Imagine reverse engineering a complex machine you have never seen, using only the scrambled text it produces. That is what American cryptanalysts did with the Japanese Type B cipher machine, codenamed Purple, which protected Japan's highest-level diplomatic traffic before and during World War II. This episode traces the origins of Purple back to the humiliation of the 1922 Washington Naval Treaty, when the American Black Chamber read Japanese diplomatic messages, and to the flawed Red machine of 1931 whose weakness the Japanese Navy hid from the Foreign Ministry.

The episode explains how Purple used telephone-exchange stepping switches rather than rotors, why it split the alphabet into a weakly protected group of six letters and a heavily scrambled group of twenty, and how that design flaw, likely rooted in telegraph tariffs, gave William F ...