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

9
The Enigma cipher machine
Historical background
In Chapter 2 we looked at simple substitution ciphers and we saw how
these can be solved by the use of frequency counts if ‘sufficient’ cipher text
is available. How many letters are always ‘sufficient’ is a matter for debate,
but it is probably true that 200 letters will normally suffice whereas 50
might not. For our purposes let us assume that if only 25 letters of cipher
are available then the cipher is safe. Since a limitation of message lengths
to no more than 25 letters would be too restrictive we conclude that the
use of a simple substitution cipher is impractical. If, however, we use not
one but several different simple substitution alphabets, switching between
the alphabets every time we encipher a letter, we can increase the security
of the system. As a rough guide: if we use N different alphabets it should
be possible to make the cipher safe for single messages of up to 25N cipher
letters; but this simple rule needs qualification. If the substitution alphabets
are related in some way the recovery of any one of them may lead to
recovery of the others. On the other hand, in some systems, additional features
may ensure that cipher messages of much greater length than 25N
are secure. In the specific case of Jefferson’s cylinder, for example, the
sender and receiver could
either agree that the cipher text will be read from the row of letters at a
specified distance from the row of plaintext letters (the distance
possibly being given by some form of indicator),
or have no indicator, and use a different distance each time a row is
enciphered.
Whilst the latter procedure involves the recipient in looking at all 25
rows of the cylinder to see which of them makes sense, the security of the
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