Security enhancement of color image cryptosystem by optical interference principle and spiral phase encoding

Tailored dual polarization encryption-coherence modulation-based decryption scheme for a predefined uniformly distributed noisy output image

In this paper, we propose a tailored dual polarization encryption scheme (T-DPES) based on a polarization encoding of input target images into a same predefined uniformly distributed noisy output image, using Mueller-Stokes's formalism. The optical encryption setup, based on a polarizer and a pixelated polarizer, uses a random image key as a first key and a pixelated polarizer as a second key. This key depends on the input image, the image key and the predefined uniformly distributed noisy output image. The decryption process is carried out, for the first time to our knowledge, optically by combining the coherence modulation to the DPES scheme. The performance evaluation and the crypto-analyze of T-DPES show their strength and robustness against attacks, thus overcoming limitations of previous optical encryption techniques.

Cryptoanalysis on optical image encryption systems based on the vector decomposition technique in the Fourier domain

In this paper, the security of optical cryptosystems based on the vector decomposition technique in the Fourier domain is analyzed. Compared to the conventional cryptosystem based on the equal modulus decomposition (EMD) technique, an additional EMD structure is introduced in the cascaded EMD-based cryptosystem; hence, the mask including the phase information of the Fourier spectrum is further encoded in the second EMD structure to enhance the security level. However, it is shown that the number of the private keys has not been increased in the cascaded EMD-based cryptosystem, which makes it possible to crack the cascaded EMD-based cryptosystem. Therefore, a chosen-plaintext attack (CPA) and a special attack with an arbitrarily given private key are proposed to retrieve information from encoded images obtained by the cascaded EMD-based cryptosystem. In addition, the security of the cryptosystem based on the random modulus decomposition (RMD) technique is also analyzed. Compared to the EMD-based cryptosystem in which the Fourier spectrum is decomposed into two vectors with equal moduli, the security level of the cryptosystem has been improved by using the RMD technique to decompose the spectrum into vectors with unequal moduli to decrease the number of the amplitude constraints. However, it is found that the arbitrarily given ciphertext provides the attackers enough information to retrieve the precise information of the plaintext without any knowledge of the private keys. A special attack is proposed to crack the RMD-based cryptosystem. This is the first time to report that these two cryptosystems based on the vector decomposition technique are attacked successfully. Numerical simulation is conducted to validate the feasibility and effectiveness of the proposed attacks.

Ciphertext-only attack on optical cryptosystem with spatially incoherent illumination: from the view of imaging through scattering medium

Security analysis is important and necessary for a new cryptosystem. In this paper, we evaluate the security risk of the optical cryptosystem with spatially incoherent illumination from the view of imaging through scattering medium and then demonstrate that it is vulnerable to ciphertext-only attack. The proposed ciphertext-only attack method relies on the optical memory effect for speckle correlations, which reveals a fact that the ciphertext’s autocorrelation is essentially identical to the plaintext’s own autocorrelation. Furthermore, by employing of an improved dynamic hybrid input-output phase-retrieval algorithm, we show that a plaintext image can be directly reconstructed from the autocorrelation of its corresponding ciphertext without any prior knowledge about the plaintext or the phase keys. Meanwhile, the theory analysis and experiment results will also be provided to verify the validity and feasibility of our proposed ciphertext-only attack method. To the best of our knowledge, this is the first time to report optical cryptanalysis from the point of view of imaging through scattering medium and we believe this contribution will open up an avenue to deepen the investigation of optical cryptosystems.

Polarization-controlled images based on double-exposure polarization holography in an azobenzene liquid-crystalline polymer

Polarization-controlled optical image operations were demonstrated based on the double-exposure polarization holographic method. Two images were stored in the same volume of an azobenzene liquid-crystalline polymer by recording superimposed holograms, a pair of polarization gratings with one spatial frequency, using two orthogonal circularly polarized 532 nm beams and were reconstructed with a 650 nm laser. The recorded polarization holographic gratings were investigated to show distinctive polarization selectivity, high diffraction efficiency, and good stability. The brightness and the polarization of the diffracted images were found to be dependent on the polarization of the readout beam, and two images could be reconstructed individually or simultaneously.

