Vigenère Cipher
Classic polyalphabetic cipher encode/decode with a key.
—The Vigenère cipher shifts each letter by the corresponding key letter (A=0, B=1, …). Non-letters pass through untouched.
About the Vigenère Cipher
Encode and decode messages with the classic Vigenère polyalphabetic cipher. Each letter of your text is shifted by the corresponding letter of a repeating keyword — A shifts by zero, B by one, and so on — while digits and punctuation pass through unchanged. Switch between encode and decode with one dropdown.
How to use the Vigenère Cipher
- Enter a keyword in the Key field — letters only, for example SECRET.
- Pick Encode or Decode from the Mode dropdown.
- Type your message into the text area and read the Result box.
- Use the same key in the opposite mode to reverse the operation.
Vigenère Cipher FAQ
What key should I use?
Any word of letters works, and longer keys are harder to break. Non-letter characters in the key are ignored, and a key with no letters at all leaves the text unchanged with a warning.
How is the cipher applied?
Each text letter is shifted forward by the alphabet position of the next key letter (A=0, B=1, and so on), wrapping around the key as needed; decoding shifts backward by the same amount.
Is this secure for real secrets?
No. The Vigenère cipher is centuries old and broken by routine cryptanalysis — use it for puzzles, games and teaching, and a modern option like AES for genuine confidentiality.