🗝️ 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.
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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.