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

44
chapter 4
Likewise the second cipher row
becomes
G I N L W
N G W I L
and the rest of the message confirms the decryption.
This is a very simple example in that the key was short and its length
was the first obvious one to try, but it illustrates the method of solution.
It also indicates that access to a table of digraph frequencies, though not
strictly essential, will make the task much easier. Had the cipher text
not been a multiple of 5 it would not have been so likely that the key
length might be 5 (or 7, in this case) and other key lengths might have to
be tried. For keys of length no longer than 5 even a brute force attack is
feasible since there are only a moderate number of possible orderings of
the columns (120 when the key length is 5). As key lengths increase
beyond 5 a brute force attack soon becomes very tedious and, eventually,
impractical by hand, whereas the digraph method used above is realistic
for all key lengths that are likely to be encountered in practice. The
cryptographer, knowing all this, would therefore attempt to disguise
the key length as far as possible and might also resort to other measures,
such as
Double transposition
The weakness of the simple transposition method is that when the cipher
message is written out column by column ‘on the width of the key’, i.e. in
rows containing as many letters as the key length, letters which were adjacent
in the plaintext will tend to fall in the same row and a search for
‘good’ digraphs may reveal the transposition order of the columns.
This becomes very obvious if we replace the plaintext in the example
above by the numbers 1, 2, 3, ..., 35, underlining the first five numbers for
ease of identification, and apply the transposition key that we used
before, thus (see Table 4.4) we get the ‘cipher’ text
12 7 12 17 22 27 32 4 9 14 19 24 29 34 1 6 11 16 21 26
31 5 10 15 20 25 30 35 3 8 13 18 23 28 33