Two Novel Multidimensional Affine Variations of the Hill Cipher

Abstract

Two novel symmetric multidimensional affine nested variations of the Hill Cipher are presented. The Hill Cipher is a block polygraphic substitution encryption scheme based on a linear transformation of plaintext characters into ciphertext characters. In the time since Hill first published his encryption scheme, variations, modifications, and improvements of theoretical and practical importance have been published every year indicating that the Hill Cipher is an active area of cryptography research. The first variation presented in this paper incorporated invertible key matrices of orders 2, 4, and 8 such that the matrix values of the <i>2×2</i> matrix rotate positions with each block of characters in a similar manner to the rotating letter wheels of a German Enigma Encoder, then results of the <i>2×2</i> key matrices output are passed to <i>4×4</i> key matrices, and <i>8x8</i> key matrix, <i>4×4</i> key matrices, and rotative-value <i>2×2</i> key matrices. The second variation is configured with invertible key matrices of orders 4, 8, and 16 without rotation of matrix values in a similar manner to the first variation. In both variations, plaintext characters of each block are operated on by exclusive-or (XOR) vectors prior to multiplication with the matrices to create the affine ciphers. Strengths, weaknesses, and other considerations are provided in the discussion. Two proposals are also argued with rationale for a more robust character set for encryption and the increase in modulus that the character set allows, and the possible advantages and disadvantages of affine XOR vectors.