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

From a cryptographic point of view a one-part code offers the cryptanalyst
too great an advantage by enabling him to guess at the meanings of
as yet unidentified code groups simply by looking for plausible words in a
dictionary which are close to identified words. This weakness can be
removed by making the numerical ordering of the code groups unrelated
to the alphabetical or numerical order of the codewords. We then have a
two-part code and the users need two code-books, one for encipherment
and one for decipherment. The codewords above might then appear like
Table 6.3 in the encipherment book, whilst the decipherment book might
begin as in Table 6.4 and so on.
Table 6.4
0005 TOMORROW
0009 ATTACK
0014 COME
In all cases, it is likely that very common codewords would be allocated
more than one code group and the users instructed to use each of the
alternatives in a ‘random’ manner.
Although codes which have not been subjected to further encipherment
do not offer a high level of security they have been used in wartime;
the Italian Navy used a one-part code, known as Mengarini [6.2], for some
very low grade messages, and the Japanese Navy used a two-part code,
known to them as OTSU and to the British as JN4 [6.3], during the Second
World War. A somewhat different code using two letters followed by four
digits was used by U-boats of the German Navy to report their positions
in the Atlantic and to receive instructions for attacking Allied convoys.
The letters were subjected to digraph substitution tables and the digits
could also be modified [6.4].
Code plus additive
No matter how many code groups a code contains, a cryptanalyst, given
enough messages, will eventually find certain groups occurring more
than once, even when the same plaintext word or phrase has several alternative
code groups allocated to it. Also, if the code-book is captured by the
enemy, decryption of all messages becomes trivial. To overcome these
weaknesses the code groups themselves are usually enciphered. A standard
way of doing this is to apply an additive key to the code groups using
non-carrying, or modular, addition. Although this has been mentioned
Codes 67