How do I decode a Vigenère cipher without the key?

Paste the ciphertext into this tool. The key cracker guesses the key length with the index of coincidence, then solves each column as a Caesar cipher with English letter frequencies. It works best with 100 or more letters of English text.

Can I use this Vigenère cipher in my own code?

Yes. Install the free, MIT-licensed npm package @gykh/vigenere-cipher. It supports strings, buffers and streams, plus automatic key cracking, with TypeScript types.

Vigenère Cipher Decoder & Encoder: Crack the Key Online

Q: What is a Vigenère cipher?

A Vigenère cipher encrypts text with a keyword. Each letter of the key picks a different Caesar shift (A = 0, B = 1 … Z = 25), and the key repeats along the message. With the key LEMON, ATTACKATDAWN becomes LXFOPVEFRNHR.

Q: Is the Vigenère cipher secure?

No. It was called the indecipherable cipher for three centuries, but frequency analysis breaks it in milliseconds today. Use it for puzzles, CTFs and teaching, never for real secrets.

Encrypt and decrypt text with a Vigenère keyword cipher, or paste a secret message and crack the key automatically with frequency analysis. Everything runs in your browser; nothing is uploaded.

Keyword Length: 5 Letters A-Z only. A = shift 0, B = 1 … Z = 25.

Mode & Presets

Plaintext Input 0 chars

Ciphertext Output

Key Cracker

Recovers the key of the ciphertext above without knowing it: the index of coincidence finds the key length, then each column is solved as a Caesar cipher.

Recovered key

…

Index of coincidence by key length (English ≈ 0.066, random ≈ 0.038)

Decrypted with the recovered key

What is a Vigenère cipher?

The Vigenère cipher encrypts text with a keyword. Each key letter picks its own Caesar shift (A = 0, B = 1 … Z = 25) and the key repeats along the message, so the same letter encrypts differently each time. With the key LEMON, ATTACKATDAWN → LXFOPVEFRNHR.

Spaces, digits and punctuation stay unchanged and do not advance the key.

How to decode a Vigenère cipher

Know the key? Choose Decrypt, type the keyword, and paste the message.
No key? Choose Decrypt and paste it anyway. The Key Cracker finds the key length from the index of coincidence, then solves each column with English letter frequencies.
Wrong result? Cracking is statistical. Give it more text; 100+ letters of English works reliably.

Use it in your code

Everything on this page is available as a free, MIT-licensed, zero-dependency npm package with TypeScript types.

npm install @gykh/vigenere-cipher

import { encryptString, crack } from "@gykh/vigenere-cipher";
encryptString("ATTACKATDAWN", "LEMON"); // "LXFOPVEFRNHR"
crack(secret).key; // "LEMON"

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Frequently asked questions

How is Vigenère different from a Caesar cipher?

A Caesar cipher uses one shift for the whole message, so there are only 25 keys to try. Vigenère uses a different shift for each key letter, which hides single-letter frequencies. A one-letter Vigenère key is exactly a Caesar cipher. Try the Caesar cipher decoder too.

Is the Vigenère cipher secure?

No. It was called le chiffre indéchiffrable for three centuries, but as the cracker above shows, frequency analysis recovers the key in milliseconds. Use it for puzzles, CTFs, escape rooms and teaching, never for real secrets.

Is my text sent to a server?

No. All encryption, decryption and cracking runs locally in your browser. Challenge links carry only the encrypted text, inside the link itself.

How do I share a secret message?

Encrypt your text, press Share as a challenge, and send the link. Your friend sees only the scrambled text and has to crack the key. To get a short, tidy link and see how many people opened it, shorten it with MiniFyn.