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

The Enigma cipher machine 111
system would be considerably enhanced. With the ‘fixed distance’ system
and a cylinder of, say, 40 discs, cipher letters 40 positions apart would
come from identical simple substitution alphabets. It follows that a collection
of messages containing more than, say, 2000 letters would be vulnerable
to attack based upon monograph frequency counts, since all the
messages would be ‘in depth’ and we would have a sample of 50 cipher
letters from each alphabet. In the variable distance system the messages
would not be ‘in depth’ and thousands more cipher characters might be
needed to solve the system; the number needed would obviously depend
upon how randomly the variable distances were selected.
Evidently, a system based upon N substitution alphabets has a security
level that increases with N but, on the other hand, if the encipherment is
to be done by hand the tediousness of using the system, and the possibility
of error, would also increase with N. So, as so often happens in life, we
have conflicting requirements. In this case we would like to make N large
to increase the security, but we would also like to keep N small for ease of
use, and we can’t do both.
During the 1914–18 war radio began to be used by military units for
sending messages to each other and to their headquarters. Radio transmission
had the advantage that communication with units at considerable distances
from base, including ships and submarines at sea, could be achieved
almost immediately, but the disadvantage that the messages could also be
intercepted by the enemy. It was therefore essential to encipher such messages
in a secure system and cipher systems of some complexity were
devised; unfortunately, the more complex the system the greater the
burden on the cipher clerks, and the greater the risk of errors with, possibly,
disastrous consequences. Some ‘user-friendly’ but highly secure
cipher systems were needed if the conflicting requirements were to be met.
Following the War a number of people in various countries decided
that the only way of providing a high level of security without obliging
cipher clerks to carry out lengthy, tedious and error-prone processes was
to use machines to do the encipherment/decipherment. One such person
was Arthur Scherbius, co-founder of a German engineering firm. In the
early 1920s Scherbius designed a number of cipher machines, all of which
were intended to provide a very large number of substitution alphabets. A
different alphabet would automatically be used every time a letter was
enciphered, and no substitution alphabet would recur until thousands of
letters had been processed. Having decided upon a particular design he
constructed the machine and called it Enigma.