Cryptography Reference
In-Depth Information
encryption process can be lessened but not eliminated,
as letter pairs are themselves highly correlated. The
best-known digraph substitution cipher is the Playfair,
invented by Sir Charles Wheatstone but championed
at the British Foreign Office by Lyon Playfair, the first
Baron Playfair of St. Andrews. Below is an example
of a Playfair cipher, solved by Lord Peter Wimsey in
Dorothy L. Sayers's
Have His Carcase
(1932). Here, the
mnemonic aid used to carry out the encryption is a
5 × 5-square matrix containing the letters of the alpha-
bet (I and J are treated as the same letter). A key word,
MONARCHY in this example, is filled in first, and the
remaining unused letters of the alphabet are entered in
their lexicographic order:
Plaintext digraphs are encrypted with the matrix by
first locating the two plaintext letters in the matrix. They
are (1) in different rows and columns; (2) in the same row;
(3) in the same column; or (4) alike. The corresponding
encryption (replacement) rules are the following:
• When the two letters are in different rows and
columns, each is replaced by the letter that is
in the same row but in the other column; i.e.,
to encrypt WE, W is replaced by U and E by G.
