Nihilist Cipher: Encrypt and Decrypt Online Nihilist cipher is a polyalphabetic substitution cipher and a variant of Polybius square. First, the cipher creates a Polybius square for a given key, and then an additional key for creating the ciphertext. It uses an alphabet written into a 5 by 5 block where addresses of each letter are used for the cipher, for example, 23 and 37. The order does not matter but there should be no repetition. Also, the alphabet often starts with the keyword. Use the above Nihilist cipher decoder and encoder to encrypt/decrypt simple messages. If this tool was helpful, tweet it or share it.