Cipher
Vigenere Cipher Encoder and Decoder
The Vigenere cipher uses a keyword to shift each letter by a different amount, which makes it far stronger than a single Caesar shift. Enter your text and a key, then encrypt or decrypt.
Result appears here
For centuries it was called the indecipherable cipher. Try it live below; the keyword and message stay in your browser.
How the Vigenere Cipher works
The keyword is repeated across the length of the message. Each plaintext letter is shifted by the alphabet position of the matching key letter, so the same letter can encrypt differently depending on where it sits.
Decryption reverses each shift using the same key. The longer and less predictable the key, the harder the cipher is to break.
Examples
History and origins
The method is named after Blaise de Vigenere, a 16th-century French diplomat, though a similar idea was described earlier by Giovan Battista Bellaso. It resisted casual code-breaking for around 300 years.
Charles Babbage and later Friedrich Kasiski found ways to break it in the 19th century by spotting the repeating key length. Even so, it remains a superb tool for understanding polyalphabetic encryption.
Frequently asked questions
What makes the Vigenere cipher stronger than Caesar?
Caesar uses one fixed shift, so the same letter always encodes the same way. Vigenere changes the shift for every position using a keyword, hiding the letter-frequency patterns that give Caesar away.
How do I decrypt a Vigenere cipher?
Enter the same keyword you used to encrypt, switch the tool to decode, and paste the ciphertext. Without the key, breaking it requires frequency analysis of the repeating pattern.
Does the key need to be a real word?
No. Any sequence of letters works as a key. Longer, more random keys are harder to crack; a key as long as the message is theoretically unbreakable.
Learn more
Go deeper on the ideas behind this tool.