A Hill-PlayFair Hybrid Based on High order Matrices and 16 x 16 PlayFair tables

Cryptographic systems are either symmetric or asymmetric. Symmetric cryptographic systems use one common key for both encryption and decryption. Asymmetric cryptographic models use different keys, one for encryption and another for decryption. A product cipher is a hybrid cipher built from combining two or more ciphers. A product cipher built from the combination of symmetric and asymmetric ciphers, supposedly is much stronger than the ciphers used to build it. This paper explores the creation of a product cipher built from the combination of the Hill (asymmetric) cipher and the Playfair (symmetric) cipher after some modification to each. The Hill cipher commonly use matrix keys to encrypt the 26 letters of the alphabet through substitution by other alphabetic characters. In this paper, we propose the use of dynamically generated high order matrix keys to encrypt the 256 ASCII characters. Determining the inverse matrices of higher order matrix keys is a complex task to many, making brute force attack harder. On the contrary, the Playfair model traditionally uses a 5 x 5 table to encrypt 25 alphabetic characters, reading I and J as one character. We propose a revised Playfair model which supports the 256 characters by using a 16 x 16 table. An experiment was administered to evaluate the performance of the hybrid model with respect to RAM demands and CPU time. Results indicate that the performances of the product cipher are better than the sum of the performances of its building blocks.