high robustness could be achieved in this cryptosystem. In the decryption process, the image without a separation and the two POMs used as keys for encryption are all treated as encoded data. In this cryptosystem, the keys for encryption are different from those for decryption.In Chapter6, we propose an optoelectronic image encryption and decryption technique based on coherent superposition principle and digital holography. With the help of a chaotic random phase mask that is generated by using logistic map, a real-valued primary image is encoded into a phase-only version and then recorded as an encoded hologram. The bifurcation parameters, the initial values for the logistic maps, the numbers of the removed elements and the reference wave parameters are kept and transmitted as private keys. As for multiple-image encryption, only one digital hologram is to be transmitted as the encrypted result by using the multiplexing technique changing the reference wave angle. Both the encryption and decryption processes can be implemented in opto-digital manner or fully digital manner. Simulation results are given for testing the feasibility of the proposed approach.While in the last Chapter, we conclude our works and point out some existed problems in our studies, which should be improved in our future work.