Encrypted Hologram Research Articles

Amplitude‐Phase Dual‐Channel Encrypted Acoustic Meta‐Holograms

AbstractEncrypted acoustic meta‐holograms have potential applications in confidential acoustic information communication, noise control, acoustic cloaking, acoustic illusion, etc. Conventional encrypted optical and acoustic meta‐holograms only focus on the encrypted hologram in a single channel, viz. modulating spatial amplitude to project a holographic image. Here, the concept of an amplitude‐phase dual‐channel encrypted acoustic meta‐hologram (DrEAM) is proposed, capable of generating, encrypting, and decrypting both amplitude and phase holographic images. This unique concept of “phase holographic image” introduces a new degree of freedom for meta‐holograms, leading to dual‐channel encrypted acoustic holography. Acoustic metasurface with simultaneous amplitude and phase modulations is employed to realize this dual‐channel encryption. The findings may offer the possibility to increase the capacity of the encrypted acoustic meta‐holograms, which can lead to practical applications in, for example, multi‐channel acoustic field communications, complex noise control, and acoustic illusions.

Terahertz Metasurfaces for Thermally Controlled Optical Encryption

AbstractTerahertz (THz) metasurfaces have emerged as powerful tools to modulate the wavefronts of THz radiation fully. Smart designs and fabrication are essential for enhancing the flexibility and encryption security of THz metasurfaces. In addition to digital coding metasurfaces and microelectromechanical systems, one method to realize dynamic THz metasurfaces is to utilize an active material. In this paper, a dynamic THz metasurface, which is combined with the phase‐change material VO2 and can be thermally controlled to achieve optical encryption, is proposed. Based on the electromagnetically induced transparency effect and by arranging the antennas in advance according to a specific hologram, a secret image can be encoded into the metasurface. At room temperature, the transmitted light field is an irregular light spot with no useful information. If the temperature increases above the phase‐change temperature, the encrypted hologram can be reconstructed. Moreover, owing to the distinct characteristics of VO2, the phase‐change temperature required during decryption is not very high, and the entire process is reversible. It is expected that, in combination with updated processing technology, such metasurfaces can be practically applied to the next generation of optical encryption or optical anticounterfeiting in the future.

Asymmetric cryptosystem based on optical scanning cryptography and elliptic curve algorithm

We propose an asymmetric cryptosystem based on optical scanning cryptography (OSC) and elliptic curve cryptography (ECC) algorithm. In the encryption stage of OSC, an object is encrypted to cosine and sine holograms by two pupil functions calculated via ECC algorithm from sender’s biometric image, which is sender’s private key. With the ECC algorithm, these holograms are encrypted to ciphertext, which is sent to the receiver. In the stage of decryption, the encrypted holograms can be decrypted by receiver’s biometric private key which is different from the sender’s private key. The approach is an asymmetric cryptosystem which solves the problem of the management and dispatch of keys in OSC and has more security strength than the conventional OSC. The feasibility of the proposed method has been convincingly verified by numerical and experiment results.

Iterative multiplane hologram generation with mixed constraint.

In this work, we introduce a mixed complex and phase only constraint to the Gerchberg-Saxton (G-S) algorithm, leading to improvements in the generation of holograms from multiplane light field distributions. To achieve this, we determine the optimal weight factor for the complex and phase only part of a light field in every plane to achieve the best accuracy. We also demonstrate how this approach can be used to generate encrypted holograms that can only be reconstructed by illumination with a determined phase profile. In this way, we enable the possibility for secure, high-quality multiplane projection and display. We show numerical results for the generation of standard and encrypted seven-plane holograms, as well as the comparison with the conventional G-S algorithm.

