Vigenere cipher decoder
Turn Vigenere cipher text back into plain text with its key, for puzzles and cryptography exercises. It is a puzzle cipher, not protection.
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Processing and privacy: The result is calculated inside your browser and your data is not sent to any server.
How to use it
- Enter encoded text and key.
- The result appears straight away and updates as you edit.
- Copy the result, or copy a link that keeps the same values to share it.
How it is calculated
Original = (y - k) mod 26, where y is the encoded letter index and k the matching key letter index (A = 0).
Example
Inputs: Key = LEMON. Result: Decoded text: attack at dawn.
lxfopv ef rnhr
attack at dawn
Limits and caveats
- This is a puzzle and teaching cipher that is easy to break, so never use it to protect passwords or sensitive data.
- You need the exact key, and the text must have been encoded with the key advancing on letters only.
Questions about this tool
What if I do not know the key?
This tool needs the key. Breaking Vigenere without it takes frequency analysis and key length detection.
Does the key case matter?
No. The keys lemon and LEMON give the same result.
Why is my output unreadable?
Usually the key is wrong, or the sender advanced the key on spaces as well.