Cryptography Reference
In-Depth Information
M,O , and Q , respectively. Later, we will see how another later cryptographer,
Blaise de Vigenere (see page 55), was inspired by this tableau to create one that
took the idea further.
Figure 1.24: Polygraphia.
Image courtesy of the National Cryptologic Museum of
The National Security Agency, Rare topics Collections.
See http://www.nsa.gov/museum/books.html .
The attentive reader will have noticed that the Trithemius tableau (neces-
sarily a square since there are exactly as many rows (cipher alphabets) as there
are letters in the alphabet) has an advantage over Alberti's method since the
cipher alphabet is changed with each letter enciphered, rather than after an
arbitrary number of enciphered words as with Alberti's method. Moreover, the
ordered table makes a quick look-up possible at a glance for each of the cipher
alphabets.
Trithemius also gave examples where he switched alphabets after exhausting
24 letters of plaintext rather than starting over with the first row of the above
tableau again. This is a variation of the above simple scheme. Moreover, the
aforementioned method is the first cipher to use a progressive key where all
possible cipher alphabets are exhausted before any are used again. Modern
ciphers have used more variations on this theme since we now have computers
to employ such key progressions. Moreover, the substitution table that he used
is now a standard feature of modern-day cryptography.
Giovanni Battista Belaso
Our next ally and proponent of the advancement of polyalphabeticity is
another from Italy, Giovanni Battista Belaso. Neither Alberti nor Trithemius
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