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

74
chapter 7
have been underlined in the message as sent! Thus the message reads,
after insertion of spaces,
as before.
ATTACK DUE ON THIRD
If a stencil is used several times it might be possible for a cryptanalyst to
discover where the holes are placed, and in what order they are used,
but it would depend on whether any likely plaintext phrases or words
were available and on the number and length of the messages. If
the stencil was changed after every page it would be impossible to read
the messages unless there was some relationship between the make-up
of a message stencil and its successor, for without knowing the stencil
many possible messages might be found within a page as is illustrated
by
Problem 7.1
Verify that all but one of the following messages can be found within the
text of the example above and so are ‘possible solutions’ given an appropriate
stencil:
(1) MERRY CHRISTMAS;
(2) COME AT ONCE;
(3) GO AWAY QUICKLY;
(4) THE AUTHOR OF OTHELLO IS BACON.
Hundreds of other ‘possible solutions’ could be found within the text
since it contains over a hundred letters and any anagram of any subset
could be picked out with an appropriate stencil. Without further information
such as, for example, that a stencil will not have more than one
hole in any row or column, such a message is ‘unbreakable’ since many
solutions are possible. The same situation can apply to other cipher
systems where insufficient material is available to provide a unique solution.
Even a simple substitution system is unbreakable if it is used only
once for a single short message. In the extreme case of a ‘one-time pad’
the system is unbreakable no matter how many messages of any lengths
are sent, as we shall see later. If the same system (simple substitution,
transposition, stencil) is used more than once it may cease to be
‘unbreakable’; even a ‘one-time pad’ may lose its security if the same pad is
used twice.