Information security with optical means, such as double random phase encoding, has been investigated by various researchers. It has been demonstrated that optical technology possesses several unique characteristics for securing information compared with its electronic counterpart, such as many degrees of freedom. In this paper, we present a review of optical technologies for information security. Optical security systems are reviewed, and theoretical principles and implementation examples are presented to illustrate each optical security system. In addition, advantages and potential weaknesses of each optical security system are analyzed and discussed. It is expected that this review not only will provide a clear picture about current developments in optical security systems but also may shed some light on future developments.

Advances in optical security systems

Ptychography is a particularly powerful coherent diffraction imaging technique. In ptychography, a localized beam that illuminates the object is scanned in a step-wise fashion, resulting in an array of partially overlapping probing spots on the object. The intensity diffraction pattern from each spot is recorded separately. Then, a complex-valued image is computationally constructed from the set of measured diffraction patterns. Ptychography is based on scanning, hence it results with a long overall acquisition time, in the order of a second or more. Also, the scanning limited resolution, vibration stability, drift, and dynamic range weaken the performances of ptychographic microscopes. We propose and analyze single-shot ptychography, where tens or hundreds of quasi-localized, partially overlapping beams illuminate the object simultaneously. Various schemes for single-shot ptychography, in both the transmission and reflection modes, with coherent and partially coherent illumination and for single-shot Fourier ptychography are proposed. Experimentally, we demonstrate single-shot ptychography with a 180 ms exposure time (limited by the CCD minimal acquisition time) using a sub-milliwatt diode laser that simultaneously illuminates the object with 49 partially overlapping beams. Single-shot ptychography, which combines the celebrated power of ptychography with (ultra)fast imaging, will surely open up new opportunities in microscopy.

Single-shot ptychography

Information security is a critical issue in modern society and image watermarking can effectively prevent unauthorized information access. Optical image watermarking techniques generally have advantages of parallel high-speed processing and multi-dimensional capabilities compared to digital approaches. This paper provides a comprehensive review of the research works related to optical image hiding and watermarking techniques conducted in the past decade. The past research works have focused on two major aspects: various optical systems for image hiding, and the methods for embedding the optical system output into a host image. A summary of the state-of-the-art works is made from these two perspectives.

Review on optical image hiding and watermarking techniques

We propose a novel nonlinear image-encryption scheme based on a Gerchberg-Saxton (G-S) phase-retrieval algorithm in the Fresnel transform domain. The decryption process can be performed using conventional double random phase encoding (DRPE) architecture. The encryption is realized by applying G-S phase-retrieval algorithm twice, which generates two asymmetric keys from intermediate phases. The asymmetric keys are generated in such a way that decryption is possible optically with a conventional DRPE method. Due to the asymmetric nature of the keys, the proposed encryption process is nonlinear and offers enhanced security. The cryptanalysis has been carried out, which proves the robustness of proposed scheme against known-plaintext, chosen-plaintext, and special attacks. A simple optical setup for decryption has also been suggested. Results of computer simulation support the idea of the proposed cryptosystem.

Fresnel domain nonlinear optical image encryption scheme based on Gerchberg–Saxton phase-retrieval algorithm

Quantum cryptographic algorithm for color images using quantum Fourier transform and double random-phase encoding

Ptychography is combined with optical image encryption for the first time. Due to the nature of ptychography, not only is the interferometric optical setup that is usually adopted not required any more, but also the encryption for a complex-valued image is achievable. Considering that the probes overlapping with each other is the crucial factor in ptychography, their complex-amplitude functions can serve as a kind of secret keys that lead to the enlarged key space and the enhanced system security. Further, since only introducing the probes into the input of common system is required, it is convenient to combine ptychography with many existing optical image encryption systems for varied security applications.

Optical image encryption via ptychography.

The well-known diffractive-imaging-based optical cryptosystem is breached in the paper. The decryption key of the system can be easily accessed by the opponent by using a new type of powerful phase retrieval method. Our result, to our best knowledge, is the first work to show the security risk of the diffractive-image cryptosystem. Meanwhile, we provide a set of numerical simulations to demonstrate the feasibility and robustness of the presented method.

Security risk of diffractive-imaging-based optical cryptosystem.

A novel optical encoding method based on single-shot ptychography is proposed for the application of optical watermarking. For the inherent properties of single-shot ptychography, the watermark is encoded into a series of tiny diffraction spots just in one exposure. Those tiny spots have high imperceptibility and compressibility, which are quite suitable for the optical watermarking application. The security of the proposed watermarking is mainly supported by the strong imperceptibility, as well as the introduction of compression encoding and scrambling encoding. In addition, the diversity of the multi-pinhole array and the structural parameters can also be served as security keys. Both numerical simulation and optical experiment demonstrate the high security and the easy implementation of the single-shot-ptychography-based optical watermarking. Further, the compression encoding can largely improve the embedding capacity that enables the multiple-watermarking for more transmitted information and higher security.

Optical watermarking based on single-shot-ptychography encoding.

We present a novel technique for 3D object hiding by applying three-dimensional ptychography. Compared with 3D information hiding based on holography, the proposed ptychography-based hiding technique is easier to implement, because the reference beam and high-precision interferometric optical setup are not required. The acquisition of the 3D object and the ptychographic encoding process are performed optically. Owing to the introduction of probe keys, the security of the ptychography-based hiding system is significantly enhanced. A series of experiments and simulations demonstrate the feasibility and imperceptibility of the proposed method.

https://iopscience.iop.org/article/10.1088/2040-8978/18/9/095701/pdf

3D object hiding using three-dimensional ptychography

The ptychographic scanning operation is introduced into digital holography to expand the field-of-view (FOV) An optical image encryption method based on this technique is further proposed and analyzed The plaintext is moved sequentially in the way of ptychographic scanning and corresponding pairs of phase-shifted interferograms are recorded as ciphertexts Then the holographic processing and the ptychographic iterative reconstruction are both employed to retrieve the plaintext Numerical experiments demonstrate that the proposed system possesses high security level and wide FOV The proposed method might also be used for other potential applications, such as three-dimensional information encryption and image hiding

Tuo Li

Papers

Optical image encryption via ptychography.

Security risk of diffractive-imaging-based optical cryptosystem.

Optical watermarking based on single-shot-ptychography encoding.

3D object hiding using three-dimensional ptychography

Optical encryption of unlimited-size images based on ptychographic scanning digital holography