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

7
Ciphers for spies
A spy operating in country X on behalf of country Y has the problem
of communicating with his controller in such a way as to protect both
himself and the contents of his messages. No matter how he sends his messages
they will have to be ‘modified’ somehow so that their true meaning
is hidden from anyone but the intended recipient. There are methods,
such as the use of microdots or ‘invisible’ ink, which do not, per se, involve
encipherment although some ‘modification’ of the text even in such cases
would probably be used to provide extra security. When we say that a text
has been ‘modified’ we do not necessarily mean that it has been enciphered
but that the ‘secret’ text is not simply sent in an unaltered form: it
might, for example, be hidden inside an apparently innocuous message.
Hiding a secret text inside an innocuous one has the advantage that,
being apparently unenciphered, it will not automatically attract the interest
of unintended recipients or interceptors, such as the security forces of
country X. A disadvantage is that it may not be too easy to construct a
realistic non-secret text in which to embed it. Here is a simple illustration.
Example 7.1 (‘Part of a letter from Agent 63’)
As I was walking through the centre of town yesterday morning at about
eleven thirty I chanced to see Ron Kingston. He was alone, driving a
newish-looking ultramarine car, a Ford Escort. Previously he’s had only
second-hand cars, not often less than three years old. Perhaps he has had
an inheritance from some rich relative who has recently died?
Secret message
If we take the first letter of the message and the first letter of every fourth
word after that, taking hyphenated words as two separate words, we get
[72]
ATTACKDUEONTHIRD