🗝️ Vigenère Cipher Encoder / Decoder
Shift each letter by a different amount driven by a repeating keyword, the Vigenère cipher, far stronger than Caesar. Encode and decode locally.
Transformed in your browser, nothing you type is uploaded.
How the vigenère cipher encoder / decoder works
The Vigenère cipher uses a keyword to vary the shift from letter to letter: the keyword is repeated across the message, and each of its letters gives the Caesar shift for the letter beneath it (A = shift 0, B = 1, and so on). Because the same plaintext letter encrypts differently depending on its position, simple letter-frequency analysis fails, which is why it was long nicknamed "le chiffre indéchiffrable" (the indecipherable cipher).
It held out for roughly three centuries until Friedrich Kasiski published a general method to break it in 1863, by finding the key length from repeated patterns. Vigenère is a beautiful step up from Caesar for learning how polyalphabetic ciphers work, but it is still not secure by modern standards; for real confidentiality, use proper encryption.
Frequently asked questions
How does the Vigenère cipher work?
A keyword is repeated across your message, and each keyword letter sets the shift for the letter below it, A shifts by 0, B by 1, up to Z by 25. So the same letter encrypts to different letters depending on where it sits, unlike a simple Caesar shift.
Why is it stronger than the Caesar cipher?
Because it uses many shifts instead of one, the letter-frequency patterns that instantly break a Caesar cipher are smeared out. Without knowing the key length, it's far harder to crack. It resisted codebreakers for about 300 years.
How was the Vigenère cipher eventually broken?
Friedrich Kasiski published a method in 1863 (anticipated by Charles Babbage) that deduces the key length from repeated sequences in the ciphertext, after which each position reduces to a solvable Caesar cipher.
Is the Vigenère cipher secure today?
No. It's breakable with pen-and-paper techniques and trivially by computer. It's excellent for learning cryptographic ideas, but real secrets need modern encryption like AES.
Does this work locally?
Yes, encoding and decoding happen in your browser; the message and keyword never leave your device.