Abstract. The double random phase encoding (DRPE) scheme, which is based on a 4f optical correlator system, is considered as a reference for the optical encryption field. In this work, we propose a modification of the classical DRPE scheme based on the use of a novel class of structured phase masks, the deterministic phase masks. In particular, we propose to conduct the encryption process by using two deterministic phase masks, which are built from linear combinations of several sub-keys. For the decryption step, the input image is retrieved by using the complex conjugate of the deterministic phase masks, which were set in the encryption process. This new concept of structured masks gives rise to encryption-decryption keys which are smaller and more compact than those required in the classical DRPE. In addition, we show that our method significantly improves the tolerance of the DRPE method to shifts of the decrypting phase mask -when no shift is applied, it provides similar performance to the DRPE scheme in terms of encryption-decryption results-. This enhanced tolerance to the shift, which is proven by providing numerical simulation results for gray-scale and binary images, may relax the rigidity of an encryptiondecryption experimental implementation set-up. To evaluate the effectiveness of the described method, the meansquare-error (MSE) and the peak signal-to-noise ratio (PSNR) between the input images and the recovered images are calculated. Different studies based on simulated data are also provided to highlight the suitability and robustness of the method when applied to image encryption-decryption processes.

Abstract. We propose an optical security technique for image encryption using triple random phase encoding (TRPE). In the encryption process, the original image is first double random phase encrypted. The obtained function is then multiplied by a third random phase key in the output plane, to enhance the security level of the encryption process. This method reduces the vulnerability to certain attacks observed when using the conventional double random phase encoding (DRPE). To provide the security enhancement of the proposed TRPE method, three attack cases are discussed: Chosen-plaintext attacks (CPA), Known-plaintext attacks (KPA) and chosen-ciphertext attacks (CCA). Numerical results are presented to demonstrate feasibility and effectiveness of the proposed method. Compared with conventional DRPE, the proposed encryption method can provide an effective alternative and has enhanced security features against the above-mentioned attacks.

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