Vigenère Cipher Decoder
What is Vigenère Cipher Decoder
This decoder reverses the Vigenère cipher: give it the ciphertext and the keyword that was used to encode it, and it returns the original message. Each letter is shifted back by the matching key letter. Letter case is preserved and non-letters pass through untouched. It all runs locally in your browser, so nothing you paste is uploaded.
How to use
- Paste the Vigenère cipher text you want to read.
- Enter the keyword that was used to encode it.
- Read the decoded message in the output box.
- Copy the result or share the link with the cipher filled in.
When to use it
Got a Vigenère message and the keyword that unlocks it? Paste both here and read the original in one click. It pairs with the Vigenère encoder for CTF challenges, cipher puzzles, and classroom exercises. Unlike a Caesar cipher, you cannot brute force it by trying 25 shifts, so the keyword is essential to decode.
Frequently asked questions
What if I do not know the keyword?
You need the keyword to decode directly. Without it, Vigenère resists the simple shift-by-shift brute force that works on a Caesar cipher, which is what makes it stronger. Attacks like Kasiski analysis exist but are not part of this tool.
Why is my output wrong?
Check that the keyword is exactly the one used to encode, including its letters and order. A different or misspelled keyword produces scrambled output. Case in the keyword does not matter.
Does decoding happen on a server?
No. The text is decoded in your browser and never uploaded. Nothing you paste is sent or stored.
Can I share this with my cipher pre-filled?
Yes. The URL updates automatically as you type. Copy it from the address bar or use the Copy link button, and anyone who opens it sees your exact cipher ready to go. The keyword is never put in the link, so share it separately.
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