TheCodeBreakers
Thus, the fc
that formerly represented NO might, at the new setting, stand for
plaintext TU. This shift in both plain and cipher equivalents dif-
*In Alberti's disk, the outer capital letters are the plaintext and the
inner lower-case letters are the ciphertext. This contradicts the
convention of this book, and is being used in the section on Alberti only
to avoid altering his text. The difference is signalized by not using italic
for the lower case.
1
ferentiates polyalphabetic from homophonic or polyphonic
substitution.
Each new setting of Alberti's disk brought into play a new cipher
alphabet, in which both the plaintext and the ciphertext equivalents are
changed in regard to one another. There are as many of these alphabets
as there are positions of his disk, and this multiplicity means that Alberti
here devised the first polyalphabetic cipher.
This achievement—critical in the history of cryptology —Alberti then
annaabi.ee 
