Polybius-square cipher with a keyword-based additive layer.
The Nihilist cipher combines a Polybius square with an additive keyword. Each plaintext letter and each keyword letter are looked up in the same Polybius square and converted to a two-digit row/column pair. The two pairs are added together to form a numeric ciphertext. The 5×5 variant uses letters only (I and J share a cell); the 6×6 variant adds the digits 0–9 so no merging is needed.