High-Quality Object Reconstruction From One-Dimensional Compressed Encrypted Signal Based on Multi-Network Mixed Learning

Conventional optical image encryption methods based on phase-shifting interferometry need at least two interferograms, and the storage or transmission of interferograms needs to occupy a lot of resources. At the same time, the low quality of reconstructed complex natural images has always been a main limiting factor in the application of optical image security. In this paper, a high-quality object reconstruction method from one-dimensional compressed encrypted signal based on multi-network mixed learning is proposed. First, an encrypted interferogram can be obtained using the double random phase encoding (DRPE) method. Then, we can obtain the one-dimensional compressed sampling signal of the encrypted hologram on the photodiode using single-pixel compressive holographic imaging method. Finally, the mapping of 1D signal to 2D object image can be learned utilizing multiple neural network models. Numerical simulation results show that the complex natural images can be reconstructed using the proposed method with high quality at lower sampling ratio.

Multiple-image encryption based on angular-multiplexing holography with quick response code and spiral phase keys.

In this research, a novel optical multiple-image encryption method based on angular-multiplexing holography, quick response (QR) code, and spiral phase keys is proposed. With this method, images are transformed into QR codes and subsequently encrypted into a series of encrypted holograms using an angular-multiplexing technique. The encrypted holograms can only be decrypted when the hologram is illuminated with a duplicate of the reference beam and correct fingerprint and spiral phase plate (SPP) keys. The multiplexing performance and key sensitivity of fingerprint and SPP order were both analyzed, showing the high strength of the security of our proposed method.

Secure storage and retrieval schemes for multiple encrypted digital holograms with orthogonal phase encoding multiplexing.

Recent developments in 3D computational optical imaging such as digital holographic microscopy has ushered in a new era for biological research. Therefore, efficient and secure storage and retrieval of digital holograms is a challenging task for future cloud computing services. In this study, we propose a novel scheme to securely store and retrieve multiple encrypted digital holograms by using phase encoding multiplexing. In the proposed schemes, an encrypted hologram can only be accessed using a binary phase mask, which is the key to retrieve the image. In addition, it is possible to independently store, retrieve, and manage the encrypted digital holograms without affecting other groups of the encrypted holograms multiplexed using different sets of binary phase masks, due to the orthogonality properties of the Hadamard matrices with high autocorrelation and low cross-correlation. The desired encrypted holograms may also be searched for, removed, and added independently of other groups of the encrypted holograms. More and more 3D images or digital holograms can be securely and efficiently stored, retrieved, and managed.

An Adaptive and Secure Holographic Image Watermarking Scheme.

A novel adaptive secure holographic image watermarking method in the sharp frequency localized contourlet transform (SFLCT) domain is presented. Based upon the sine logistic modulation map and the logistic map, we develop an encrypted binary computer-generated hologram technique to fabricate a hologram of a watermark first. Owing to the enormous key space of the encrypted hologram, the security of the image watermarking system is increased. Then the hologram watermark is embedded into the SFLCT coefficients with Schur decomposition. To obtain better imperceptibility and robustness, the entropy and the edge entropy are utilized to select the suitable watermark embedding positions adaptively. Compared with other watermarking schemes, the suggested method provides a better performance with respect to both imperceptibility and robustness. Experiments show that our watermarking scheme for images is not only is secure and invisible, but also has a stronger robustness against different kinds of attack.

Asymmetrical and Biometric Encrypted Optical Scanning Holography (ABE-OSH)

In this article, we report an optical scanning holography (OSH) system that is equipped with biometric and the asymmetrical Rivest–Shamir–Adleman (RSA) encryption. Past research works have demonstrated that either encryption alone is vulnerable to certain types of attacks if both the encrypted and source data are available. Essentially, given sufficient time and computation power, it is possible to deduce the encryption key from these two datasets. Our proposed method can be divided into two stages. In the first stage, biometric encryption is incorporated in the OSH system to convert the image of a physical object into a biometric encrypted hologram. In the second stage, the biometric encrypted hologram is encoded with differential pulse code modulation (DPCM), and the polarity of the DPCM data is encrypted with the RSA algorithm. Due to the cascade of these two stages, it is not possible to identify both the source and encrypted data in each stage, and use them to hack the decryption key. We have demonstrated fast capturing and encryption of physical objects with our proposed method, and shown that that when either the biometric or the RSA encryption key is missing, the hologram cannot be recovered and the reconstructed image is incomprehensible. While we have demonstrated our idea with OSH, the proposed technique is applicable to any digital holographic systems.

