I am curious about just how much influence the Polish specialists had on Japanese cryptography. In "The Codebreakers", Khan mentions a Captain Kowalefsky (wikipedia has him as Kowalewsky) being employed, in Japan, to help the Japanese improve their codes. Khan says it was 1919-1920. A Japanese book, 暗号を盜んだ男たち (The Men Who Stole[?] the Codes) by 桧山 良昭 (sorry, I don't know how to read this name) goes into much more detail, but I am not sure how much of that is accurate. The Japanese book is labeled 'non fiction' but there are no sources given.

Another Japanese book, 「暗号解読」入門 (An Introduction to Deciphering Codes[?]) by 高川 (Takagawa - not sure of given name), has the Japanese asking the Polish for help on some translations from Siberia in 1919-ish, but not working with Kowalefsky until 1924. I'm not sure of the translation, but it could be that he also trained some students in Poland, either before or after his time in Japan. Unfortunately, this book also has no sources.

Wikipedia says that Kowalewsky went to Japan in 1923 but does not give a source for this.

Considering how strong the Japanese machine codes were later, and how weak many of their earlier paper codes were (thinking of the Washington Naval Conference here), is anyone aware of any more information on the development of Japanese cryptography between the world wars, especially the influence of the Polish experts?