Substitution Cipher Encoder & Decoder
Encode and decode a simple (monoalphabetic) substitution cipher using your own 26-letter key alphabet, where each letter maps to a fixed replacement.
What is a substitution cipher?
A simple substitution cipher replaces each letter of the alphabet with a fixed different letter. The whole message uses the same mapping, defined by a 26-letter key alphabet, which makes it a monoalphabetic cipher.
How to use it
- Choose Encode or Decode.
- Enter a key alphabet — all 26 letters, each once.
- Type your text and copy the result.
How it works
The key alphabet is the set of letters that plain A, B, C… map to, in order. With the key starting QWERTY…, an A becomes Q, a B becomes W, and so on. Decoding reverses the mapping. There are more than 400 trillion possible keys, though letter-frequency patterns still make long messages crackable — which is exactly what cryptogram puzzles rely on.
How it differs from Caesar and Atbash
Caesar and Atbash are specific fixed substitutions (a shift and a reversal). A general substitution cipher lets you choose any scrambled alphabet you like.
More codes and ciphers
Explore the rest of the family: the Caesar Cipher, Atbash, Vigenère, and A1Z26. For other encodings, try the Morse Code Translator and the Binary Translator.
Free and private
This substitution cipher tool runs entirely in your browser — free, with no sign-up, and nothing you type is uploaded or stored.
Frequently asked questions
- What is a substitution cipher?
- A simple substitution cipher replaces each letter of the alphabet with a fixed different letter, defined by a 26-letter key alphabet. The whole message uses the same mapping, which makes it a monoalphabetic cipher.
- How do I set the key?
- Enter a key alphabet containing all 26 letters exactly once — this is the cipher alphabet that A, B, C… map to in order. Encoding uses that mapping and decoding reverses it.
- How is it different from Caesar or Atbash?
- Caesar and Atbash are specific fixed substitutions (a shift, and a reversal). A general substitution cipher lets you choose any scrambled alphabet, giving far more possible keys.
- Is it free and private?
- Yes — it runs entirely in your browser with no sign-up, and nothing you type is uploaded.
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