The Japanese have used 4 electric circuit cipher machines,
the Red, the Purple, the Coral and the Jade.
The Red was formerly used on diplomatic channels. In this
machine, the circuits were divided into two sections, one of
six circuits and the other of twenty. The sections were
handled separately by two commutator wheels which effected
a simple progressive vigenere substitution. The period of the
encipherment was 60, since both commutators stepped after each
encipherment. In addition there was an interrupter wheel of
period 47, which caused the commutators to skip certain po-
The letters were attached to the circuits by a pluggable se-
quence which was the same at both ends. However, there was
an additional feature which scrambled the plugging further.
The details of this additional scrambling have never been
completely recovered. This problem is no longer current.
The Purple was introduced to replace the Red on many diploma-
tic channels, and is still current. The circuits are divided
into groups of 6 and 20. The substitutions are effected by
telephone selectors. (A telephone selector is a device which
applies, in different positions, 25 unrelated substitutions).
The six-bank has overall period of twenty five, the circuits
traversing just one telephone selector, which steps at every
encipherment. The other twenty circuits traverse a sequence
of three telephone selector banks. The stepping of these
other banks is quasi-metric of a distinctive type, and has 6
variations. Any bank can be made fast, slow or medium speed.
The input and output are independently pluggable, but, in
practice, the sequences have always been the same or related.
Cryptanalytically, the machine can be attacked by breaking
out the six-bank and cribbing. The letters of the six bank
betray themselves by frequency count. Since there is a set-
ting list of 240 starting points and motions, and an almost
completely recovered book of 1000 sequences which are used in
a systematic fashion, the traffic is read currently without
much cryptanalysis. No special cryptanalytic machinery is