Vigenère & Autokey Cipher Tool
The polyalphabetic cipher that resisted breaking for 300 years.
About this tool
Vigenère applies a different Caesar shift per letter, cycling through a keyword — enough to earn the name «le chiffre indéchiffrable» until Kasiski broke it in 1863. The autokey variant appends the plaintext itself to the key, removing the periodicity that Kasiski exploited.
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Frequently asked questions about Vigenère Cipher
Why was Vigenère called unbreakable?+
By using a keyword to vary the shift per letter, it hides the single-letter frequency signature that breaks the Caesar cipher. It resisted cryptanalysis for about 300 years until Kasiski's method in 1863.
What is the autokey variant?+
Instead of repeating the keyword, autokey appends the plaintext itself to the key, removing the periodicity that Kasiski examination exploits.
How is it broken today?+
By finding the key length (Kasiski or index of coincidence) then solving each position as a Caesar cipher. See the frequency analysis tool.
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