Codes / Classical ciphers
Vigenère Cipher Encoder and Decoder
Encrypt and decrypt text with the Vigenère cipher and a keyword, with a table showing how each letter is shifted by the repeating key.
Vigenère Cipher Encoder and Decoder: The Vigenère cipher shifts each letter by an amount set by the matching letter of the key, A for 0 up to Z for 25, and repeats the key along the text. With the key LEMON, ATTACKATDAWN becomes LXFOPVEFRNHR. Because the shift changes from letter to letter, simple letter-frequency counting does not break it, which earned it the name le chiffre indéchiffrable, until Kasiski and Babbage found ways to crack it in the 19th century. Runs 100% locally in your browser with zero server file uploads.
- Category
- Text tools
- Runs
- In your browser
- Cost
- Free · no sign-up
- Availability
- Ready to use
Runs entirely in your browser
| Text | A | T | T | A | C | K | A | T | D | A | W | N |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Key | L | E | M | O | N | L | E | M | O | N | L | E |
| Shift | +11 | +4 | +12 | +14 | +13 | +11 | +4 | +12 | +14 | +13 | +11 | +4 |
| Result | L | X | F | O | P | V | E | F | R | N | H | R |
Each letter is shifted by the matching letter of the key, A = 0 to Z = 25, and the key repeats along the text. Spaces and punctuation are kept and do not use up key letters.
The Vigenère square
On paper, the cipher uses a 26 by 26 table of alphabets, each shifted one place further than the one above. Find the text letter along the top and the key letter down the side; the letter where they meet is the ciphertext letter.
For the single-shift version, see the Caesar cipher; for strong modern encryption, encrypt text with AES.
How it was broken
In 1863 Friedrich Kasiski published the method: find repeated groups of letters in the ciphertext, measure the distances between them, and the key length is likely a common factor. Split the text into that many columns, and each column is a Caesar cipher solved by letter frequencies.
How to use it
- Type the text and the key, such as LEMON.
- Choose Encrypt or Decrypt.
- Read the result, and the table lining up text, key, and shift for the first letters.
Privacy & limitations
Your text and key stay in your browser.
Related tools
Frequently asked questions
How strong is the Vigenère cipher?
Much stronger than a Caesar shift, but breakable: repeated patterns reveal the key length, after which each column is a simple Caesar cipher.
What happens to spaces and punctuation?
They are kept as they are and do not use up letters of the key, the usual convention.
What makes a better key?
A longer key with no pattern; a random key as long as the message, used once, becomes the one-time pad, which cannot be broken.
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