Enhanced exclusive-OR and quick response code-based image encryption through incoherent illumination

Multi-key optical encryption based on two-channel incoherent scattering imaging

Optical encryption has been extensively researched in the field of information security due to its characteristics of being parallel and multi-dimensionsal. However, most of the proposed multiple-image encryption systems suffer from a cross-talk problem. Here, we propose a multi-key optical encryption method based on a two-channel incoherent scattering imaging. In the encryption process, plaintexts are coded by the random phase mask (RPM) in each channel and then coupled by an incoherent superposition to form the output ciphertexts. In the decryption process, the plaintexts, keys, and ciphertexts, are treated as a system of two linear equations with two unknowns. By utilizing the principles of linear equations, the issue of cross-talk can be mathematically resolved. The proposed method enhances the security of the cryptosystem through the quantity and order of the keys. Specifically, the key space is significantly expanded by removing the requirement of uncorrected keys. This approach provides a superior method that can be easily implemented in various application scenarios.

Phase response optimization of a liquid crystal spatial light modulator with partially coherent light

This paper demonstrates a method to determine and calibrate the modulation characteristics of a liquid crystal spatial light modulator (SLM) for on-axis phase response with partially coherent light. A polarimetric approach has been implemented to obtain the phase characterization curve of the SLM. The corrections for phase response errors exhibited by SLM have been incorporated through encoded grayscale patterns to ensure a spatially uniform phase response and a linear relationship between addressed phase and phase delay by SLM. In this approach, corrections can be applied at selective pixels of the SLM's display without altering its gamma curve. Experimental results are presented that verify the feasibility of the proposed approach.

Cryptoanalysis and enhancement of a binary image encryption system based on interference

In this paper, cryptoanalysis on a binary image encryption system based on interference is conducted. In the cryptosystem under study, the binary plaintext image modulated by a random phase mask (RPM) is separated directly into two phase-only masks (POMs) as private keys. Phase wrapping operation is applied to modulate two separated POMs further for silhouette removal. The plaintext image can be reconstructed by compositing two phase-wrapped POMs. However, since the RPM used in the encryption process is irrelative to the plaintexts, it is possible to retrieve the RPM by a known-plaintext attack (KPA). And then with the help of the retrieved RPM, the information encoded in the arbitrarily given ciphertext can be reconstructed by a ciphertext-only attack (COA). Based on our analysis, a hybrid attack including a KPA and a COA with different constraints is proposed in this study. Besides, the cryptosystem under study can only be used to encode binary plaintexts, which would limit the application of this scheme in the information security. Consequently, an improved cryptosystem in which both binary and gray-scale plaintext images can be encoded is proposed. In addition, the RPM to generate two private keys in the enhanced system is dependent on the plaintexts, which makes the proposed encryption scheme immune to the proposed hybrid attack. The feasibility and effectiveness of the security-enhanced cryptosystem have been validated by numerical simulations.

Image encryption scheme based on alternate quantum walks and discrete cosine transform

As an important information medium, the digital image exists widely on the Internet. Quantum walks have the property of encrypting information. For the eneryption problem of optical digital images, an encryption scheme based on discrete cosine transform (DCT) and alternate quantum walks (AQW) is proposed in this paper. First, we use AQW and XOR operation to preprocess images in the spatial domain. Then, AQW are used to generate two random phase masks which can operate the preprocessed image and the DCT image, respectively. Finally, the encrypted image is obtained by using discrete cosine inverse exchange. The control parameters of AQW can replace the random phase mask as the key in the encryption and decryption process, so it is convenient for key management and transmission. The experimental simulation carried out the analysis of the image pixel histogram, the correlation of adjacent pixels, the robustness against noise and the sensitivity of secret keys, the results show that the image encryption method has strong security.

Lensless optical encryption with speckle-noise suppression and QR codes

The majority of contemporary optical encryption techniques use coherent illumination and suffer from speckle-noise pollution, which severely limits their applicability even when information encoded into special "containers" such as a QR code. Spatially incoherent encryption does not have this drawback, but it suffers from reduced encryption strength due to formation of an unobscured image right on top of the encrypted one by undiffracted light from the encoding diffraction optical element (DOE) in axial configuration. We present a new lensless encryption scheme, experimentally implemented with two liquid crystal spatial light modulators, that does not have this disadvantage because of a special encoding DOE design, which forms desired light distribution in the photosensor plane under spherically diverging illumination without a converging lens. Results of optical experiments on encryption of QR codes and successful information retrieval from decoded images are presented. Conducted analysis of encryption strength demonstrates sufficiently high key sensitivity and large enough key space to resist any brute force attacks.

Optical asymmetric cryptosystem for multi-image in cyan-magenta-yellow-black color space

In this paper, we propose an optical single-channel asymmetric cryptosystem for multi-image in cyan-magenta-yellow-black (CMYK) color space. To the best of our knowledge, this is the first time that multiple images in CMYK color space have been directly encrypted. The proposed optical asymmetric cryptosystem is based on the quick response (QR) encoding process and the designed Fresnel-linear canonical-fractional Fourier transform (FLFT) encryption process. Each FLFT encryption process consists of phase-truncated FLFT and random amplitude phase masks. The proposed cryptosystem without color space conversion can improve the quality of the decrypted images and avoid the loss of information. In addition, by utilizing the QR codes, the cross talk and quality-loss problems can be reduced efficiently. Numerical simulation results demonstrate that the proposed cryptosystem possesses high robustness against various types of attacks, high security for encrypting multiple color images, and fast encryption efficiency. Furthermore, the proposed cryptosystem outperforms the other relevant cryptosystems and can be extended to encrypt multiple color images in a straightforward way.