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Applied Optics

Lensless optical encryption with speckle-noise suppression and QR codes

Черёмхин Павел Аркадьевич, Nikolay N. Evtikhiev, Vitaly V Krasnov, Vladislav G. Rodin, I. P. Ryabcev, Anna V. Shifrina, Rostislav S. Starikov

Published Jul 20, 2021• 13 citations
Publisher

Abstract

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.

DOI
10.1364/ao.430968

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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.
Abstract Optical encryption is an actively developing field of science. The majority of encryption techniques use coherent illumination and suffer from speckle noise, which severely limits their applicability. The spatially incoherent encryption technique does not have this drawback, but its effectiveness is dependent on the Fourier spectrum properties of the image to be encrypted. The application of a quick response (QR) code in the capacity of a data container solves this problem, and the embedded error correction code also enables errorless decryption. The optical encryption of digital information in the form of QR codes using spatially incoherent illumination was implemented experimentally. The encryption is based on the optical convolution of the image to be encrypted with the kinoform point spread function, which serves as an encryption key. Two liquid crystal spatial light modulators were used in the experimental setup for the QR code and the kinoform imaging, respectively. The quality of the encryption and decryption was analyzed in relation to the QR code size. Decryption was conducted digitally. The successful decryption of encrypted QR codes of up to 129 × 129 pixels was demonstrated. A comparison with the coherent QR code encryption technique showed that the proposed technique has a signal-to-noise ratio that is at least two times higher.
State of the art micromirror DMD spatial light modulators (SLM) offer unprecedented frame rate of up to 32000 frames per second. This, in conjunction with fast digital camera, makes it possible to create high speed optical encryption systems. Experimental setup for optical encryption under spatially incoherent illumination based on two DMD SLMs had been created. Input information is displayed by first SLM, encoding hologram - by second SLM. Results of optical experiment demonstrate successful encryption and decryption of digital information in the form of QR code image.
We introduce for the first time the concept of an information "container" before a standard optical encrypting procedure. The "container" selected is a QR code which offers the main advantage of being tolerant to pollutant speckle noise. Besides, the QR code can be read by smartphones, a massively used device. Additionally, QR code includes another secure step to the encrypting benefits the optical methods provide. The QR is generated by means of worldwide free available software. The concept development probes that speckle noise polluting the outcomes of normal optical encrypting procedures can be avoided, then making more attractive the adoption of these techniques. Actual smartphone collected results are shown to validate our proposal.
Abstract Exclusive-OR (XOR) operation is a popular technique in optical cryptography. The linear and inverse properties of XOR operation provide a simple way of encryption but in some cases fail to provide strong security. In XOR-based image encryption, each pixel is individually encrypted, therefore secret information can be traced with partial knowledge of the key. In this paper, we describe a simple and practical method to improve the strength and effectiveness of XOR-based encryption. A higher level of security is achieved by implementing a phase retrieval algorithm with XOR operation. Another advantage of the proposed scheme is that the optical XOR operation is demonstrated using incoherent light sources, which make the system compact, speckle free, and cost effective. Also, to overcome the problem of quality degradation while decrypting, quick response code-based ciphertext has been implemented. Both simulation and experimental results are demonstrated as a proof-of-concept.
In recent years, optical information processing has become increasingly important due to its ability to handle large amounts of data efficiently. Amplitude, phase, spatial frequency, wavelength, and polarization are the physical dimensions used for information encoding into the light beam. Information encoding using orbital angular momentum (OAM) carried by a vortex beam is gaining interest in this regard. Such beams provide high-dimensional orthogonal states for encoding and have shown more stability in turbulent media than Gaussian beams, especially in the case of free space propagation. This study demonstrates an image encryption method utilizing an array of OAM light modes. The approach uses a non-interferometric intensity recording-based encoding scheme, which offers ease of implementation. Quick response codes have been utilized for encoding to avoid information loss from non-uniform distribution of light and aberrations. The encryption scheme is highly secure because the encrypted light beam contains only partial information about the object, preventing complete information retrieval without proper key information.