You'll want to read analysis of a VIGENERE cipher (a polyalphabetic substitution, proposed by Blaise de Vigenere from the court of Henry III of France in the sixteenth century).

Resources:

• Mathematical Discussion and College Assignment
• Cryptanalysis of the Vigenere with other reading
• Cryptanalysis of the Vigenere II
• Cryptanalysis III
• A modified Viggy that has yet to be completely solved

C-.

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