Vigenère Cipher
Encode and decode the Vigenère cipher using a keyword. This classic polyalphabetic cipher shifts each letter by a different amount based on your key, making it stronger than a simple Caesar cipher. Great for puzzles, CTFs, and learning cryptography.
Whether you call it vigenère cipher or vigenere cipher, this free tool handles it instantly in your browser.
Options
Example usage
Example input
Hello
Example output
Rijvs
Batch Processing
Enter multiple inputs, one per line. Each line will be processed separately.
Why this tool?
Everything runs right in your browser — no uploads, no sign-up, no waiting. Your text never leaves your device.
Quick tips
- •Outputs update live as you type.
- •Use the sticky copy button to grab results quickly.
- •Bookmark this page for fast access.
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How it works
- Enter the text you want to encode or decode.
- Type a keyword made of letters — it repeats across your text to shift each letter.
- Switch to decode and use the same keyword to recover the original message.
Use cases
Le chiffre indéchiffrable
The Vigenère cipher was described by Giovan Battista Bellaso in 1553 and later misattributed to Blaise de Vigenère, whose name stuck. Its innovation was using a repeating keyword so each letter gets a different shift: with the key LEMON, the first letter shifts by L (11), the second by E (4), and so on, restarting after five. That single change defeats the frequency analysis that breaks Caesar ciphers instantly, because the same plaintext letter encodes differently depending on its position.
It earned the nickname "le chiffre indéchiffrable" — the indecipherable cipher — and held that reputation for roughly three centuries, protecting real diplomatic and military traffic.
How it finally fell — and how to use it now
Charles Babbage broke it in the 1850s and Friedrich Kasiski published a general method in 1863. The weakness is the repeat: because the key cycles, identical plaintext fragments spaced a multiple of the key length apart produce identical ciphertext. Measuring the gaps between repeated sequences reveals the key length, and once you know that, the message splits into several ordinary Caesar ciphers that fall to frequency analysis one by one.
Practically, that means key length is everything. A three-letter key on a long message is nearly transparent; a key as long as the message and never reused becomes a one-time pad, which is genuinely unbreakable. For puzzles, note that only letters are shifted here — spaces and punctuation pass through unchanged, which preserves word boundaries and makes solving considerably easier. If your keyword is wrong, output will be noise, so guessing keys is part of the sport.
Encoding
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FAQ
What is the Vigenère cipher?
It is a classic cipher that uses a repeating keyword to shift each letter by a different amount, making it harder to crack than a single-shift Caesar cipher.
Do I need the keyword to decode?
Yes. Decoding requires the same keyword that was used to encode. Without it, the output will not be readable.
What characters does it change?
Only letters are shifted, and their case is preserved. Spaces, numbers, and punctuation are kept unchanged.