Missing Pieces: - Royal Australian Navy

codebreaking
appendix -
australian sigint and intelligence
contributions in wwii
It has become accepted fact that the important breaks into Japanese codes before and
during WWII were made by the United States, with some assistance from the British.
Recent research has uncovered information which demonstrates that the British role
was rather more important, and that the Australian contribution to the struggle to break
and derive intelligence from Japanese communications was considerable.
The first break into an IJN code was made by Paymaster Lieutenant Nave, RAN, in 1925
during his attachment to the British China Fleet. 859 Nave continued his groundbreaking
work in the China Fleet, in London with GC&CS, and later with FECB. His efforts were
supported by the RAN’s Radio Operators Special, who undertook a number of important
tasks of monitoring Japanese communications in the Mandated Islands, including a
complete analysis of traffic in 1927 from the steam yacht Franklin.
The Japanese may have used as many as 700 different codes. 860 The IJN alone used at
least 24 major codes, which does not include callsign and address codes, date/time
codes used internally in messages, and other internal codes to conceal map references
and other sensitive information. Overwhelmingly, Japanese messages were protected
by codes and not by ciphers. 861 This marks an important difference from the German
use of the machine cypher Enigma.
Throughout the 1930s the IJN continued to introduce new and improved codes, which
were promptly broken by the British. The new five-figure code, Naval Code D, was
introduced in early 1939. Designated ‘JN-25’ by the Americans, Code D was being
broken by the FECB Sigint unit led by Nave in Singapore by December that year. 862
The USN Corregidor RI Unit was also making some progress, but the Americans
were surprised at the extent of British penetration of the code when direct contact
between CAST and FECB was approved in March 1941. With both organisations sharing
information, the second ‘B’ edition new code was partially (10 per cent) readable by
December 1941. 863 The real breakthrough came in March 1942, when it was realised
that the 4 December change of cipher to the B-8 version was only a minor variation
on B-7. This vastly speeded recovery of JN-25, and by May 1942 USN cryptanalysts
were delivering transcripts of intercepts as soon as six hours after transmission by