TheCodeBreakers
repeating the alphabets over and over again in groups of 24.
The great advantage of this procedure over Alberti's is that a new
alphabet is brought into play with each letter. Alberti shifted alphabets
only after three or four words. Thus the ciphertext would mirror the
obvious pattern of repeated letters of a word like Papa ("Pope"), or in
English, attack, and the cryptanalyst could seize upon this reflection to
break into the cryptogram. The letter-by-letter encipherment obliterates
this clue.
If the first two steps in polyalphabeticity were made by men who were
giants in their time, the third was taken by a man who was so
unexceptional that he left almost no traces. This is Giovan Batista
Belaso; the sum total of knowledge about him consists of the facts that
he came from Brescia of a noble family, served in the suite of one
Cardinal Carpi, and, in 1553, brought out a little booklet entitled La cifra
del. Sig. Giovan Bastista Belaso
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