What two keys does the Nihilist cipher use?

It uses a keyword to scramble the 5×5 Polybius square and a second additive keyword. The square turns letters into two-digit coordinates, and the additive keyword's own coordinates are added to them.

Why are I and J combined?

A 5×5 square has only 25 cells but the alphabet has 26 letters. The standard fix is to merge I and J into one cell, so the letter J is encoded and decoded as I.

How is the keyword added?

Both the message and the keyword are first converted to Polybius numbers. The keyword numbers repeat across the message, and each is added to the matching message number using ordinary carrying addition, so sums can reach three digits.

Why are the ciphertext numbers separated by spaces?

Because adding the keyword can carry into the hundreds, the resulting numbers have a variable number of digits. Spacing them keeps each group distinct so the decoder can subtract correctly.

Is the Nihilist cipher secure?

No. It is a classical additive cipher and can be broken with enough ciphertext, especially because the additive keyword repeats. It is studied for history and puzzles and as the conceptual ancestor of the much stronger VIC cipher.

Nihilist Cipher Encoder & Decoder

The Nihilist cipher was used by Russian revolutionaries in the 1880s. It combines a Polybius square — which converts letters to numbers — with an additive keyword, producing a numerical ciphertext. It is an early example of layering substitution and addition, an idea that culminated decades later in the Soviet VIC cipher.

How it works

First a keyed Polybius square is built: write the unique letters of a keyword into a 5×5 grid (merging I and J), then fill the rest of the alphabet. Every letter is now identified by its row and column, both numbered 1–5, giving a two-digit number from 11 to 55.

Encryption then proceeds in three steps:

Convert the plaintext letters to their Polybius numbers.
Convert the additive keyword’s letters to Polybius numbers and repeat them across the message.
Add each keyword number to the matching plaintext number using normal carrying addition:

C = P + K

Decryption subtracts the same keyword numbers, P = C - K, and looks each result back up in the square. Because the addition carries, sums can reach into the hundreds, so the ciphertext numbers are written space-separated.

Example

With square keyword ZEBRAS and additive keyword RUSSIA, the message DYNAMITE WINTER PALACE enciphers to:

37 106 62 36 67 47 59 63 74 53 73 60 26 65 64 36 66 30 36 63

Subtracting the repeating RUSSIA coordinates from each group and reading the square recovers the original message.

Notes

The letter J is treated as I in both directions because they share a square cell.
Security depends heavily on the additive keyword length; a short repeating key leaks structure to cryptanalysts, just as it does in Vigenère.
The Nihilist family inspired the Soviet VIC cipher, which replaces the simple square with a straddling checkerboard and adds transposition for far greater strength.