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Journal of Optics

Security enhancement of double-random phase encryption by iterative algorithm

Sheng-Xia Qian, Yongnan Li, Ling‐Jun Kong, Simin Li, Zhi‐Cheng Ren, Chenghou Tu, Hui‐Tian Wang

Published Jul 25, 2014• 7 citations
Publisher

Abstract

We propose an approach to enhance the security of optical encryption based on double-random phase encryption in a 4f system. The phase key in the input plane of the 4f system is generated by the Yang–Gu algorithm to control the phase of the encrypted information in the output plane of the 4f system, until the phase in the output plane converges to a predesigned distribution. Only the amplitude of the encrypted information must be recorded as a ciphertext. The information, which needs to be transmitted, is greatly reduced. We can decrypt the ciphertext with the aid of the predesigned phase distribution and the phase key in the Fourier plane. Our approach can resist various attacks.

DOI
10.1088/2040-8978/16/8/085401

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We propose an approach to enhance the security of optical encryption based on double-random phase encryption in a 4f system. The phase key in the input plane of the 4f system is generated by the Yang–Gu algorithm to control the phase of the encrypted information in the output plane of the 4f system, until the phase in the output plane converges to a predesigned distribution. Only the amplitude of the encrypted information must be recorded as a ciphertext. The information, which needs to be transmitted, is greatly reduced. We can decrypt the ciphertext with the aid of the predesigned phase distribution and the phase key in the Fourier plane. Our approach can resist various attacks.
The modified iterative phase retrieval algorithms developed in Part I [Guo et al., Appl. Opt.54, 4698 (2015)] are applied to perform known plaintext and ciphertext attacks on amplitude encoding and phase encoding Fourier-transform-based double random phase encryption (DRPE) systems. It is shown that the new algorithms can retrieve the two random phase keys (RPKs) perfectly. The performances of the algorithms are tested by using the retrieved RPKs to decrypt a set of different ciphertexts encrypted using the same RPKs. Significantly, it is also shown that the DRPE system is, under certain conditions, vulnerable to ciphertext-only attack, i.e., in some cases an attacker can decrypt DRPE data successfully when only the ciphertext is intercepted.
There is growing interest of the successful application of double random phase encoding in optical encryption techniques. Nevertheless, this method has some weaknesses such as the problem of optical axis alignment. A few modified schemes have been proposed in order to improve the misalignment but not for the DRPE. This paper addresses a simulated solution of DRPE scheme based on 4f optically system to improve the shift tolerance of the decrypting key by using deterministic phase masks instead of random phase masks. The main advantage of deterministic phase masks is the compact and tiny size of the keys and they are long enough so that an attacker cannot try any possible combinations. Initial feasibility studies have been undertaken and show a sufficient enhancement to axis alignment as compared to the conventional DRPE. We describe the principle of masks generation and present the simulations results which have been made to evaluate its performance in terms of data loss and shift of the decrypting key in the Fourier plane.
A new nonlinear optical image encryption scheme based on a modified amplitude phase retrieval algorithm is proposed. With two random phase masks that serve as the public encryption keys, one iterative amplitude and phase retrieval process is employed to encode the primary image into a stationary white noise. The two private keys generated in the encryption process are randomly distributed binary matrices to be used to perform one-way binary phase modulations. The proposed encryption process is nonlinear and offers enhanced security. Numerical simulations are presented to demonstrate the feasibility and security of the proposed system. The results also illustrate that the computing efficiency of the algorithm is improved and the number of iterations is much less than that of cryptosystem based on Yang-Gu algorithm, which has two iteration processes.
A phase-only encryption/decryption scheme with the readout based on the zeroth-order phase-contrast technique (ZOPCT), without the use of a phase-changing plate on the Fourier plane of an optical system based on the 4f optical correlator, is proposed. The encryption of a gray-level image is achieved by multiplying the phase distribution obtained directly from the gray-level image by a random phase distribution. The robustness of the encoding is assured by the nonlinearity intrinsic to the proposed phase-contrast method and the random phase distribution used in the encryption process. The experimental system has been implemented with liquid-crystal spatial modulators to generate phase-encrypted masks and a decrypting key. The advantage of this method is the easy scheme to recover the gray-level information from the decrypted phase-only mask applying the ZOPCT. An analysis of this decryption method was performed against brute force attacks.
We propose a new optical encryption technique using the fractional Fourier transform. In this method, the data are encrypted to a stationary white noise by two statistically independent random phase masks in fractional Fourier domains. To decrypt the data correctly, one needs to specify the fractional domains in which the input plane, encryption plane, and output planes exist, in addition to the key used for encryption. The use of an anamorphic fractional Fourier transform for the encryption of two-dimensional data is also discussed. We suggest an optical implementation of the proposed idea. Results of a numerical simulation to analyze the performance of the proposed method are presented.
Abstract Security in optical encryption techniques is a subject of great importance, especially in light of recent reports of successful attacks. We propose a new procedure to reinforce the ciphertexts generated in double random phase encrypting experimental setups. This ciphertext is protected by multiplexing with a ‘salt’ ciphertext coded with the same setup. We present an experimental implementation of the ‘salting’ technique. Thereafter, we analyze the resistance of the ‘salted’ ciphertext under some of the commonly known attacks reported in the literature, demonstrating the validity of our proposal.