Vigenere Cipher Encoder and Decoder

Each letter is shifted by an amount taken from the next letter of the keyword (A shifts 0, B shifts 1, and so on). The keyword repeats across the message, so the same plaintext letter can map to different letters depending on its position. To decode, the same shifts are subtracted with the same keyword.

Only the letters A-Z and a-z are shifted, and their case is preserved. Spaces, digits, punctuation and line breaks pass through unchanged, and they do not advance the keyword, so the keyword only lines up with the letters it actually transforms.

No. The keyword is read for its letters only, so KEY, Key and key all produce the same shifts. Any non-letters in the keyword are ignored when deriving the shift amounts.

No. This is a classical cipher meant for puzzles, history and learning, not for protecting secrets. It was broken in the 1800s and is easy to crack with frequency analysis, especially with a short keyword. For real protection use a tool that applies modern, password-based encryption such as AES.

Yes. The encoding and decoding run entirely in your browser. Your text and keyword are never uploaded to a server and nothing is stored.

The shift amounts come from the letters in the keyword, so a keyword with no letters (empty or only digits and symbols) gives the cipher nothing to work with. The tool asks for at least one letter and otherwise reports an error instead of returning the text unchanged.