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

42
chapter 4
Cryptanalytic attack
Simple though it is, a transposition cipher may not be easy to solve. A frequency
count of the individual letters (‘monographs’) will reveal that they
have not been changed but the frequencies of pairs of letters (‘digraphs’)
such as TH, HE and QU will be different from what would be expected in
an English text. The cryptananlyst would therefore suspect that a transposition
system is being used and his first task in trying to solve it would
be to determine the length of the key.
Since the message above is 35 letters in length all 7 groups have 5
letters. The cryptanalyst would not know whether the message was genuinely
35 letters in length or had been ‘padded out’ with some ‘dummy
letters’ in order to produce full cipher groups, all of which have 5 letters.
In either case however he has a possible clue as to the length of the key.
Since 35�5�7 it is worth looking at the cipher text on the assumption
that the key is of length 5 or 7. He is making the assumption that the
transposition box is ‘regular’, i.e. that all the columns are of equal length;
they may not be, but this is a reasonable first step.
Assuming that the key is of length 5, two letters which were adjacent
in the original message will be 7, 14, 21 or 28 positions apart in the cipher
text unless one of the pair was at the end of one row and the other was at
the beginning of the next. The cryptanalyst would therefore write out the
cipher text in 5 columns of 7 letters, which has the effect of giving letters
which were next to each other in the plaintext a good chance of being in
the same row of the cipher text arranged in this way. Using this arrangement
the cipher text becomes as shown in Table 4.2.
Table 4.2
E T M I E
G I N L W
B O L N E
R D F A I
A E Y L T
V N E T E
I T H Y R
The next step is to look at the various pairs of letters in each row to see
which of them look to be the most likely digraphs. For this purpose access
to a frequency count of digraphs in English, such as can be found in the
Brown corpus and in some books on cryptography, is a great help. In the