How the Dutch broke
the Japanese Blue Code
in the late 1930s

Joost Rijneveld

Abstract: In a prelude to the Second World War, Japan sought to expand its territories. The Japanese navy used several code systems to encrypt the contents of their communication telegrams, most notably a code the intelligence department of the Dutch East Indies navy referred to as the '.' code. Through breaking this code, Johannes Frans Willem Nuboer was able to provide the Dutch with a valuable tactical insight in Japanese naval communications in the late thirties. This thesis will explain how the code worked and how the Dutch intelligence department broke the ‘.’ code. Through various examples, the inner workings of the code will be made clear. Additionally, an implementation of the algorithm that was used to break the cipher will be provided. Furthermore, evidence will be provided that indicates that the ‘.’ code is the very same code that the United States Navy refers to as the Blue Book or Blue Code.

Thesis: 2013-07-05

Summary paper: 2014-11-26

Source code: 2013-07-05

Related talks:
How the Dutch broke the Japanese Blue Code in the late 1930s
2013-07-01 – Bachelor thesis presentation, Radboud University –