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

Book ciphers
A spy must avoid arousing suspicion and so any cipher equipment he has
in his house must not be obvious. Even a single stencil might appear suspicious
to an investigator and a stack of stencils could be incriminating.
Likewise a spy would be unlikely to use a code if it meant having to have a
large code-book in the house. A cipher that requires no unusual equipment
is therefore a very attractive proposition so far as a spy is concerned
and a book cipher is precisely that; all that is required is a book on any
topic which does not employ non-Latin alphabetic characters. The book
could, for example, be an English novel or a biography or historical work
but probably not one dealing with organic chemistry.
Using a book cipher
In order to use a book cipher it is necessary to be able to ‘add and subtract’
pairs of letters of the alphabet. This is done, as explained in Chapter 1, by
numbering the letters of the alphabet A�0, B�1, C�2,..., Z�25 and
adding or subtracting (mod 26) and re-converting the answers to letters.
Since this is a tedious process it is better to make up tables once and for all
and look up the result of adding or subtracting in the appropriate table,
but to show how to do it without such tables let us carry out the process on
a few letters.
Example 7.3
Convert the alphabet to numbers beginning with A�0, B�1 etc. Then
‘add’ together the two texts below (mod 26) and re-convert the resulting
numbers to letters.
Text 1 THEXCURFEWXTOLLSX
Text 2 ONCEXUPONXAXTIMEX
Solution
We repeat Table 1.1 as Table 7.1.
Table 7.1
Ciphers for spies 75
A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25