A ciphertext-only attack (COA) on a joint transform correlator (JTC) encryption system is proposed. From the perspective view of optical cryptanalysis, we find out that the issue to be solved in the COA scheme could be transferred into a phase retrieval problem with single intensity measurement. And in this paper, the hybrid input-output (HIO) algorithm is employed to handle this issue with the help of an inartificial signal domain support and a given frequency domain constraint. Meanwhile, we provide a set of numerical simulations to demonstrate the validity and feasibility of the presented method.

A compressive optical steganography based on single-pixel imaging (SPI) is proposed. The SPI system, which employs a digital light projector to illuminate the host image with modulated patterns, is similar to a wireless broadcast system. Therefore, it is suitable for covert communication naturally. By leveraging the compressive sensing technique and a generalized phase retrieval algorithm, the secret message is sparse-sampled and then encoded into the illumination patterns, which are projected onto the host image in an SPI architecture. The resulting reflected light signals travel in the free space as a broadcast system, and the signals would be captured by the authorized receivers and the potential eavesdroppers. By implementing an inverse Fourier transform, a stegoimage will be received, which is almost the same as the host image. However, only the authorized receivers, who possess the secret key, could extract the desired data from the stegoimage and then reconstruct the secret message by using a convex optimization algorithm. Because the secret message is sparse-sampled before embedding, the imperceptibility is well preserved while the capacity also be kept in a high level. A series of simulation and experimental results verifies the validity and feasibility of the proposed method.

A novel optical cryptosystem based on phase-truncated Fresnel diffraction (PTFD) and transport of intensity equation (TIE) is proposed. By using the phase truncation technique, a phase-encoded plaintext could be encrypted into a real-valued noise-like intensity distribution by employing a random amplitude mask (RAM) and a random phase mask (RPM), which are regarded as two secret keys. For decryption, a generalized amplitude-phase retrieval (GAPR) algorithm combined with the TIE method are proposed to recover the plaintext with the help of two keys. Different from the current phase-truncated-based optical cryptosystems which need record the truncated phase as decryption keys, our scheme do not need the truncated phase because of the introducing of the TIE method. Moreover, the proposed scheme is expected to against existing attacks. A set of numerical simulation results show the feasibility and security of the proposed method.

A novel compressive optical cryptosystem based on single-pixel imaging is proposed. By using random sparse sampling in the encryption process, the plaintext is encrypted and compressed into a noise-like intensity sequence. As a result, the ciphertext is much less than the plaintext in volume. For decryption, a convex optimization algorithm is employed to reconstruct the plaintext with the help of the correct key based on the compressive sensing theory. Computer simulations and optical experiments are conducted and a series of results verify the validity and feasibility of the proposed method.

A high capacity steganography based on optical phase-encoding and transport of intensity equation (TIE) is proposed. The secret image is phase-encoded and then attached a real-valued host image to form a complex light field, which diffracts a short distance and generate a slightly different complex amplitude. Its intensity is recorded and the corresponding amplitude is calculated as the stegoimage. For extraction, the secret image could be reconstructed near perfectly by using the TIE method. It is well known that the capacity and the imperceptibility are two fundamental but contrary attributes in any a steganography scheme. However, in this work, the high capacity could be attained while the imperceptibility is well preserved based on the phase-encoding and TIE method. As the steganography is applied mainly on the digital media which are transmitted through the Internet, the optical experiments are not essential in this situation, and we only provide the simulation results to verify the validity and feasibility of our method.