Optical interference-based multiple-image encryption using spherical wave illumination and gyrator transform

A new optical interference-based multiple-image encryption using spherical wave illumination and gyrator transform is proposed. In this proposal, each secret color image is divided into normalized red, green, and blue component images and independently encoded into corresponding phase-only component images. Then each phase-only component image of all the images are combined together to produce a single-phase-only component image as an input component image, which is bounded with a random phase mask to form a complex image. The two phase-only masks are analytically obtained from the inverse Fourier transformation of the complex image. The host image is chosen as the first phase-only mask, and the complex image hidden in the host image is regarded as the second phase-only mask. The spherical wave is generated to simultaneously illuminate phase-only masks. Then two modulated masks are gyrator transformed. The corresponding transformed images are phase truncated to obtain encrypted images and amplitude truncated to construct decryption keys. The decryption keys, angles of gyrator transform, wavelength and radius of the spherical wave, and individual decryption keys for authorized users are sensitive keys, which enhance the security layers of the system. The proposed system can be implemented by using optoelectronic architecture. Numerical simulation results demonstrate the flexibility of the system.

Double-image encryption without information disclosure using phase-truncation Fourier transforms and a random amplitude mask

We present a study about information disclosure in phase-truncation-based cryptosystems. The main information of the original image to be encoded can be obtained by using a decryption key in the worst case. The problem cannot be thoroughly solved by imaginary part truncating, keeping the encryption keys as private keys, or applying different phase keys for different plaintexts during each encryption process as well as the phase modulation in the frequency domain. In order to eliminate the risk of unintended information disclosure, we further propose a nonlinear spatial and spectral encoding technique using a random amplitude mask (RAM). The encryption process involving two security layers can be fully controlled by a RAM. The spatial encoding of the plaintext images and the simultaneous encryption of the plaintext images and the encryption key greatly enhance the security of system, avoiding several attacks that have cracked the phase-truncation-based cryptosystems. Besides, the hybrid encryption system retains the advantage of a trap door one-way function of phase truncation. Numerical results have demonstrated the feasibility and effectiveness of the proposed encryption algorithm.

Optical asymmetric cryptography based on elliptical polarized light linear truncation and a numerical reconstruction technique

We demonstrate a novel optical asymmetric cryptosystem based on the principle of elliptical polarized light linear truncation and a numerical reconstruction technique. The device of an array of linear polarizers is introduced to achieve linear truncation on the spatially resolved elliptical polarization distribution during image encryption. This encoding process can be characterized as confusion-based optical cryptography that involves no Fourier lens and diffusion operation. Based on the Jones matrix formalism, the intensity transmittance for this truncation is deduced to perform elliptical polarized light reconstruction based on two intensity measurements. Use of a quick response code makes the proposed cryptosystem practical, with versatile key sensitivity and fault tolerance. Both simulation and preliminary experimental results that support theoretical analysis are presented. An analysis of the resistance of the proposed method on a known public key attack is also provided.

Information authentication system using interference of two beams in gyrator transform domain

A novel information authentication system, i.e., an asymmetric optical interference of two beams in the gyrator transform (GT) domain, is proposed. In this algorithm, the input color image is divided into R, G, and B channels. The complex field of each channel is constructed by the inverse Fourier transform of the channel attached with a random phase function. The phase-only mask (POM) and amplitude-only mask (AOM) are analytically derived from the complex field. The POM and AOM are separately gyrator transformed. The two asymmetric phase keys and two encrypted images are obtained by the amplitude truncations and phase truncations of the transform images, respectively. Finally, the encoded image is produced by the interference of two encrypted images. The two asymmetric keys and two angle parameters of the GT are regarded as additional keys for each channel to enhance the security level of the cryptosystem. The noniterative authentication procedure is devoid of any silhouette problem. The proposed system can be implemented digitally or optically, and its architecture is free from optical misalignment problems. The theoretical analysis and numerical simulation results both validate the proposed technique.