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Nihilist Cipher

Encrypt or decrypt numeric Nihilist ciphertext with a keyed Polybius square and repeated additive key.

Learn how the Nihilist cipher works

Plaintext Input

Letters 0 Chars 0

Numeric Ciphertext

Status Valid Numbers 0 Chars 0

Rule: cipher number = plaintext coordinate + key coordinate. Classical 5×5 squares merge I/J.

Type text or paste ciphertext numbers to see the first eight calculations.

Cipher Properties

Type: Polybius + Additive Alphabet: 25 cells I/J: Combined Key: Repeated Mode: Encrypt

Default check: ATTACK AT DAWN → 29 96 89 59 52 47 / 29 96 / 66 59 84 50.

Using This Tool: Guide & Notes Show guide

The Nihilist cipher adds Polybius coordinates from a repeated key to the coordinates of each plaintext letter.

How to use it

  • Choose Encrypt or Decrypt.
  • Set the Polybius square keyword and the repeated additive key.
  • Enter plaintext letters for encryption or space-separated ciphertext numbers for decryption.
  • Check the square and calculation preview before copying the result.

Options and settings

  • The square keyword removes repeated letters and fills the remaining 5x5 square alphabetically.
  • The additive key is converted through the same square and repeated across message letters.
  • Mark word breaks writes plaintext spaces as slashes so they can be restored during decryption.
  • Grouping inserts a visual divider after a chosen number of ciphertext values without changing the cipher.
  • The separator controls how numeric values are displayed; digits and slashes are reserved and cannot be separators.

Notes

  • Classical 5x5 squares merge I and J, so decrypted J characters return as I.
  • Nihilist is a historical hand cipher for learning and puzzles, not secure modern encryption.
  • Decryption requires the same square keyword and additive key used for encryption.
See all tools
Bifid Cipher Mix Polybius square coordinates in fixed-size blocks to create a fractionating classical cipher. ADFGX Cipher Use a keyed ADFGX square followed by keyed columnar transposition. ADFGVX Cipher Use a keyed ADFGVX square with letters and digits, then apply keyed columnar transposition. One-Time Pad / Vernam Cipher Encrypt or decrypt letters with an equal-length key, securely generate a random pad in your browser, and inspect every modulo-26 step.
Related Article

Nihilist Cipher: Polybius Squares, Repeated Keys, and Number Codes

How a simple 5×5 letter square became a numerical cipher with two keys, repeated addition, and a revolutionary history.

Read the article
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A note from the team

We build these tools because we love cryptography, encoding, and making difficult ideas easier to explore. If they've helped you, even a small coffee means the world to the project. Thank you!

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