Vigenère decode

The exact key, letters only. This tool subtracts the key letters instead of adding them, so the same key that Vigenere encode used brings the text back. There is no automatic key recovery here, and a wrong key produces readable-looking nonsense rather than an error, because every key gives valid letters.

The key is checked against a letters-only test and anything else is refused with "key must contain letters only". Spaces, digits, hyphens and accented characters all fail. If your key was written with spaces, remove them, and bear in mind that removing them changes the letter sequence the encoder used.

No. Each key letter is lowercased before it is turned into a shift, so KEY, Key and key behave identically. Case in the text is a different matter: it is preserved through the transformation, so an uppercase ciphertext letter decodes to an uppercase plaintext letter.

No. There is no cryptanalysis on this page, no key-length detection and no scoring of candidates. Caesar brute force handles single-shift ciphers only, and XOR brute force covers single-byte XOR keys. A Vigenere key has to be supplied by you.

The most likely difference is key advancement. Here the key position moves only on letters, so spaces and punctuation do not consume a key letter. A tool that advances the key on every character will produce a different result from the same key and the same text. Check that before assuming the key is wrong.

In a Web Worker in this tab, over the file you dropped. The key you type and the text you decode both stay in page memory for the length of the run and are then gone. Nothing is uploaded, nothing is stored, and the site works offline once loaded, so you can decode with the network disconnected.