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The Trithemius cipher, however, provided a progressive, rather rigid and predictable system for switching between cipher alphabets. [note 1] In 1586 Blaise de Vigenère published a type of polyalphabetic cipher called an autokey cipher – because its key is based on the original plaintext – before the court of Henry III of France. [7]
Once the length of the keyword is discovered, the cryptanalyst lines up the ciphertext in n columns, where n is the length of the keyword. Then each column can be treated as the ciphertext of a monoalphabetic substitution cipher. As such, each column can be attacked with frequency analysis. [6]
where N is the length of the text and n 1 through n c are the frequencies (as integers) of the c letters of the alphabet (c = 26 for monocase English). The sum of the n i is necessarily N. The products n(n − 1) count the number of combinations of n elements taken two at a time. (Actually this counts each pair twice; the extra factors of 2 ...
The polyalphabetic cipher was most clearly explained by Leon Battista Alberti around AD 1467, for which he was called the "father of Western cryptology". [1] Johannes Trithemius , in his work Poligraphia , invented the tabula recta , a critical component of the Vigenère cipher.
1854 – Charles Wheatstone invents Playfair cipher; c. 1854 – Babbage's method for breaking polyalphabetic ciphers (pub 1863 by Kasiski) 1855 – For the English side in Crimean War, Charles Babbage broke Vigenère's autokey cipher (the 'unbreakable cipher' of the time) as well as the much weaker cipher that is called Vigenère cipher today ...
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The Alberti cipher by Leon Battista Alberti around 1467 was an early polyalphabetic cipher. Alberti used a mixed alphabet to encrypt a message, but whenever he wanted to, he would switch to a different alphabet, indicating that he had done so by including an uppercase letter or a number in the cryptogram.