Vigenère Cipher Decoder

Common questions

How do I decode a Vigenère cipher?

Put the ciphertext in the Ciphertext box, the keyword in the Key field, and leave the mode on Decode. The key is repeated across the message one letter per letter, and each key letter shifts its letter back instead of forward. Elew aa tf tks kdk buwzyl aw aevuijvp. under SHADOW comes back as Meet me by the old bridge at midnight.

Can I decode a Vigenère cipher without the key?

This workflow requires a keyword. Unknown-key Vigenère solving needs key-length and key-recovery analysis, which is not provided here. The additional Caesar Crack mode ranks single shifts; it does not solve a general Vigenère key.

What happens if the key is wrong?

Every valid keyword gives a result, so a wrong key can produce letters without an error. Readable output is evidence to check against the puzzle context, not proof of the original key. The tool verifies the arithmetic for the key you supplied.

Does the key have to be one word?

It has to be letters. Spaces, digits and punctuation are not part of a Vigenère key, so the field asks you to take them out and tells you what the key becomes without them. Case does not matter: lemon and LEMON are the same key.

What is autokey, and do I need it?

Plaintext autokey starts with the keyword and then continues with message letters instead of repeating the keyword. Select it only when that is the cipher convention. The decoder recovers plaintext in sequence and uses those recovered letters as the later key.

Is the ciphertext sent anywhere?

The cipher runs in your browser. This page holds its verified ciphertext, SHADOW keyword, decode mode and repeating-key setting when it opens. Editing one field remembers only that choice and preserves your other saved fields. Clear resets the settings and remembers an empty message.