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"On This Date in History" Calendar

1941: Eligible readers of "Magic" intercept are established.

Monday, January 23, 2023

23 January 1941: Intelligence chiefs of the U.S. Army and Navy established a list of eligible readers of the "Magic" intercept. Follow the link at the bottom of the page to learn about the Magic of Purple exhibit at the NCM.

The Japanese "Purple" code did not break easily. For eighteen months cryptanalysts working for S.I.S. struggled with this difficult Japanese diplomatic code. Then, one day in September 1940, Genevieve Grotian made a discovery that would change the course of history. By analyzing and studying the intercepted coded messages, she found a correlation that no one else had yet detected. This breakthrough enabled other cryptanalysts to find similar links.

William Friedman and members of the S.I.S. built a crude model that was a remarkable imitation of Purple. Soon this product of American engineering and mathematical insight helped with reproducing the most guarded Purple communications. So impressed was one USN Rear Admiral that he called the process Magic, and the nickname stuck.

Almost sixty years later, Frank Rowlett, a cryptologic pioneer and head of the "Purple" team, remembered that historic day when the code broke. "What [Genevieve] Grotjan did was a big step forward and was very significant in the solution of Purple." Her discovery, and the work of other team members, allowed the United States to read secret Japanese diplomatic messages and to continue reading them throughout World War II. Genevieve Grotjan's contribution to the Allied victory cannot be measured.


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