How to use this Vigenère decoder
- Pick Decode (ciphertext to plaintext) or Encode (plaintext to ciphertext).
- Enter the keyword — letters only, case does not matter.
- Paste your text. The result updates as you type.
- Copy the result with the Copy button.
What is the Vigenère cipher?
The Vigenère cipher encrypts text with a keyword instead of a single shift. Each letter of the keyword sets the shift for one letter of the message. With the keyword KEY, the first letter shifts by K (10), the second by E (4), the third by Y (24), then the keyword repeats. So HELLO becomes RIJVS — and the same letter L becomes two different letters, J and V. That is what makes Vigenère far harder than a Caesar cipher: a single-shift cipher always maps the same letter to the same letter, but Vigenère does not.
For about 300 years the Vigenère cipher was called “le chiffre indéchiffrable” — the indecipherable cipher. It was finally broken in the 1800s by Charles Babbage and Friedrich Kasiski, who noticed that repeated words in the plaintext leave repeated patterns in the ciphertext. Those patterns reveal the key length, and once the key length is known, each column of the message is just a simple Caesar cipher.
Examples
- RIJVS with key KEY → HELLO
- LXFOPV EF RNHR with key LEMON → ATTACK AT DAWN (the classic textbook example)
- HELLO with key KEY → RIJVS
Frequently asked questions
What is the Vigenère cipher?
The Vigenère cipher encrypts text with a keyword instead of a single shift. Each letter of the keyword sets the shift for one letter of the message, so the same plaintext letter can become different ciphertext letters.
How do I decode a Vigenère cipher?
Enter the keyword, paste the ciphertext, and this tool shifts each letter back. If you know the keyword, decoding takes one second.
How do I decode a Vigenère cipher without the key?
Use the Kasiski examination and the index of coincidence to guess the key length, then solve each column like a Caesar cipher. Our Kasiski tool (coming soon) will automate the first step.
Is the Vigenère cipher secure?
Not by modern standards. It resisted code-breakers for 300 years and was called “le chiffre indéchiffrable”, but Kasiski and Babbage broke it in the 1800s. Never use it for real secrets.
Related tools
Caesar Cipher Decoder — Vigenère with a one-letter keyword is just a Caesar cipher. ROT13 Decoder — the shift-13 special case. Kasiski Examination (coming soon) — find an unknown Vigenère key length. Index of Coincidence Calculator (coming soon) — the other classic key-length test.