Problem statement: The Hill cipher is the first polygraph cipher which has a few advantagesin data encryption. However, it is vulnerable to known plaintext attack. Besides, an invertible key matrixis needed for decryption. It may become problematic since an invertible key matrix does not alwaysexist. Approach: In this study, a robust Hill algorithm (Hill++) has been proposed. The algorithm is anextension of Affine Hill cipher. A random matrix key, RMK is introduced as an extra key for encryption.An algorithm proposed for involutory key matrix generation is also implemented in the proposedalgorithm. Results: A comparative study has been made between the proposed algorithm and theexisting algorithms. The encryption quality of the proposed algorithm is also measured by using themaximum deviation factor and correlation coefficient factor. Conclusion/Recommendations: Theproposed algorithm introduced a random matrix key which is computed based on the previous ciphertextblocks and a multiplying factor. A modified of Hill Cipher is free from the all-zero plaintext blocksvulnerability. Usage of involutory key for encryption and decryption managed to solve the non invertiblekey matrix problem. It also simplify the computational complexity in term of generating the inverse keymatrix.
