Optical Asymmetric Image Encryption Algorithm Research

With the rapid development of computer network technology, the distribution and exchange of information become more quickly and easily. How to prevent unauthorized users from transmitting and modifying important information has become a very important issue. Thus, how to ensure the security of important information has been a very challenging topic. As the image is one of the main carriers of information, image security is a significant topic in information security.Various optical image encryption techniques have been proposed. However, most of the existing methods belong to the symmetric cryptosystem, that is, the function dependency between plaintext and ciphertext is relatively simple, and the decryption key and encryption key are the same, so that the encryption system is potentially insecure against some common attacks. Therefore, it is important to establish an asymmetric encryption system to enhance the anti-attack ability of the encryption system. In addition, compared with the grayscale image, the information of the color image is larger, how to reduce the storage and transmission burden while ensuring the security has become an important research hotspot in the field of color image information security. In this paper,asymmetric encryption schemes based on gyrator transform (GT) for grayscale images and color images are proposed respectively, and asymmetric encryption scheme based on singular value decomposition (SVD) for color images is proposed. The research work is summarized as follows:An asymmetric image encryption method is proposed by using the gyrator transform and vector operation. The original grayscale image is encrypted into two phase masks with vector operation. One phase mask is then encrypted with the proposed encryption algorithm deduced from numerical implementation of GT to obtain the ciphertext, and the other phase mask is used as a phase key. The decryption keys are different from the encryption keys. Numerical simulation results have been shown to demonstrate the efficiency and security of the proposed cryptosystem.A new asymmetric image encryption method is proposed by using the gyrator transform. The red, green and blue components of the original image is modulated by a random phase mask respectively and then combined using convolution. The convolution result is then encrypted with the proposed encryption algorithm deduced from numerical implementation of GT. In this encryption algorithm, the decryption keys are different from the encryption keys, and the ciphertext is gray which can bewilder unauthorized users. Computer simulations have demonstrated its performance and security.A novel asymmetric color image encryption approach by using singular value decomposition (SVD) is proposed. The red, green and blue components are subsequently encoded into a complex function. The SVD is then applied to the complex function and the obtained orthogonal matrices U and V are multiplied to obtain the data matrix of the ciphertext. Diagonal entries of matrices S are ed and scrambling combined to obtain the colormap. The ciphertext is obtained after the colormap is added to the data matrix. In the proposed method,private keys obtained from phase-truncation guarantee the safety, and private keys obtained from the orthogonality of V establish an additional security layer, and the original color image with 3×N×N pixels is encrypted into an indexed image with N×N+N×3 pixels, which decreases the burden of storage and transmission.Numerical simulation results have been shown to prove the efficiency and security of the proposed cryptosystem.