This paper aims to review the encountered technical contradictions when an attacker meets the cipher-images encrypted by the image encryption schemes (algorithms) proposed in 2018 from the viewpoint of an image cryptanalyst. The most representative works among them are selected and classified according to their essential structures. Almost all image cryptanalysis works published in 2018 are surveyed due to their small number. The challenging problems on design and analysis of image encryption schemes are summarized to receive the attentions of both designers and attackers (cryptanalysts) of image encryption schemes, which may promote solving scenario-oriented image security problems with new technologies.

When an attacker meets a cipher-image in 2018: A year in review

Asymmetric encryption of multiple-image based on compressed sensing and phase-truncation in cylindrical diffraction domain

When an attacker meets a cipher-image in 2018: A Year in Review

Owing to the characteristics of existing spatial light modulators (SLMs), the computer-generated hologram (CGH) with continuous complex-amplitude is conventionally converted to a quantized amplitude-only or phase-only CGH in practical applications. The quantization of CGH significantly affects the holographic reconstruction quality. In this work, we evaluated the influence of the quantization for both amplitude and phase on the quality of holographic reconstructions by traversing method. Furthermore, we considered several critical CGH parameters, including resolution, zero-padding size, reconstruction distance, wavelength, random phase, pixel pitch, bit depth, phase modulation deviation, and filling factor. Based on evaluations, the optimal quantization for both available and future SLM devices is suggested.

Optimal quantization for amplitude and phase in computer-generated holography

In this paper, we propose a visual-cryptographic image hiding method based on visual cryptography (VC) and volume grating optics. The secret image is converted to the encrypted visual keys (VKs) according to the normal VC algorithm. Then the VKs are further embellished to the QR-code-like appearance and hidden as the holographic optical elements (HOEs), which are fabricated by the holographic exposure of photopolymer. In the decryption process, the fabricated HOE-VKs are illuminated with the laser beam. The reconstructed VKs are overlaid to extract the hidden information directly in the optical facility, without additional computation. Optical experiments verify that the VKs in HOE mode improves the system with high security and good robustness on the random noise attack. Besides, the volume grating nature also enlarges the system bandwidth by using the multiplexing technique. The proposed method may provide a promising potential for the practical image hiding systems.

Visual-cryptographic image hiding with holographic optical elements.

An encryption method based on interleaved computer-generated holograms (CGHs) displayed by a spatial light modulator (SLM) is demonstrated. Arbitrary decrypted complex optical wave fields are reconstructed in the rear focal plane of two phase-only holograms, generated from original image using a vector decomposition algorithm. Two CGHs are encoded into one hologram by interleaving the column of pixels, which optically combines the optical wave fields of two neighboring phase-only modulated pixels. The designed image encryption system may avoid the inherent silhouette problem and alleviate the precise alignment requirements of interference encryption. Video encryption and real-time dynamic decryption is demonstrated using one SLM.

Image Encryption Based on Interleaved Computer-Generated Holograms

Three-dimensional information hierarchical encryption based on computer-generated holograms

An advanced iterative phase retrieval algorithm is applied to perform a ciphertext-only attack on the fractional Fourier transform-based double random phase encryption system. With the given complex amplitude of ciphertext and definite support of the object image, the original object image can be recovered by estimating the energy of support area in the recovered image. The encryption system can be attacked by analyzing the sensibility of fractional Fourier transform order keys and evaluating the energy of the object image support area. The proposed algorithm can obtain encrypted fractional order and retrieve two random phase keys. Numerical results demonstrate the efficacy of the proposed attacking method.

Phase retrieval for attacking fractional Fourier transform encryption

A holographic display method based on complex wavefront modulation using single spatial light modulator is proposed. The holographic display is achieved from complex wavefront encoded by double phase hologram. The modulated beam by single phase-only spatial light modulator passes through a 4f optical system to synthesize the expected complex modulated wavefront on the output plane, with a low-pass filter in the Fourier plane. The performance of holographic display is also improved by complex wavefront modulation, compared with the holographic display based on phase-only wavefront modulation. The proposed encoding and display technique is theoretically demonstrated, as well as validated in numerical simulations.

Xueju Shen

Papers

Image Encryption Based on Interleaved Computer-Generated Holograms

Three-dimensional information hierarchical encryption based on computer-generated holograms

Phase retrieval for attacking fractional Fourier transform encryption

Complex wavefront modulation and holographic display using single spatial light modulator