STORY2: A Fully Parameterised, Story-Key Driven Substitution-Permutation Network Cipher

Abstract

We present STORY2, a symmetric block cipher built on a substitution-permutation network (SPN) architecture in which every cryptographic parameter --- including the S-box selection and all round keys --- is derived deterministically from a natural-language narrative supplied by the user as the primary key. This paper introduces a novel 16 × 16 Cauchy Maximum Distance Separable (MDS) diffusion matrix over GF_2⁸. The matrix achieves a branch number of 17, the theoretical maximum for any 16 × 16 linear map over GF_2⁸, and enforces complete 16-byte diffusion in a single pass. The construction is proven invertible by the Cauchy determinant theorem. To our knowledge, no published block cipher employs a full-state 16 × 16 Cauchy MDS matrix as its sole diffusion layer. We also introduce a two-level, privately held S-box pool architecture that functions as an independent secondary security factor: if the story key is disclosed, a private pool whose contents are unknown to the adversary requires a brute-force search of 256! ≈ 2¹⁶⁸⁴ bijections for decryption. Empirical analysis across 2,000 story keys confirms near-ideal statistical behaviour: avalanche effect mean of 50.007%, and SAC convergence at round 1 --- four rounds earlier than an equivalent 4 × 4 column-restricted baseline.