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ToolWrenEncrypt or decrypt text with the Vigenère cipher, which uses a repeating keyword to shift letters.
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Vigenère is a free, private online tool that lets you encrypt or decrypt text with the Vigenère keyword cipher. It runs entirely in your browser, so nothing you enter is uploaded to a server.
Privacy: this tool runs entirely in your browser. Your input is never sent to, received by, or stored on any server — there are no uploads and no tracking of what you enter.
It extends the Caesar cipher by using a keyword: each letter is shifted by the corresponding letter of the repeating key, making it much harder to break by frequency analysis.
Enter a keyword (letters only), choose encrypt or decrypt, and type your message.
Once called 'le chiffre indéchiffrable' (the unbreakable cipher), it resisted casual cryptanalysis for centuries.
The Vigenère cipher is a polyalphabetic substitution cipher that encrypts text using a repeating keyword. Each letter is shifted by an amount determined by the corresponding letter of the key, so the same plaintext letter can encrypt to different letters depending on its position - a big step up from simple ciphers like Caesar.
The keyword is repeated to match the length of the message. Each plaintext letter is shifted by the alphabetical position of the key letter above it (A shifts by 0, B by 1, and so on). For example, with the key LEMON, the word ATTACKATDAWN encrypts to LXFOPVEFRNHR. Decryption reverses the shifts using the same key.
Because a letter's encryption depends on its position, simple frequency analysis fails - the letter E does not always map to the same ciphertext letter. For three centuries this defeated casual codebreakers, earning it the nickname 'le chiffre indéchiffrable' (the indecipherable cipher), until methods like the Kasiski examination revealed how to break it by finding the key length.
By modern standards Vigenère is not secure - given enough ciphertext, the key length and key can be recovered - but it is far stronger than Caesar or Atbash and is a superb teaching tool for understanding polyalphabetic encryption, key reuse and why key length matters. A longer, more random keyword resists analysis better than a short or repetitive one.
Choose encrypt or decrypt, enter a keyword (letters only), and type your message - the result updates live. Use it for puzzles, cryptography study and historical interest. Everything runs in your browser, so your text and key never leave your device.