Vigenère Cipher Encoder and Decoder
Encrypt and decrypt text with the Vigenère cipher using a keyword, with an autokey option, a visible key stream and a tabula recta guide. Free and in your browser.
How to use Vigenère Cipher
- 1Type or paste your text and choose Encrypt or Decrypt.
- 2Enter a keyword using the letters A–Z only. Turn on Autokey if you want the message to extend the key.
- 3Copy the result, and open Show key stream to see which key letter was applied to each letter.
What makes it better
- Encrypt and decrypt with any letters-only keyword
- Autokey variant: the key is your keyword followed by the plaintext
- Key stream shown aligned under the text so each shift can be checked by hand
- Keeps case, spaces and punctuation, or outputs upper-case letters only
- Collapsible tabula recta grid explains the method
About Vigenère Cipher
The Vigenère cipher shifts each letter by a different amount taken from a repeating keyword, so the same plaintext letter can encrypt to different letters. This tool encrypts and decrypts live, can show the key stream lined up under your text, and supports the autokey variant where the message itself extends the key. It all runs in your browser.
Common uses
- Solve a Vigenère clue in a puzzle hunt or crypto challenge when you know the keyword.
- Check hand-worked encryption exercises from a cryptography course.
- Demonstrate polyalphabetic substitution and why it resisted simple frequency analysis for centuries.
- Create keyword-protected clues for a game or scavenger hunt.
Example
Input
ATTACKATDAWN
Result
LXFOPVEFRNHR
Good to know
- Classical ciphers offer no real security: a computer breaks them in milliseconds. For real encryption use AES-256 (see AES Encrypt).
- Only the letters A–Z advance the key. Spaces, digits, punctuation, accents and emoji are copied through unchanged, or dropped if you turn off Keep case and punctuation.
- A short repeating keyword can be recovered with the Kasiski examination and frequency analysis. Autokey avoids repetition but is also breakable.
Vigenère Cipher FAQ
How does the Vigenère cipher work?
Write the keyword repeatedly under the message. Each letter is shifted by the position of the key letter above it, where A means 0 and Z means 25. Decrypting subtracts the same shifts.
What is the Vigenère keyword limit?
The keyword must use only the letters A to Z, with no spaces or digits. Longer keywords that do not repeat words give better scrambling, but any keyword is breakable by modern methods.
What is the autokey variant?
Instead of repeating the keyword, autokey appends the plaintext to it, so the key is the keyword followed by the message itself. The key never repeats, but anyone who guesses the keyword length can still attack it.
What is a tabula recta?
It is a 26 by 26 grid where each row is the alphabet shifted one place further than the row above. To encrypt, find the plaintext letter in the top row, the key letter in the side column and read the letter where they cross.
Why did my non-letters not use up the key?
The key advances only on letters, which is the standard convention. A space or comma between words does not consume a key letter, so results match textbook examples such as LEMON with ATTACKATDAWN.