ffee All tools
EN 中文 FR DE

Substitution Cipher

Build a keyed alphabet from a keyword, see the mapping and encode or decode your text.

Keyed substitution cipher

Key alphabet: not built yet.
The keyword letters are written first without repeats, then the rest of the alphabet follows in order, giving a full substitution alphabet.

How to use the Substitution Cipher

  1. Type a keyword to build your own substitution alphabet.
  2. Check the key alphabet shown under the output box.
  3. Encrypt your text, then switch to decrypt to read a message back.

Features

Advertisement

More tools

Caesar Cipher ROT13 Cipher Vigenere Cipher Morse Code Encoder Morse Code Decoder Rail Fence Cipher Bacon Cipher Atbash Cipher Affine Cipher Playfair Cipher Hill Cipher Columnar Transposition

Frequently Asked Questions

Common questions about the free online substitution cipher.

Every letter of the alphabet is replaced by another letter according to a fixed mapping. The key alphabet here comes from a keyword you choose.
The unique letters of the keyword are written first, then the remaining letters of the alphabet follow, giving a full twenty six letter mapping.
Use the same keyword, switch to decrypt mode and paste the ciphertext. The tool looks each letter up in the key alphabet and returns the original.
Yes. The receiver needs the same keyword, and it should be shared through a safe channel, because anyone with the keyword can read the message.
Only weakly. Guessing a keyword is hard, but the cipher still keeps letter frequencies, so longer texts can be broken by frequency analysis.
Only the first appearance of each letter is used, so the keyword zebra gives the same alphabet as zebras, because the second a is ignored.
No. The mapping is built and applied inside your browser, so nothing you type is transmitted or stored anywhere.
Yes. The tool is completely free, works offline once loaded, and has no account, no sign-up and no usage limits.

Last updated: October 10, 2026