Cryptanalyzing an image encryption algorithm based on scrambling and Veginère cipher

Recently, an image encryption algorithm based on scrambling and Veginère cipher has been proposed. However, it was soon cryptanalyzed by Zhang et al. using a method composed of both chosen-plaintext attack and differential attacks. This paper briefly reviews the two attack approaches proposed by Zhang et al. and outlines their mathematical interpretations. Based on these approaches, we present an improved chosen-plaintext attack to further reduce the number of chosen-plaintexts required, which is proved to be optimal. Moreover, it is found that an elaborately designed known-plaintext attack can efficiently compromise the image cipher under study. This finding is confirmed by both mathematical analysis and numerical simulations. The cryptanalyzing techniques developed in this paper provide some insights for designing secure and efficient multimedia ciphers.