Secure Hybrid Cryptosystem Based on Playfair Cipher Using 16×16 Key Matrix and XOR Operation

Abstract

One of the most popular and commonly used substitution cipher is the Playfair Cipher. Here, the cryptosystem uses a 5×5 key matrix to encrypt/decrypt data. However, this approach has some main drawbacks where it encrypts/decrypts only uppercase letters. As well as, it produces the same ciphertext when a slight modification applied the key.  Therefore, this paper presents a hybrid cryptosystem to overcome the flaws of the traditional Playfair cipher and to enhance the security of the overall system. So, it proposes hybrid cryptosystem of a modified Playfair cipher (MPC) with XOR cryptosystem based SHA-512 algorithm. Where, the modified Playfair cipher can deal with all characters in the Unicode range which is between 0 and 65535. While, combining the MPC with XOR cryptosystem will make the avalanche effect as high as possible and consequently, the frequency analysis tables of the original plaintext language will be more complicated. A C# program has been used to implement and evaluate the proposed hybrid cryptosystem using Different file sizes. The Experimental results have revealed that the proposed hybrid cryptosystem has better performance in terms of cracking probability as compared to the existing algorithms in related works.

Keywords: Playfair Cipher, Cryptography, SHA-512 Algorithm, Avalanche Effect, Cracking probability.