PLT-Net: A 2-Layer SP-Network using a Modified Playfair Cipher and the Logistic-Tent Map

The Playfair cipher is a classic digraph substitution cipher. Over the years, it has been modified in many ways to increase its security. Most of its modifications are basically substitution ciphers with limited confusion and diffusion, and are vulnerable to chosen-plaintext attacks, ciphertext-only attacks, and even frequency analysis. In this paper, we introduce a modification of the Playfair cipher, Playfair Logistic-Tent Network (PLT-Net), that uses two 5X19 key tables simultaneously to randomly encrypt the text. The key tables consist of all printable characters. This modification has been used in combination with the chaotic logistic-tent map as a permutation cipher, to create a 2-layer substitution-permutation network with better security, where the encrypted text is scrambled in each layer to break the digraphs. The combination of a substitution cipher and a permutation cipher has produced an algorithm that is free from the drawbacks of both. Cryptanalysis has been performed on the PLT-Net to showcase its strengths. It has better confusion and diffusion, as changing a single character, either in the key or in the plaintext, completely changes the ciphertext.