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Optics Letters

Optical image encryption via ptychography

Yishi Shi, Tuo Li, Yali Wang, Qiankun Gao, Sanguo Zhang, LI Hai-fei

Published Apr 23, 2013• 216 citations
Publisher

Abstract

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.

DOI
10.1364/ol.38.001425

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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.
An optical cryptosystem via Fourier ptychography with double random phase masks is proposed. The encryption process cannot be precisely simulated except by optical experiment due to vignetting, which is space variant and can act as a one-way function from the perspective of pure optics and improve the security of our system. In addition, the encryption for a high-resolution, large field-of-view, and complex-valued image is achievable. Optical experiments are presented to prove the validity and security of the proposed system. Our method will give more insight into separating optical cryptography from computer cryptography in nature.
Optical cryptography has attracted extensive interest because of the inherent nature of parallel and multidimensional capability of optical information processing compared with computer cryptography. However, the linear space-invariant (LSI) cryptosystems are easy to be simulated and may be vulnerable to different attacks. To resist attacks, several phasetruncated Fourier transforms based asymmetric cryptosystem are proposed to utilize the nonlinear operations in the LSI system, but they are proved to be vulnerable due to the inherent nature of LSI system. Notice that several works misunderstand the concept of nonlinear operations to the nonlinear systems. But the nonlinear systems are not easy to be achieved. Herein, an optical cryptosystem based on Fourier ptychography (FP) with double random phase masks is proposed. The encryption process cannot be precisely simulated but only by optical experiment due to the vignetting effect, which is linear space-variant (LSV) and can act as an one-way function from the perspective of optics purely and guarantee the security of our system. In addition, the encryption for a high resolution, large field-of-view and complexvalued image is achievable. Optical experiments are presented to prove the validity and the security of the proposed system. Our method would give more insights to separate the optical cryptography from computer cryptography in nature.
The principle of ptychography is applied in known plain text attack on the double random phase encoding (DRPE) system. We find that with several pairs of plain texts and cipher texts, the model of attack on DRPE can be converted to the model of ptychographical imaging. Owing to the inherent merits of the ptychographical imaging, the DRPE system can be breached totally in a fast and nearly perfect way, which is unavailable for currently existing attack methods. Further, since the decryption keys can be seen as an object to be imaged from the perspective of imaging, the ptychographical technique may be a kind of new direction to further analysis of the security of other encryption systems based on double random keys.
A new scheme for parallel optical decryption and the display of encrypted image information is presented. The scheme is based on a common path interferometer configuration providing a simple and robust optical setup. Images are encrypted directly during recording by use of a combined phase encoding and phase scrambling method. The encoding and encryption does not require sophisticated, iterative and time consuming optimization algorithms. Pixels are independently encoded and encrypted by use of a simple look-up table technique that can be performed in milliseconds on a standard personal computer. Optical decryption can subsequently be implemented in the common path interferometer by use of single phase or if desired a combined phase/amplitude key. An advantage of the presented method is that the encrypted image may selectively require recording and decryption of phase values or amplitude values or a combination thereof. Another advantage is that decryption is performed in a plane adjacent to the encrypted mask or an equivalent plane whereby generation of speckles in the decrypted image is strongly suppressed. Finally, there is no requirement of positioning a decrypting mask or spatial light modulator in the optical Fourier plane whereby an accurate three dimensional positioning requirement can be avoided.
We report a simultaneous image compression and encryption scheme based on solving a typical optical inverse problem. The secret images to be processed are multiplexed as the input intensities of a cascaded diffractive optical system. At the output plane, a compressed complex-valued data with a lot fewer measurements can be obtained by utilizing error-reduction phase retrieval algorithm. The magnitude of the output image can serve as the final ciphertext while its phase serves as the decryption key. Therefore the compression and encryption are simultaneously completed without additional encoding and filtering operations. The proposed strategy can be straightforwardly applied to the existing optical security systems that involve diffraction and interference. Numerical simulations are performed to demonstrate the validity and security of the proposal.
A simultaneous optical triple-image encryption and verification method based on sparse information strategy was proposed. The information of the encoded images could be integrated into a sparse representation. Only sparse (partial) data from the encrypted information were adopted for verification process. In sparse strategy, the partial data was extracted from the encrypted distribution. For complex-valued distribution, the sparse strategy was applied to the amplitude; thus, only the sparse amplitude pixels kept the phase data for verification process. Therefore, the proposed method can effectively facilitate optical information transmission and storage, as only part of the encrypted information was adopted. In addition, the simultaneous encryption and authentication verification of three images was achieved. The results proved that the proposed method was feasible and effective, and can provide an additional security layer for optical security systems. Moreover, the method that demonstrated secure optical triple-image can lead to a high flexibility and variety compared to that of photon counting technique which required a specific device.