Vigenère Cipher Decoder
Decode Vigenère and autokey ciphers when you know the key.
Output appears here as you type — decoded on your device.
Input is processed on your device. Nothing you paste is sent anywhere.
What is the Vigenère Cipher Decoder?
The Vigenère cipher encrypts text with a repeating keyword, shifting each letter by the alphabetic position of the corresponding key letter, which makes it polyalphabetic — the same plaintext letter encrypts differently depending on where it falls.
For three centuries the Vigenère cipher was known as 'le chiffre indéchiffrable'. Its strength over Caesar is that the shift changes letter by letter according to a repeating key, so simple frequency analysis fails: the letter E maps to a different ciphertext letter each time the key advances.
The weakness is that repetition. Because the key cycles, the cipher is really N interleaved Caesar ciphers where N is the key length. Kasiski examination finds repeated ciphertext sequences to estimate N, then each of the N slices falls to ordinary frequency analysis. Babbage and Kasiski broke it independently in the 1800s.
The autokey variant fixes exactly this: after the keyword is exhausted, the plaintext itself continues the key, so nothing repeats. It is genuinely stronger, and this decoder supports both modes — choose the one that matches how the message was encrypted.
Vigenère Cipher Decoder at a glance
- Type
- Polyalphabetic substitution
- Key
- A repeating keyword; A = shift 0, B = shift 1
- Autokey
- Plaintext extends the key — no repetition
- Classic attack
- Kasiski examination, then per-slice frequency analysis
How to use the Vigenère Cipher Decoder
- 1Paste the ciphertext and enter the keyword in the Key field.
- 2Choose Vigenère for a repeating key, or Autokey if the plaintext extended it.
- 3Read the plaintext; if it is gibberish, the key length or the mode is wrong.
When you would reach for it
- Decrypting a Vigenère challenge once the keyword has been found.
- Demonstrating why polyalphabetic ciphers resist naive frequency analysis.
- Testing candidate keywords quickly during a puzzle solve.
Vigenère Cipher Decoder — frequently asked questions
Last reviewed · questions or corrections: contact@decoder.tools