Thread by @GershomG

#OnThisDay (yesterday, actually, but I don't tweet on Rosh Hashanah): Sept. 20, 1940: Genevieve Grotjan, mathematician in US Army's Signal Intelligence Service, makes crucial break in Japan's Purple diplomatic cipher:
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@sheeraf @SpitfireFilly @ShiraOvide

Gene Grotjan was 26. She'd entered civil service b/c she hadn't been able to get a college teaching job, apparently b/c colleges wouldn't hire women.

The only law always followed in war is the Law of Unintended Consequences. Grotjan's rejection by universities led to her career in codebreaking - and to the discovery that allowed U.S. to read Japan's most secret diplomatic cipher.
@sarahposner @danephron

In sweltering heat of the Munitions Building - predecessor to the Pentagon - Genevieve Grotjan studied Purple messages and realized they'd been put through a triple scrambler similar to German Enigma.
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It was one week before German, Italy, Japan signed Tripartate Pact binding them to mutual defense. Negotiations likely increased flow of Japanese diplomatic traffic, helping breakthrough.

Under master codebreaker William Friedman, Signal Intelligence Service in Washington had broken Japan's earlier Red cipher. But in early 1939, Japan switched to Purple machine. Friedman's codebreakers then launched effort to break Purple.

Three weeks after Grotjan's insight, team under Lt. Leo Rosen builds ersatz Purple Machine. Japanese are using only few hundred of countless possible settings for Purple.

As usual, the encryption is no better than the care people take in using it. Taking off from Genevieve Grotjan's stroke of genius, Friedman's team rapidly finds settings that Japanese use on Purple cipher machine.

Big fear is that War Secretary Henry Stimson will shut down operation. Years earlier, as Herbert Hoover's Sect of State, Stimson shut U.S. codebreaking, famously explaining, "Gentlemen don't read other's mail."

In 1940, though, Stimson - a Republican - has joined FDR's cabinet to help prepare for likely war. He likes the break of Purple. "What you do in war & what you do in peace are two entirely different things," Stimson will explain.
@natsecHeather @artgoldhammer @ArielliNir

Thanks to Genevieve Grotjan, now FDR, Stimson & US military leaders can read Japanese diplomatic messages. But even decoded messages hold unclear warnings. Missing their meaning can have disastrous consequences.
@dovalfon @NimrodNovik @calder_walton

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