Japan's PURPLE Code Machine

Jeffrey Herman jeffreyh at HAWAII.EDU
Sun Jul 5 06:20:35 EDT 1998

I don't usually read the obituaries, but the following was in
very large type which caught my eye. It talks about the person who
broke Japan's PURPLE Code. Like everyone else, I've heard of Germany's
Enigma and the effort to break it, but this was the first I'd heard of the
PURPLE machine. Can anyone provide some info?
Jeff KH2PZ

P.S. I hope crypto BAs aren't off topic for this list; they were
periodically discussed on the other BA list...
   Frank B. Rowlett, a premier U.S. cryptographer whose solution to
a major Japanese cipher machine saved hundreds of American lives during
WWII, died yesterday of heart failure in Gaithersburg, MD. He was 90.
   Rowlett supervised the half-dozen Army code-breakers who, after
18 months of effort, cracked the chief Japanese diplomatic cipher
machine - called PURPLE by U.S. officials - in September 1940.
   Though the solution gave no warning of the Pearl Harbor attack
- since no messages alerting anybody to the attack were ever sent -
it paradoxically helped U.S. forces in the European theater.
   The PURPLE machine encrypted the dispatches to Toyko of the
Japanese ambassador in Berlin. One of these detailed German defenses
against the expected Allied invasion, sent after his November 1943
tour of the Atlantic Wall, a line of fortifications. A U.S. radio post
in Ethiopia intercepted it; U.S. code-braekers read it.
   It revealed that tank ditches were "built in a triangular cross
section with a span across the top of 5 meters and a depth of 3.5
meters." Turrets were "built in a continuous line, close to the
shore, each equipped with two or three machine guns as well as
grenade launchers." Rowlett said he believed that the information
from this message alone saved thousands of U.S. lives.
   Rowlett, a soft-spoken, courteous Virginian who had been a
mathematics teacher, joined what was then the Army's Signal Intel-
ligence Service as a junior crypanalyst on April 1 1930. He said in
an interview that he felt his greatest contribution to the PURPLE
attack was his confidence that the machine would be solved.
   He has been awarded the National Security Medal, the Legion
of Merit and the Order of the British Empire.
   Rowlett is survived by his son, Thomas M. Rowlett of Frederick, MD.

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