Encrypt or decrypt with a repeating keyword, the classic polyalphabetic cipher. Historically important, breakable since the 1860s, and educational only.
Show calculation steps
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How to use the Vigenere Cipher Encoder and Decoder
- Paste your message in the first panel.
- Enter a keyword in the second panel – it repeats across the message.
- Choose encrypt or decrypt, and whether punctuation advances the key.
- Click "Apply Vigenère".
Frequently asked questions
How does the Vigenère cipher work?
Each letter of the key selects a different Caesar shift, and the key repeats across the message. Because the same plaintext letter encrypts differently depending on its position, simple frequency analysis fails – which earned it the nickname "le chiffre indéchiffrable" for three centuries.
How was it eventually broken?
Charles Babbage and later Friedrich Kasiski showed that repeated sequences in the ciphertext reveal the key length, because they occur at multiples of it. Once the key length is known, the message splits into that many separate Caesar ciphers, each trivially broken by frequency analysis.
Does a longer key make it secure?
Only in the limiting case. A truly random key as long as the message, never reused, is the one-time pad – which is provably secure but impractical for most purposes. Any repeating key is vulnerable to the analysis above.
What does the punctuation option change?
Whether spaces and punctuation consume a key letter. Classical practice strips them entirely; keeping them and not advancing the key preserves readability of the layout while matching most textbook examples.
