Vigenère Cipher Encoder & Decoder
Encode and decode Vigenère cipher messages with any keyword. Letters keep their case, everything that is not a letter passes through unchanged, and you can show the letter-by-letter working to see exactly how each shift is applied.
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History of the Vigenère Cipher
The cipher is named after Blaise de Vigenère, a 16th-century French diplomat, although the scheme it popularised was first described by Giovan Battista Bellaso in 1553. Instead of one fixed shift, a repeating keyword selects a different Caesar shift for every letter — the key letter A means shift 0, B means shift 1, and so on. That simple change defeats basic frequency analysis, because the same plain letter no longer always encrypts to the same cipher letter.
For roughly three hundred years it was considered unbreakable and earned the nickname le chiffre indéchiffrable — the indecipherable cipher. That reputation ended in 1863, when the Prussian officer Friedrich Kasiski published the first general attack: repeated fragments in the cipher text reveal the likely key length, after which each key position can be broken separately as an ordinary Caesar cipher. Try our Caesar cipher tool to see the single-shift building block the Vigenère cipher is made of.
Frequently Asked Questions
- What is a Vigenère cipher?
- A Vigenère cipher is a polyalphabetic substitution cipher that shifts each letter of the message by an amount determined by a repeating keyword. The first letter is shifted by the first key letter, the second by the second, and so on, so the same plain letter can encrypt to different cipher letters.
- How is the Vigenère cipher different from a Caesar cipher?
- A Caesar cipher uses one fixed shift for the whole message, while a Vigenère cipher uses a keyword to apply a different Caesar shift to each letter. You can think of the Vigenère cipher as many interleaved Caesar ciphers, one per key letter.
- What happens to spaces, numbers, and punctuation?
- Only the letters A-Z and a-z are shifted, and each letter keeps its original case. Spaces, digits, and punctuation pass through unchanged, and they do not consume a key letter, so the keyword only advances on actual letters.
- How do I decode a Vigenère cipher?
- If you know the keyword, choose Decode, enter the keyword, and paste the cipher text — each letter is shifted backwards by the matching key letter. Without the keyword, attacks like Kasiski examination look for repeated cipher-text patterns to guess the key length and then break each column as a simple Caesar cipher.
- Is the Vigenère cipher secure?
- No. It resisted attack for about three centuries and was nicknamed 'the indecipherable cipher', but Friedrich Kasiski published a general method for breaking it in 1863. It is great for puzzles and learning about cryptography, but never use it to protect real secrets.