Vigenère Cipher
Encrypt and decrypt text with a keyword using the classic polyalphabetic Vigenère cipher.
Encrypt with the Vigenère cipher
The Vigenère Cipher tool encrypts and decrypts text using a keyword, applying the classic polyalphabetic substitution cipher. Enter a message and a secret key, choose encrypt or decrypt, and copy the result. It's a fun, hands-on way to explore a cipher that baffled codebreakers for centuries — and it runs entirely in your browser.
How the Vigenère cipher works
Unlike a simple Caesar cipher that shifts every letter by the same amount, the Vigenère cipher uses a keyword to vary the shift letter by letter. Each letter of your message is shifted by the position of the corresponding letter in the repeating key. Because the same letter can encrypt to different letters depending on its position, simple frequency analysis fails — which is why it was once called "le chiffre indéchiffrable" (the indecipherable cipher).
How to use it
- Choose Encrypt or Decrypt.
- Enter your secret key (letters only).
- Type your message and copy the result. The recipient needs the same key.
An example
| Plaintext | Key | Ciphertext |
|---|---|---|
| HELLO | KEY | RIJVS |
Each letter of HELLO is shifted by the repeating key K-E-Y-K-E, producing RIJVS. Decrypting with the same key reverses it.
How secure is it?
For its time, the Vigenère cipher was remarkably strong, but it's no longer secure by modern standards. Once the key length is discovered (using techniques like the Kasiski examination), it can be broken. Today it's valued for education — it beautifully illustrates the leap from simple substitution to polyalphabetic encryption. For real security, use modern cryptography, never a classical cipher.
Tips
- The longer and more random the key, the stronger the encryption.
- Only letters are shifted; numbers, spaces, and punctuation are unchanged.
- Share the key separately and securely — anyone with it can decrypt.
Private and free
All encryption runs in your browser — nothing is uploaded. The tool is completely free with no sign-up.
Frequently asked questions
How is Vigenère different from a Caesar cipher?
A Caesar cipher shifts every letter equally; Vigenère uses a keyword so the shift varies letter by letter, defeating simple frequency analysis.
Is the Vigenère cipher secure today?
No. It's breakable once the key length is found. It's great for learning, but use modern cryptography for real security.
Does the key need to be a real word?
No. Any sequence of letters works, and a longer, more random key is stronger.
Is it private?
Yes. Everything runs in your browser and nothing is uploaded.