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Vigenère Cipher Encoder & Decoder

Encode/decode Vigenère cipher text.

How it works

  • Enter a letter-only key. Each plaintext letter is shifted according to the repeating key; decoding applies the inverse shifts.
  • Spaces and punctuation are preserved and do not consume key letters.

Example

  • Using key LEMON, the classical plaintext ATTACKATDAWN encodes to LXFOPVEFRNHR.

Assumptions and limitations

  • This implementation uses the classical A–Z Vigenère alphabet and ignores non-letter characters when advancing the key.
  • Vigenère is a historical cipher and is not secure by modern standards.

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