TheCodeBreakers
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. In this he proposed the use of a literal,
easily remembered, and easily changed key—he called it a
"countersign"—for a poly-alphabetic cipher. Wrote Belaso: "This
annaabi.ee 
