Abstract
In May 1945 an Imperial Japanese Army Green cipher machine was captured at Baguio in the Philippines. The machine was unlike any previous Japanese cipher machine that was known to U.S. codebreakers; GREEN made use of multiple irregularly stepping rotors and a fractionating matrix. The U.S. Army’s Signal Security Agency began an urgent effort to understand the machine and its weaknesses, design an attack, and construct an analog. However, three months after the capture of a Green Machine, the war with Japan ended without any messages enciphered by GREEN having been intercepted. Six months after the war ended, construction of a GREEN analog was completed, and the analog was delivered to the museum.
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