Vigenère Square.
Advanced substitution using a series of interwoven Caesar ciphers based on the letters of a keyword.
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The Unbreakable Cipher?
For three centuries the Vigenère cipher was considered 'le chiffre indéchiffrable' (the undecipherable cipher). It uses a keyword to shift each letter by a different amount, defeating simple frequency analysis. In 1863 Friedrich Kasiski published a method to break it by finding repeated patterns. Today it serves as an excellent introduction to polyalphabetic substitution.