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NSA Historical Figures

Leo Rosen

2010 Hall of Honor Inductee

Leo Rosen was instrumental in breaking the Japanese diplomatic code during World War II. His pioneering work in machine processing systems was the groundwork for much of the future U.S. excellence in cryptologic technology. He showed cryptologists the potential of processing equipment and helped the cryptologic services realize that potential in machine support in practical terms.

An ROTC graduate, Leo Rosen was called to active duty in 1939 with the Army Signal Intelligence Service (SIS). By the time he completed his cryptologic training, SIS workers had devised a "paper and pencil" method for solving one aspect of the Japanese machine cipher system known as "Purple."" The method was slow, cumbersome, and required many cryptanalysts.

Mr. Rosen prepared the layout of a machine that completely duplicated the function of the Japanese machine. With his equivalent, American cryptanalysts could simply type a "Purple" cipher text into a machine and retrieve a plain text decipherment from the printer. When the U.S. later recovered parts of an actual "Purple" machine, it was discovered that Japan, itself, had used stepping switches for their original device. Mr. Rosen had duplicated a machine he had never seen.

In 1940, Mr. Rosen was a member of a small team of Americans who made the initial cryptologic exchanges with the United Kingdom cryptologic organization. His technical knowledge contributed much then - and during the war years - to the good foundation for U.S.-UK technical collaboration in cryptology.

After the war, Mr. Rosen became Assistant Director of Research and chief engineer of the Army Security Agency, and later, for NSA. In 1967, NSA awarded him the Exceptional Civilian Service Award.