PURPLE was the designation given by U.S. cryptanalysts to the cipher machine used by the Japanese Foreign Office for secure communications before and during WW 2. We present the structure and internal wiring of the machine, as well as details of the keying procedures and a system of abbreviations, which was used in the messages. We have also written a computer simulation of the PURPLE machine. Operation of the simulator is demonstrated by deciphering portions of the 14-part message delivered to the United States on Dec. 7, 1941. Finally, an automated cryptanalysis method for the PURPLE system is presented.
