Question by Kevin7: what was the purple machine?
Answer by asimboral
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
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. 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.
The development of the machine was the responsibility of the Japanese Navy Institute of Technology, Electric Research Department, Section 6. In 1928, the chief designer Kazuo Tanabe (田辺一雄) and Navy Commander, Genichiro Kakimoto (柿本権一郎) developed a prototype of Red, Ō-bun taipuraita-shiki angō-ki (欧文タイプライタ暗号機) (“Roman-typewriter cipher machine”).
The prototype used the same principle as the Kryha cipher machine, having a plug-board, and was used by the Japanese Navy and Ministry of Foreign Affairs at negotiations for the London Naval Treaty in 1930.
The prototype machine was finally completed as 91-shiki injiki(九一式印字機) (“Type 91 print machine”) in 1931. The year 1931 was year 2591 in the Japanese Imperial calendar. Thus it was prefixed “91-shiki” from the year it was developed.
The 91-shiki injiki Roman-letter model was also used by the Ministry of Foreign Affairs as Angooki Taipu-A (暗号機 タイプA) (“Type A Cipher Machine”), codenamed Red by United States cryptanalysts.
The Red machine was unreliable unless the contacts in its half-rotor switch were cleaned every day. It enciphered vowels (AEIOUY) and consonants separately, perhaps to reduce telegram costs, and this was a significant weak point. The Navy also used the 91-shiki injiki Kana-letter model at its bases and on its vessels.
In 1937, the next generation 97-shiki injiki(九七式印字機) (“Type 97 print machine”) was completed. The Ministry of Foreign Affairs machine was the Angooki Taipu-B (暗号機 タイプB) (“Type B Cipher Machine”), codenamed Purple by United States cryptanalysts.
The chief designer of Purple was Kazuo Tanabe (田辺一雄). His engineers were Masaji Yamamoto (山本正治) and Eikichi Suzuki (鈴木恵吉). Eikichi Suzuki suggested use of stepping switch instead of the more troublesome half-rotor switch.
Clearly, the Purple machine was more secure than Red, but the Navy did not recognize that Red had already been broken. The Purple machine inherited a weak point from the Red machine, namely vowel-consonant separate encryption, which was called “sixes-twenties” by the US Army SIS.
Weaknesses and Cryptanalysis
An equivalent analog to the Purple machine reconstructed by the US Signals Intelligence Service. A hand-operated Red analog is also visible
In operation, the enciphering machine accepted typewritten input (in Latin letters) and produced ciphertext output, and vice versa when deciphering messages. The result was a potentially excellent cryptosystem. In fact, operational errors, chiefly in key choice, made the system less secure than it could have been; in that way the Purple code shared the fate of the German Enigma machine. The cipher was broken by a team from the US Army Signals Intelligence Service, then directed by William Friedman in 1940. Reconstruction of the purple machi
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