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

Ciphers for spies 93
simply a ‘random’ string of letters, taken from a page of ‘random letters’
which is destroyed after use so that it can never be used again, then we have what
is known as a one-time pad and the resulting cipher cannot be solved. This
may seem to be a very bold assertion, but it is a mathematical fact [M7].
Since one-time pads provide total security why are they not used for all
encipherments? Basically because a different pad has to be provided for
every pair of people who need to communicate, each of whom has one of
the only two copies produced, and although this is feasible for a few
hundred pairs, such as ambassadors communicating with their governments,
it is out of the question for large numbers of military units in
wartime. It must also be realised that the situation changes dramatically
if a ‘one-time’ pad is used more than once. We then have a ‘depth’ of two,
or more, cipher messages enciphered with the same additive key. By subtracting
one text from another the key is eliminated and the resulting differenced
text is now the difference of two unenciphered texts and a
‘crib-dragging’ or similar technique may lead to their decryption. If a
one-time pad is used to encipher code groups, rather than natural language,
the cryptanalyst’s task is much harder since he must have some knowledge
of the code itself in order to use the crib-dragging attack, but the
method is essentially the same.
For a variety of reasons, including the provision of one-time pads,
cryptographers are very interested in methods by which ‘random’ letters
(or numbers) can be generated. We look at some of the methods of doing
this in the next chapter.