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Applied Optics

Security-enhanced optical cryptosystem using nonergodic scrambling phase mask

Shuaifeng Dou, Xueju Shen, Bing Zhou, Chao Lin, Jie Liu

Published Aug 7, 2020• 1 citation
Publisher

Abstract

Cryptanalysis revealed a security flaw in the double random phase encoding (DRPE) system, and the original image was recovered via a ciphertext-only attack. In this paper, a nonergodic scrambling phase mask (NESPM) is designed, and a security-enhanced cryptosystem is established by replacing the bonded random phase mask (RPM) in the DRPE system with the designed NESPM. The original image in the security-enhanced cryptosystem is optically scrambled at "macropixel" level by the bonded NESPM, instead of being diffused by the RPM, as in the DRPE system. Due to such a scrambling operation, the ergodic property of the Fourier-plane speckle pattern in the designed cryptosystem is removed, and the original image cannot be recovered via the attack. Experimental results demonstrate that the designed cryptosystem is resistant to the attack. Security enhancement is achieved.

DOI
10.1364/ao.400902

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Cryptanalysis revealed a security flaw in the double random phase encoding (DRPE) system, and the original image was recovered via a ciphertext-only attack. In this paper, a nonergodic scrambling phase mask (NESPM) is designed, and a security-enhanced cryptosystem is established by replacing the bonded random phase mask (RPM) in the DRPE system with the designed NESPM. The original image in the security-enhanced cryptosystem is optically scrambled at "macropixel" level by the bonded NESPM, instead of being diffused by the RPM, as in the DRPE system. Due to such a scrambling operation, the ergodic property of the Fourier-plane speckle pattern in the designed cryptosystem is removed, and the original image cannot be recovered via the attack. Experimental results demonstrate that the designed cryptosystem is resistant to the attack. Security enhancement is achieved.
This paper offers a security analysis of an optical image cryptosystem that integrates interference and unequal modulus decomposition (EMD). In contrast to the conventional interference-based cryptosystem, which utilizes a single phase-only mask (POM) derived from the encryption process as the private key, the enhanced cryptosystem employs two random phase masks (RPMs) and two POMs derived from the phase-reserved (PR) structure as private keys. This modification addresses the silhouette problem inherent in the conventional interference-based cryptosystem by expanding the key space, thereby enhancing security strength. However, it is determined that the RPM R 2 ( u , v ) and the POM P 2 ( x , y ) exhibited low key sensitivity, thereby limiting their impact on the security enhancement. Additionally, due to the independence from the plaintext, the RPMs employed as private keys can be recovered using a designed known-plaintext attack (KPA). Once the RPMs are recovered, the number of unknown keys is reduced to two. This enables the recovery of plaintext information from its ciphertexts via a subsequent ciphertext-only attack (COA) without any prior knowledge of the POMs. In light of these findings, a novel hybrid attack strategy, to our knowledge, is proposed, integrating four cascaded KPAs and a COA, to effectively compromise the advanced interference and EMD-based cryptosystem. To validate the feasibility and effectiveness of the proposed hybrid attack, numerical simulations have been conducted.
In this paper, the security strength of an enhanced cryptosystem based on interference and the phase-retrieval technique is evaluated. The security strength of the optical cryptosystem was improved through the phase-retrieval technique used to generate a phase-only mask (POM) as the ciphertext. Due to the complex mathematical model of the phase-retrieval technique, it seems that a silhouette problem existing in the conventional interference-based scheme was removed. However, we noted that the random phase mask (RPM) regarded as the only private key was fixed in the encryption path, which is not related to the plaintext and makes it possible to be recovered using a known-plaintext attack (KPA). Moreover, we also found that the RPM has high key sensitivity, and it should be recovered precisely to retrieve information of plaintexts during the attack. Thus, a hybrid KPA where three pairs of known plaintexts and their ciphertexts are regarded as the amplitude and phase constraints to obtain the precise estimation of the RPM is proposed. Then, with the help of the estimated private key, information of the original plaintexts encoded using the cryptosystem under study could be retrieved from an arbitrarily given ciphertext without any knowledge of the private key. Our cryptoanalysis shows that the cryptosystem based on interference and the phase-retrieval technique is vulnerable to the proposed attack, and there is a security leak in it. Numerical simulations have been carried out to demonstrate the performance of our proposed attack.
Abstract Background: The double random phase encoding techniques have received considerable attention from researchers in recent years because of its advantages of parallel and high speed processing capability. Meanwhile, the security of cryptosystem is also one of the major concerns. Methods : We experimentally demonstrated the ciphertext redundancy vulnerability of the coherent double random encryption system (DRPE). Based on the statistical ergodicity of speckles and the consistency of the power spectral density (ESD), we have proved that the most plaintext information can be retrieved from partial ciphertext alone. Results: In this paper, the simulation and experiment result were performed to verify whether the algorithm is effectiveness. The ciphertext redundancy of the DRPE system is analyzed from the results of ciphertext occlusion test. There is a risk of plaintext leakage of this scheme, as long as the average ESD can be estimated from the sub-images. The results will help to open up deeper understanding of limitation of current optical security techniques. Conclusions: The DRPE system has potential redundancy risk. Even one-time-pad manner is not secured in DRPE system. This vulnerability allows a cryptanalyst to estimate the plaintext information with only a half or less ciphertext.
In this paper, the security of an optical cryptosystem based on double random phase encoding (DRPE) with perfect forward secrecy (PFS) is analyzed for a particular session. In the cryptosystem, the PFS strategy is utilized to enhance the security and key management of the traditional DRPE scheme. Our analysis reveals that the use of PFS has certain advantages in the key management approach, but the method is still vulnerable against modified plaintext attacks when the attack is performed in the same session. Also, it is noted that the method is safe against conventional plaintext attacks, but it is vulnerable to the modified chosen and known plaintext attacks. The original plaintext can be easily retrieved with the proposed attack algorithms. Numerical simulation results are presented to validate the effectiveness of the proposed attack algorithms.
The speckle correlation technique is applied to ciphertext-only attack (COA) on optical cryptosystem based on double random phase encoding. According to the inherent merits of speckle correlation, we have revealed a fact that the ciphertext’s autocorrelation is essentially identical to the plaintext’s own autocorrelation. Then, a plaintext image can be directly reconstructed from the autocorrelation of its corresponding ciphertext by employing a iterate phase-retrieval algorithm. This could then lead to a potential security flaw because an unauthorized user could directly retrieve the plaintext from an intercepted ciphertext by performing proposed COA approach. Meanwhile, a series of numerical simulations will also be provided to verify the validity and feasibility of our proposed COA method.
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.