Japanese Cryptology From The 1500s To Meiji - Analysis of The Two-Letter Code

Analysis of The Two-Letter Code

The two-letter code Japanese diplomats were using in 1919 consisted of two English-letter groups. This allows for a maximum of 676 (26*26) groups. That is far too small for a diplomatic code in 1819 much less 1919. Worse, the Japanese cryptographers did not use all of the available groups because Yardley says that the groups were either vowel-consonant or consonant-vowel, with “y” counting as both. If Yardley is correct about this, it means that the Japanese cryptographers limited themselves to only 252 of the 676 possible groups. After using anywhere from 54 to 100 groups for the kana and ten groups for the numbers zero to nine, there were at most 188 unassigned code groups remaining.

Yardley made his original break into the code by realizing that wi ub po mo il re re os ok bo was a i ru ra n do do ku ri tsu (Ireland independence). The doubled re re suggests the do do of airurando dokuritsu. This guess is confirmed when he discovers that the recovered groups re ub bo work elsewhere for do i tsu (Germany).

The initial break into the code is further confirmed when as fy ok makes sense as o wa ri (stop). This is exactly how one breaks a simple substitution cipher --- letter frequencies and repetitions in the text suggest possible plaintext letters. The cryptanalyst plugs in those letters and sees what yields meaningful text and what does not. Meaningful text suggests new letters to try and the cryptanalyst starts the cycle over again.

As can be seen from the description of Yardley's original break into the code, groups were assigned to kana like “do” and “bo” which in Japanese are not part of the regular alphabet but are created from other kana by adding pronunciation marks. Providing for these non-alphabet kana would require at least another 25 and possibly as many as 60 more code groups – hence the range given above for code groups for the kana – leaving only about 150 groups for words, phrases, and names. French cryptanalysts were making and breaking bigger, better codes in the 18th century. One suspects the Japanese language gave Yardley more trouble than the code itself did.

Thus the Japanese diplomatic code in use in 1919 was extremely weak and fundamentally flawed: a diplomatic code that does not contain code groups for common geopolitical names and phrases but requires them to be spelled out cannot be considered strong. Spelling out “stop” is further evidence that the code was not well designed. Even if the Japanese cryptographers devoted their 188 groups to the 188 most common phrases, the fact that they only had 188 groups to work with meant that most of their encoded messages would actually be simple-substitution enciphered messages of the sort that people had been solving for hundreds of years.