TheCodeBreakers
Outlying stations of both the Army and Navy
picked out Japanese messages bearing certain indicators, enciphered the
Japanese cryptograms in an American system, and radioed them to
Washington. The reencipherment was to keep the Japanese from
knowing of the extensive American cryptanalytic effort. Only the three
top Japanese systems were involved in this expensive radio
retransmission: PURPLE, RED (a machine system that antedated PURPLE,
which had supplanted it at major embassies, but that was still in use for
legations such as Vladivostok), and the J series of enciphered codes. The
Army did not install a teletype for intercepts from its continental posts
until the afternoon of December 6, 1941; the first messages (from San
Francisco) were received in the early morning hours of December 7.
The intercept services missed little. Of the 227 messages pertaining to
Japanese...
