Purple (cipher machine)

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In the history of cryptography, 97-shiki ōbun inji-ki (九七式欧文印字機) ("System 97 Printing Machine for European Characters") or Angōki Taipu-B (暗号機 タイプB) ("Type B Cipher Machine"), codenamed Purple by the United States, was a diplomatic cryptographic machine used by the Japanese Foreign Office just before and during World War II. The machine was an electromechanical stepping-switch device.

The information gained from decryptions was eventually code-named Magic within the US government.

The codename "Purple" referred to binders used by US cryptanalysts for material produced by various systems; it replaced the Red machine used by the Japanese Foreign Office. The Japanese also used CORAL and JADE stepping-switch systems.

Contents

Development of Japanese cipher machines

Overview

The Japanese Navy did not cooperate with the Army in cipher machine development, continuing to the war. The Navy believed the Purple machine was sufficiently difficult to break that it did not attempt to revise it to improve security. This seems to have been on the advice of a mathematician, Teiji Takagi (高木 貞治) who lacked a background in cryptanalysis[citation needed]. The Ministry of Foreign Affairs was supplied Red and Purple by the Navy. No one noticed weak points in both machines.

Just before the end of the war, the Army warned the Navy of a weak point of Purple, but the Navy failed to act on this advice.

The Army developed their own cipher machines on the same principle as Enigma, 92-shiki injiki (九二式印字機), 97-shiki injiki (九七式印字機) and 1-shiki 1-go injiki(一式一号印字機) from 1932 to 1941. The Army judged that these machines had lower security than the Navy's Purple design, so the Army's two cipher machines were less used.

Prototype of Red

Japanese diplomatic communications at negotiations for the Washington Naval Treaty were broken by the American Black Chamber in 1922, and when this became publicly known, there was considerable pressure to improve their security. In any case, the Japanese Navy had planned to develop their first cipher machine for the following London Naval Treaty. Japanese Navy Captain Risaburo Ito (伊藤利三郎), of Section 10 (cipher & code) of the Japanese Navy General Staff Office, supervised the work.

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