Vigenère Cipher Encoder & Decoder

Polyalphabetic substitution cipher with a keyword — encrypt or decrypt instantly.

The Vigenère cipher uses a repeating keyword to shift each letter independently, defeating simple frequency analysis used to crack Caesar. Type your message, choose a keyword, and switch between encrypt and decrypt modes.

Common use cases

Demonstrate polyalphabetic ciphers in cryptography lessons
Solve Vigenère challenges in CTFs and crypto puzzles
Encode personal notes with a keyword only you and a friend know
Compare Vigenère vs Caesar resistance to frequency analysis

Frequently asked questions

Is Vigenère unbreakable?▾

No. Once thought 'le chiffre indéchiffrable', it was broken by Babbage and Kasiski using key-length detection. It's not modern crypto, but stronger than Caesar.

What characters does it operate on?▾

Only A–Z and a–z. The keyword is normalised to uppercase letters; everything else passes through unchanged.

Does the keyword length matter?▾

Longer keywords are harder to crack. A keyword as long as the message (one-time pad) is theoretically unbreakable.