Japanese Cryptology From The 1500s To Meiji - Code Improvements in The Nineteen Twenties and Thirties

Code Improvements in The Nineteen Twenties and Thirties

According to Yardley, the Japanese codes his Black Chamber broke in 1919 were improved by a Polish cipher expert about a year later. His exact words are :

Now the Japanese had no intention of permitting us to rest on our laurels, for from 1919 until the spring of 1920 they introduced eleven different codes.

We learned that they had employed a Polish cipher expert to revise their code and cipher systems. It took all our skill to break the new codes that this man produced, but by now we had developed a technique for the solution of Japanese codes that could read anything. Theoretically the Japanese codes were now more scientifically constructed; practically they were easier to solve than the first code, although some of them contained as many as twenty-five thousand kana, syllables and words.

The Polish cryptographer seemed to specialize on army codes, for the Japanese Military Attaché's codes suddenly became more difficult than those of any other branch of the Japanese Government.

Yardley was right about a Polish expert visiting Japan but he was mistaken about the timing. The Japanese army did bring in a Polish expert, Jan Kowalefsky, but he did not arrive in Japan until September 1924. If Japanese codes improved significantly between 1919 and 1924, as Yardley claims, the improvements were the work of Japanese cryptologists.

An interesting possibility, which is ripe for further research, is that Japanese cryptologists studied one or more of the books on codes and ciphers that were occasionally published in Europe and America. For example, Parker Hitt's 1916 book Manual for the Solution of Military Ciphers was hugely popular, selling around 16,000 copies in America. Also, Japanese military attachés might have been aware that Winston Churchill, in his 1923 The World Crisis, admitted that Britain had read German naval messages during World War I.

It is possible that Yardley is simply wrong and Japanese codes did not improve significantly between 1919 and 1924. Kahn found that one improvement Yardley mentions – three letter code groups mixed in with two letter groups – was not actually present in the Japanese telegram that Yardley claimed it was.

Japanese cryptographers supposedly improved their codes through sectioning – breaking the message into parts and rearranging them prior to encoding. This buries stereotypical openings and closings, which makes it harder for cryptanalysts to make initial breaks into a code by guessing at probable words. The technique is known as bisecting, Russian copulation, trisecting, tetrasecting, etc. depending on how many pieces the text is broken into. Sectioning was not a new or revolutionary technique in the 1910s.

If, as Yardley claims, some Japanese codes did have as many as 25,000 code groups at the time of the Washington Naval Conference, it would indicate a healthy appreciation of cryptological realities. Cryptographers have long known that bigger codes are better – all else being equal, a 25,000 group code is stronger than a 2,500 group code. In fact, many commercial code books as far back as the 1850s had 50,000 groups – but governments were often reluctant to pay for the production of large codebooks. This limited the size and thus strength of government and military codes for many years. To be fair, the secure production, storage, and distribution of codebooks is not easy nor is it cheap.

However, it seems unlikely that the Japanese government was using codebooks with 25,000 groups in the early 1920s. Jumping from the weak code used for the Washington Naval Conference to a book code of 25,000 in just a few years seems too fast, especially without some external indication that their codes had been compromised. Further, as shown below, even in 1926 the Army's top cryptologist was developing a cipher system that had only about 2,500 groups and those were actually just 10 charts of about 250 groups each.

Thus, the situation between the Washington Naval Conference and the mid-1920s was not that of a Polish officer helping to make Japanese codes much more secure. Rather, Japanese cryptographers were working to bring their codes up to the level of other major governments.

The Polish cipher expert, Jan Kowalefsky, might not have helped improve Japanese codes before the Washington Naval Conference but he did have a strong influence on Japanese cryptography between the conference and World War II. He trained what seems to be the first generation of professional Japanese cryptographers.