Vigenère Cipher
Encrypt and decrypt text with the classic Vigenère keyword cipher. Enter a keyword and a message, choose encrypt or decrypt, and convert instantly. Free, private and processed entirely in your browser.
How to use the Vigenère cipher tool
- Enter your text. Type or paste the message you want to protect or reveal into the text box.
- Choose a keyword. Enter a secret keyword such as LEMON. The same keyword is needed to both encrypt and later decrypt the message.
- Encrypt or decrypt. Pick the mode and the converted text appears instantly below, ready to copy with one click.
About this tool
The Vigenère cipher is a polyalphabetic substitution cipher invented in the 16th century and long known as le chiffre indéchiffrable — “the indecipherable cipher.” Instead of shifting every letter by the same amount like a Caesar cipher, it uses a repeating keyword: each letter of the keyword defines a different shift, so the letter A of your message might be enciphered one way in one position and a completely different way in the next. This masks the letter-frequency patterns that make simple ciphers so easy to break, which is why Vigenère resisted cryptanalysis for roughly 300 years.
In this tool, only letters are enciphered — spaces, digits and punctuation pass through unchanged, and the case of each letter is preserved. The keyword advances only across letters, so formatting never throws the alignment off. Everything runs locally in your browser, so your message and keyword are never uploaded or stored. Note that Vigenère is a historical cipher: it is wonderful for learning and puzzles, but it can be broken by modern methods such as the Kasiski examination and should never be relied on for real security.
Frequently asked questions
How is the Vigenère cipher different from the Caesar cipher?
A Caesar cipher shifts every letter by the same fixed amount. Vigenère uses a repeating keyword where each keyword letter sets a different shift, so the same plaintext letter can encrypt to different letters depending on its position. That variation is what makes it far harder to crack.
Do I need the same keyword to decrypt?
Yes. Decryption reverses the exact shifts applied during encryption, so you must use the identical keyword. A different keyword produces unreadable output.
What happens to spaces, numbers and punctuation?
They are left completely unchanged. Only the 26 letters A–Z are shifted, letter case is preserved, and the keyword advances only when it encounters a letter, so spacing and symbols never disrupt the alignment.
Is the Vigenère cipher secure today?
No. Although it went unbroken for centuries, it is vulnerable to techniques like the Kasiski examination and frequency analysis once the key length is found. Use it for education, puzzles and fun — never for protecting genuinely sensitive information.