Multi-section images parallel encryption based on optical scanning holographic cryptography technology

In this paper, the function of parallel encrypting multiple images and reproducing arbitrary layers of images is realized by improving the double pupil function in optical scanning holography. In an optical scanning holography (OSH) system, a dual-pupil heterodyne incoherent image processing technique is used to record holographic images. By adjusting the two pupil functions in the optical system, the interference fringes can be modified to achieve different imaging effects. In this paper, the ring pupil and random phase plate are used to act as two pupil functions to interfere to form a ring random phase plate, and thus realizing the fast scanning of multi-layer images. Then the multi-layer images can be quickly encrypted by one imaging technique. The scanned signals are quickly collected by photoelectric detectors, and they synthesize encrypted holograms by computer. By using the digital holography to decrypt the holograms, the precise reproduction of any layer image can be achieved. The OSH system with random phase pupil is strongly dependent on the longitudinal position of the system in digital reconstruction. The defocusing noise can be converted into random noise and the effect of defocusing layer on imaging can be effectively suppressed. However, in practice, it is necessary to average multiple images to achieve better imaging effect, and the accuracy of random phase plate is required. In this paper, most of the random noise can be filtered with the aid of ring pupil, and all the information about multi-layer graphics can be recorded and reconstructed by one scan. In the process of reconstruction, the influence of defocusing image can be effectively eliminated, and the decryption of any layer image can be realized. This method collects encrypted image by photoelectric detector, and does not need complex algorithm reconstruction nor phase iteration, which greatly reduces the time expended in the encryption process. In the process of encryption, the key space of the system is increased, and the encrypted image obtained has high security. In this paper, correlation coefficient is used to evaluate the encryption effect of this method, and the effectiveness and security of this method are verified by simulation experiments. For cutting resistance, when 75% of the information is lost, the correlation coefficient can still reach more than 0.5. For the sensitivity of information, the integrity of decrypted image will be seriously damaged when the wavelength and distance shift very little. For the anti-noise ability, under the influence of Gauss noise and salt and pepper noise, the correlation coefficient and image recognition degree are high. This method is very time-saving, and the result of encryption has high security, high sensitivity, strong ability to resist clipping and noise.

Optical cryptography with biometrics for multi-depth objects

We propose an optical cryptosystem for encrypting images of multi-depth objects based on the combination of optical heterodyne technique and fingerprint keys. Optical heterodyning requires two optical beams to be mixed. For encryption, each optical beam is modulated by an optical mask containing either the fingerprint of the person who is sending, or receiving the image. The pair of optical masks are taken as the encryption keys. Subsequently, the two beams are used to scan over a multi-depth 3-D object to obtain an encrypted hologram. During the decryption process, each sectional image of the 3-D object is recovered by convolving its encrypted hologram (through numerical computation) with the encrypted hologram of a pinhole image that is positioned at the same depth as the sectional image. Our proposed method has three major advantages. First, the lost-key situation can be avoided with the use of fingerprints as the encryption keys. Second, the method can be applied to encrypt 3-D images for subsequent decrypted sectional images. Third, since optical heterodyning scanning is employed to encrypt a 3-D object, the optical system is incoherent, resulting in negligible amount of speckle noise upon decryption. To the best of our knowledge, this is the first time optical cryptography of 3-D object images has been demonstrated in an incoherent optical system with biometric keys.

Four-image encryption scheme based on quaternion Fresnel transform, chaos and computer generated hologram

A novel four-image encryption scheme based on the quaternion Fresnel transforms (QFST), computer generated hologram and the two-dimensional (2D) Logistic-adjusted-Sine map (LASM) is presented. To treat the four images in a holistic manner, two types of the quaternion Fresnel transform (QFST) are defined and the corresponding calculation method for a quaternion matrix is derived. In the proposed method, the four original images, which are represented by quaternion algebra, are processed holistically in a vector manner by using QFST first. Then the input complex amplitude, which is constructed by the components of the QFST-transformed plaintext images, is encoded by Fresnel transform with two virtual independent random phase masks (RPM). In order to avoid sending entire RPMs to the receiver side for decryption, the RPMs are generated by utilizing 2D–LASM, which results that the amount of the key data is reduced dramatically. Subsequently, by using Burch’s method and the phase-shifting interferometry, the encrypted computer generated hologram is fabricated. To improve the security and weaken the correlation, the encrypted hologram is scrambled base on 2D–LASM. Experiments demonstrate the validity of the proposed image encryption technique.

