PURPLE Revealed: Simulation and Computer-aided Cryptanalysis of Angooki Taipu B Wes Freeman,1 Geoff Sullivan 2 and Frode Weierud 3 ADDRESSES: (1) 2527 Mardell Way, Mt View CA 94043 USA. Email:
[email protected]; (2) 64 Tennyson Road, Headless Cross, Redditch, Worcs., B97 5BJ UNITED KINGDOM. Email:
[email protected]; (3) Le Pre Vert, 1041 Rte de Mategnin, F-01280 Prevessin-Moens FRANCE. Email:
[email protected] ABSTRACT: 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. KEYWORDS: Pearl Harbor, PURPLE, RED, Rowlett, Stepping Switch, Computer Simulations, Hill Climbing, Roma-ji, Romazi. Introduction On December 6, 1941, the Japanese Government sent a message, which was divided for transmission into 14 parts (the 14-part message), to its embassy in Washington. An accompanying message directed that the 14-part message be delivered to the United States Government on December 7 at 1:00 p.m., which corresponded to 7:30 a.m. at Pearl Harbor, Hawaii. The 14-part message, which broke off negotiations between Japan and the United States, was enciphered on the PURPLE machine.