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

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chapter 3
would typically be between 20 and 40. If, for example, there were 30 discs
in the set the message to be enciphered would first be broken up into
blocks each containing 30 letters. The 30 discs on the cylinder would then
be rotated so that the first 30 letters of the message appeared in a horizontal
line and then the 30 letters of any of the other 25 horizontal lines
would be sent as the cipher. The discs would then be rotated so that the
second block of 30 plaintext letters lined up and, again, any one of the
other 25 horizontal lines chosen as the cipher text. Decipherment would
involve the recipient lining up the discs to show the 30 letters of each
block of cipher in a horizontal line and then looking at the other 25 horizontal
lines and finding the one that made sense. This raises the question:
‘Is it possible that there might be more than one line that makes sense?’
Such a situation cannot arise with most cipher systems, where there is a
unique relationship between the plaintext and cipher text, but the
Jefferson cylinder is an exception. If the plaintext message is in a natural
language the possibility of finding more than one sensible line among the
25 is negligible. On the other hand if the plaintext message consists of
code groups, such as are described in Chapter 6, it is not impossible that
more than one valid decipherment could be found. It would depend upon
how many of the theoretically available code groups were actually used in
the code. If all possible code groups were being used every one of the 25
lines would produce a valid decrypt, though it is unlikely that more than
one of them would make sense when converted back to natural language.
The security of the Jefferson cylinder would be considerably enhanced
if the order in which the discs were placed on the common axis could be
changed regularly, e.g. daily, but this would necessitate the sender(s) and
recipient(s) agreeing on the ordering in advance. In this case the ordering
of the discs would be ‘the indicator’ for the messages but, instead of being
given in the preamble or embedded in the text of the message, it would
probably be given in a printed sheet which had been produced and distributed
some time before the start of the cipher-period.
Without knowing the shuffled alphabets on the discs or the order in
which the discs themselves were placed on the common axis the cryptanalyst
would have considerable difficulty in decrypting messages and
would probably need many messages sent in the same cipher-period (i.e.
using the same ordering of the discs) to achieve success. If the order of the
discs cannot be changed the cipher-period is infinite and all messages sent
using this system can be considered together but decryption will still not
be easy since, although the messages can all be lined up vertically in