On the afternoon of December 7, 1941, Secretary of State Cordell Hull had to act surprised when Japanese diplomats delivered a message he had already read in translation. This episode explores the Allied cryptanalysis project known as Magic, from a stolen Japanese naval codebook in 1923 through the Blue system broken in 1932 to the machine cipher the Americans called Purple, which William Friedman and Frank Rowlett's team reverse engineered without ever seeing the hardware. It explains how cribs from formal diplomatic greetings let them deduce the machine's internal wiring.

The episode then looks at the dysfunctional system built around this triumph: an Army and Navy rivalry so bitter that the services alternated days handling decrypts, intercepts sometimes mailed to Washington, translation bottlenecks, and courier briefcases chained to wri ...