Multiple information encryption by user-image-based gyrator transform hologram

A novel multiple information encryption by user-image-based gyrator transform hologram is proposed. In encryption process, each channel of the user image is phase encoded, modulated by random phase function and then gyrator transformed to get the gyrator spectrum of user image. Subsequently, each channel of the secret image is normalized, phase encoded, multiplied by modulated user image, and then gyrator transformed to obtain the gyrator spectrum of secret image. The encrypted digital hologram is recorded by the interference between the gyrator spectrum of user image and the spherical wave function. Similarly, the digital hologram for decryption is recorded by the interference between the gyrator spectrum of secret image and the spherical wave function. The multiple encrypted digital holograms are multiplexed into a final encoded hologram and the corresponding digital holograms for decryption are multiplexed into a final hologram for decryption. The wavelength and radius of the spherical wave function, and angle of gyrator transform are all essential keys for decryption. The proposed system has two main features. First, the encrypted hologram has no information about secret image. Second, the hologram for decryption used as identification key. Consequently the two marked security layers of information protection are achieved. The proposal can be realized by optoelectronic system. Numerical simulation results demonstrate the feasibility and security of the proposed technique.

Compressive optical image encryption with two-step-only quadrature phase-shifting digital holography

An image encryption method which combines two-step-only quadrature phase-shifting digital holography with compressive sensing (CS) has been proposed in the fully optical domain. An object image is firstly encrypted to two on-axis quadrature-phase holograms using the two random phase masks in the Mach–Zehnder interferometer. Then, the two encrypted images are highly compressed to a one-dimensional signal using the single-pixel compressive holographic imaging in the optical domain. At the receiving terminal, the two compressive encrypted holograms are exactly reconstructed from much less than the Nyquist sampling number of observations by solving an optimization problem, and the original image can be decrypted with only two reconstructed holograms and the correct keys. This method largely decreases holograms data volume for the current optical image encryption system, and it is also suitable for some special optical imaging cases such as different wavelengths imaging and weak light imaging. Numerical simulation is performed to demonstrate the feasibility and validity of this novel image encryption method.

基于计算全息的强鲁棒性自适应水印算法

An adaptive watermarking algorithm based on computer generated holography with strong robustness is proposed.Firstly,two original watermarks are converted to holograms using the computer generated holography by the conjugate-symmetric extension.And the holograms are encrypted by the pre-process method.Then,the original image is decomposed into two pyramidal levels by Contourlet transform,and the intermediate frequency is transformed into eight directional subbands.Finally,encrypted holograms are embedded into the directional subbands adaptively.Experiments demonstrate that the PSNR of watermarked image is 38.385 7dB.The extracted watermarks can be well recognized when JPEG compression quality factor is bigger than 30 and cropping part is less than30%.The proposed method also can well resist attacks such as Gaussian noise,salt pepper noise,speckle noise,Gaussian filtering,median filtering and average filtering.And the watermarks extracted from print-scan experiment can be well recognized.In the phase of watermark extraction,original image is not needed.The watermarking technology proposed can be widely used in copyright protection of digital and printed images.

Multiple images encryption based on Fourier transform hologram

A new optical encryption method is proposed in this paper to achieve multiple images encryption. We introduce reference waves with different incident angles and random amplitude masks into a Fourier transform hologram configuration to encrypt multiple images. In the encryption procedure, different random amplitude masks (RAMs) which are placed into the reference arm vertically admit the multiplexing capability. When decrypting one of the original images, reference wave with the same incident angle as encrypting the target image is used to illuminate the encrypted hologram with the insertion of random amplitude mask whose transmissivity is reciprocal of that of the encrypting random amplitude mask in the reference arm. We also simulate and analyze the influence of partly wrong decrypting key on the decrypted results. Numerical simulation proves that the proposed encryption method is valid and of high security level.

