Double Encryption of Grayscale Images Using Hill Cipher with Self-Invertible Keys

Abstract

The Hill Cipher (HC) algorithm is a symmetric technique with a simple structure and fast computations; however, it has a low level of security because both parties need to use and share the same private key through an insecure channel. In this study, a double encryption technique for grayscale images has been proposed by combining the HC with self-invertible (SI) keys. In this method, the encryption and decryption processes are carried out using a key matrix that is SI, where the elements of the key are Integer Matrices with Integer Eigenvalues (IMIE). To assess the quality and resilience of the encryption, a thorough analysis of the grayscale image that has been encrypted twice is performed using metrics such as Entropy, Peak Signal-to-Noise Ratio (PSNR), and Unified Average Changing Intensity (UACI). As a result, the transmission of grayscale images through this new variant of the HC is found to be competitive in terms of security and efficiency compared to several existing encryption techniques, where the proposed method achieves an entropy of 7.9947, PSNR of 8.5230, and UACI of 30.0486\% for the double encrypted image, demonstrating its competitiveness compared to existing techniques.