Update

In The Japanese FUJI diplomatic cipher 1941-43 I’ve added the following:

1). In ‘Allied exploitation of the improved J series codes’:

When the new J-19 system was introduced the US codebreakers were already familiar with the basic characteristics of the cipher and Rowlett quickly made important discoveries regarding the underlying code. However solution of the daily key settings was a difficult problem, especially since more resources were put into the solution of the traffic sent on the PURPLE cipher machine.

2). In ‘Australian effort’:

Progress in 1941 was slow and up to February 1942 the only keys solved were those for messages whose content was known (for example messages reporting the departure of ships). However in 1942 things progressed rapidly.

In March ‘42 a member of the British Foreign Office from Singapore who possessed an excellent knowledge of Japanese joined the section. At the same time personnel of the unit developed elaborate cryptanalytic methods for recovering the daily settings and by May ‘42 the section was able to read virtually all FUJI traffic and ‘all bigrams, except those of very rare occurrence, and most tetragrams had been recovered’.

3). In ‘OKW/Chi effort’:

The OKW/Chi designation for FUJI was system J-13/J2B4BCüRuW (Japanese 2-letter and 4-letter code with stencil and transposition – Raster und Würfel). FUJI messages were first solved thanks to a repeat message sent from Paris to Tokyo. The first message and the repeat had the same plaintext (with small variations) and they had both been enciphered with the same key. This mistake facilitated their solution and the basic characteristics of the system were identified.

The solution of the daily transposition settings and the different stencils was taken over by personnel of the mathematical research department, specifically by the mathematician dr Werner Weber.

According to Part 3 of the report I-181 ‘Homework by Dr Werner Weber of OKW/Chi’, Weber started working on Japanese diplomatic messages in July ’41 and he identified the system as a transposed code. The underlying code for some of the messages was the previously solved LA code, thus they could be read. The rest of the messages had a new code.

Solution of the new system and recovery of the code proceeded slowly in 1941. In September ’41 Weber was allocated a small staff to help him with the Japanese traffic and by February ’42 some material could be read. During the year the new system was solved and most of the circular and European/Middle East traffic could be read. In the period summer ’42 to summer ’43 the previous year’s indicators were reused and the old transposition keys and stencils were either repeated or were modified in a predictable manner (with some exceptions).