Fast numerical generation and encryption of computer-generated Fresnel holograms

In the past two decades, generation and encryption of holographic images have been identified as two important areas of investigation in digital holography. The integration of these two technologies has enabled images to be encrypted with more dimensions of freedom on top of simply employing the encryption keys. Despite the moderate success attained to date, and the rapid advancement of computing technology in recent years, the heavy computation load involved in these two processes remains a major bottleneck in the evolution of the digital holography technology. To alleviate this problem, we have proposed a fast and economical solution which is capable of generating, and at the same time encrypting, holograms with numerical means. In our method, the hologram formation mechanism is decomposed into a pair of one-dimensional (1D) processes. In the first stage, a given three-dimensional (3D) scene is partitioned into a stack of uniformed spaced horizontal planes and converted into a set of hologram sublines. Next, the sublines are expanded to a hologram by convolving it with a 1D reference signal. To encrypt the hologram, the reference signal is first convolved with a key function in the form of a maximum length sequence (also known as MLS, or M-sequence). The use of a MLS has two advantages. First, an MLS is spectrally flat so that it will not jeopardize the frequency spectrum of the hologram. Second, the autocorrelation function of an MLS is close to a train of Kronecker delta function. As a result, the encrypted hologram can be decoded by correlating it with the same key that is adopted in the encoding process. Experimental results reveal that the proposed method can be applied to generate and encrypt holograms with a small number of computations. In addition, the encrypted hologram can be decoded and reconstructed to the original 3D scene with good fidelity.

Deterministic phase encoding encryption in single shot digital holography

We demonstrate a deterministic phase-encoded encryption system based on the digital holography and adopted a lenticular lens array (LLA) sheet as a phase modulator. In the proposed scheme the holographic patterns of encrypted images are captured digitally by a digital CCD. This work also adopt a novel, simple and effective technique that is used to suppress numerically the major blurring caused by the zero-order image in the numerical reconstruction. The decryption key is acquired as a digital hologram, called the key hologram. Therefore, the retrieval of the original information can be achieved by multiplying the encrypted hologram with a numerical generated phase-encoded wave. The storage and transmission of all holograms can be carried out by all-digital means. Simulation and experimental results demonstrate that the proposed approach can be operated in single procedure only and represent the satisfactory decrypted image. Finally, rotating and shifting the LLA is applied to investigate the tolerance of decryption to demonstrate the feasibility in the holographic encryption, as well as can also be used to provide the higher security.

Fast Optimization of Binary Encrypted Hologram Based on Error Correction Method in Optical Security Systems

In practical optical security systems we must consider various circumstances for reading and decrypting encrypted holograms. Binarization of the hologram is best suited for such applications because of the ease of handling encrypted data. However, the decrypted image is greatly degraded by binarization. Therefore, optimization of a binary hologram is essential in using such a technique. In this paper, we propose a fast optimization method of a binary encrypted hologram to obtain a good reconstruction based on the error correction algorithm. In the proposed method, multiple pixels of the binary hologram are simultaneously flipped for the optimization according to the priority for the correction. The time for the optimization is only 3% of that of the simulated annealing method.

Optimization of Binary Hologram Degraded by Periodic Lattice Structure of Liquid-Crystal Device Panel in Real Optical Security Systems

A numerically generated encryption pattern in practical optical security systems is processed through real display devices such as electronically addressed spatial liquid-crystal devices (LCDs). The pattern to be encrypted must be therefore congenial with electronic interfaces. In usual fact, the quality of a decrypted image in a practical system is greatly degraded due to the mismatch between the desired encryption pattern and the generated pattern without consideration of the device structures, such as lattice structures of LCDs. We take into account lattice structures for the displays of encryption and key patterns in real optical security systems and apply a simulated-annealing like method for the optimization of an encrypted binary hologram. We successfully demonstrate the decryption of holograms by this method.