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Le chiffre indéchiffrable

As the development of frequency analysis proceeded and passed the borders of the Arab world the security of existing substitution ciphers was destroyed. After the cryptanalysts had gained the upper hand, Blaise de Vigenére, a French diplomat and cryptographer born in 1523, prepared another strong cipher actually designed by another cryptographer, Leone Battista Alberti. His idea of the first polyalphabetic cipher was a substantial breakthrough for the cryptographic world so far. Nevertheless the cipher bears its name from Vigenére, who developed it to its final form. The strength of the cipher lies in its using not one, but 26 distinct cipher alphabets to encrypt a message. Thus every letter of the message is enciphered using a different permutation of the alphabet according to a secret keyword. Although the cipher seems to be invulnerable to frequency analysis, it was largely neglected for the next two centuries, because of the complexity of usage. The existing monoalphabetic substitution cipher was perfectly adequate for most purposes, because it was quick, easy to use and secure against people without knowledge of cryptanalysis. However the Vigenére cipher was applied for more serious military and government communications and was considered unbreakable. Hence it made history as le chiffre indéchiffrable.

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