H-S-X Cryptosystem and Its Application to Image Encryption

Information security is an important issue. Today´s encryption technologies can be traced back to the earliest ciphers, and have grown as a result of evolution. The first ciphers were cracked, so new, stronger ciphers emerged. Code breakers set to work on these and eventually found flaws, forcing cryptographers to invent better ciphers and so on. Hill Cipher is one of the most famous symmetric cryptosystem that can be used to protect information from unauthorized access. Hill cipher is a polygraph substitution cipher based on linear algebra. it was the first polygraph cipher which was practical to operate on more than three symbols at once, Hill Cipher has many advantages in data encryption. First, it is resistant to the frequency letter analysis. It´s also very simple since it uses matrix multiplication. Finally, it has high speed and high throughput. However, noninvertible key matrix over Z_m is the main disadvantage of Hill Cipher, because few of the matrices have inverses over Z_m. This means that the encrypted text can´t be decrypted. Moreover, Hill cipher algorithm cannot encrypt images that contain large areas of a single color. Thus, it does not hide all features of the image which reveals patterns in the plaintext. Moreover, it can be easily broken with a known plaintext attack revealing weak security. The objective of this paper is to encrypt an image using a technique different from the conventional one. In this paper, a novel encryption technique has been proposed which we name H-S-X (Hill-Shift-XOR) encryption. The scheme is relatively slow but quite reliable technique where cryptanalysis is quite difficult. It also injects more diffusion and confusion which are the two important attributes of a powerful encryption technique. A comparative study of the proposed encryption scheme and the existing Hill cipher scheme is made. The output encrypted images reveal that the proposed technique is quite reliable and robust.