Optical image encryption scheme based on quantum s-box and meaningful ciphertext generation algorithm

With the development of quantum computers, difficult computational problems of classical cryptosystems could potentially become solvable. Hence, due to the powerful computing power of quantum computers, all current cryptographic algorithms based on computational complexity may become ineffective. Furthermore, most of the ciphertexts generated by traditional encryption schemes are noise-like, and such ciphertexts are easily suspected by cryptanalysts during transmission. To solve these problems, a novel image encryption scheme is proposed by visually meaningful ciphertext generation and quantum walks (QWs). The proposed scheme is composed of three phases: (I) Permutation, (II) Diffusion, and (III) Visually meaningful ciphertext generation. In phase I, the quantum substitution box (s-box) is firstly constructed by QWs, and the original image is permutated by the quantum s-box to reduce the correlation between adjacent pixels. In phase II, the permutated quantum image is diffused by a key image, which is constructed by the probability matrix generated by the same QWs. In phase III, the diffused image is disguised as a meaningful image to avoid the suspicions of cryptanalysts. Compared to traditional encryption algorithms, the introduction of quantum technology effectively improves the security of the cryptosystem. Compared to previous quantum encryption methods, our algorithm generates meaningful ciphertexts that can further enhance security by preventing cryptanalysts from suspecting noise-like ciphertexts. Thus, our encryption scheme is secure even in the future quantum computer age. Besides, the decryption time of our proposed scheme can also be effectively reduced since the decryption process can be optically implemented, partially. Therefore, real-time scenarios can be satisfied. Simulation results and numerical analysis show the effectiveness of the proposed scheme.