Code and ciphers: Julius Caesar, the Enigma and the internet

plaintext digraphs according to some rule. A nineteenth century system
of this type is the method of
Playfair encipherment
The alphabet, ignoring J, is written into a 5�5 square in some order,
either starting with a keyword, with the remaining letters following in
order, or in a ‘random’ order. If a keyword has any repeated letters the
second and later occurrences are ignored. Thus if TOMORROW is the
keyword it would go into the square as TOMRW.
Encipherment is then carried out digraph by digraph according to the
following rules.
(i) If the two letters of a digraph are at diagonally opposite corners of a
rectangle in the 5�5 square they encipher to the pair at the other two
corners. In the example below we adopt the convention that each letter
is replaced by the corner letter in the same row as itself. The alternative
convention is to replace each letter by the letter in the same column as
itself and this is the usual practice with double Playfair ciphers, as we see
below.
(ii) If the two letters are in the same column of the 5�5 square they
encipher as the letters below them, where row 1 is considered to be
below row 5 if necessary.
(iii) If the two letters are in the same row of the 5�5 square they encipher
as the letters to the right of them, where column 1 is considered to be
to the right of column 5 if necessary.
(iv) If the two letters are identical a ‘dummy’ letter, such as Q, is inserted
between them.
(v) If necessary, a ‘dummy’ letter is inserted at the end of the plaintext.
Example 5.6
Encipher the message
SUPPORT NEEDED URGENTLY
using Playfair encipherment with the keyword WALKING.
Table 5.5
W A L K I
N G B C D
E F H M O
P Q R S T
U V X Y Z
Two-letter ciphers 59