Code breaking (Enigma).

The attack on the Japanese Purple code was led by William F. Friedman of the Army Signal Intelligence Service and the crew of talented cryptanalysts he assembled at Arlington Hall Junior College across the Potomac from Washington, D.C. The difficult technical problem Friedman’s team faced was figuring out the Purple machine scrambling patterns. Leo Rosen, an electrical engineer at the Massachusetts Institute of Technology (MIT), discovered the way the six most common letters were rotated through twenty-five contacts—a major advance, but of no help with the other twenty letters. Then, one year later, on September 20, 1940, Genevieve Grotjan had the crucial insight into the manner by which the many thousands of patterns were linked in their own pattern of cycles. Whereas a single scrambler mixed up the six common letters, three scramblers interconnected by hundreds of wires transmitted the other twenty letters. Finally, after three weeks, Rosen and Frank Rowlett soldered the last of thousands of connections, flipped a switch, typed in a Purple cipher text, and watched the deciphered message roll out of the printer.