Purple (cipher machine)

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Fragment of an actual Purple machine from the Japanese embassy in Berlin, obtained by the United States at the end of World War II.

This article is about the Japanese cipher Purple, also known as AN-1 (citation for AN-1 nomenclature for Purple?). For other Japanese ciphers, such as JN-25 which played a part in the Battle of Midway, see Japanese naval codes.

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; there had been a Red machine used by the Japanese Foreign Office, and purple was the next available color. The Japanese also used CORAL and JADE stepping-switch systems. Purple was a successor to, and improvement on, both the Red machine and what the Americans called the "M machine" (used in some embassies and consulates by attachés).

[edit] Development of Japanese cipher machines

[edit] 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 did its Purple design, so they were less used.

[edit] 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.

[edit] Red

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 pre-fixed "91-shiki" from the year it was developed.

The 91-shiki injiki Roman-letter model was also used 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.

[edit] Purple

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.

[edit] 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.^[1] Reconstruction of the purple machine was based on ideas of Larry Clark. Advances into the understanding of Purple keying procedures were made by Lt Francis A. Raven, USN. Raven discovered that the Japanese had divided the month into three 10-days periods, and within each period they used the keys of the first day with small predictable changes.

The Japanese believed it to be effectively unbreakable throughout the war —and even for sometime after the war—, even though they had been informed otherwise by the Germans. In April 1941, Hans Thomsen, a diplomat at the German embassy in Washington, D.C., sent a message to Joachim von Ribbentrop, the German foreign minister, informing him that "an absolutely reliable source" had told Thomsen that the Americans had broken the Japanese diplomatic cipher (that is, Purple). That source apparently was Constantin Oumansky, the Soviet ambassador to the US, who had deduced the leak based upon communications from Sumner Welles. The message was duly forwarded to the Japanese; but use of the code continued.^{[2] [3]} The United States obtained portions of a Purple machine from the Japanese Embassy in Germany following Germany's defeat in 1945 (see image above) and discovered that the Japanese had used precisely the same "stepping switch" in its construction that Leo Rosen of SIS had chosen when building a "duplicate" (or Purple analog machine) in Washington in 1939 and 1940. The "stepping switch" was a common type of unit used in high-tech telephone exchanges in countries like the United States, Canada, the UK and Japan, that had good dial-telephone systems in their large cities. These fast switches were at the heart of the systems.

Apparently, all of the other Purple machines at Japanese embassies and consulates around the world (e.g. in Axis countries, Washington, London, Moscow, and in neutral countries) and in Japan itself, were destroyed and ground into particles by the Japanese. It is certain that the American occupation troops in Japan in 1945-52 searched for any remaining units.^{[citation needed]}

The Purple machine itself was first used by Japan in June 1938, but US and British cryptanalysts had broken some of its messages well before the attack on Pearl Harbor. US cryptologists decrypted and translated the 14-part Japanese diplomatic message to its Washington Embassy (ominously) breaking off relations with the United States at 1 p.m. Washington time on 7 December 1941 before the Japanese Embassy in Washington could do so. Decryption and typing difficulties at the Embassy were a major reason the diplomatic "Nomura note" was delivered late.

[edit] Other Factors

During WW-II, the Japanese ambassador in Berlin, Baron Oshima, who was a military man, avidly studied German military developments and deployments and reported on them at length back to Tokyo via Purple-enciphered messages sent via radio. A good example of this was that he reported on the emplacement of the Atlantic Wall fortifications that the German Wehrmacht was building along the coasts of France and Belgium. Thus, unbeknownst to either the Germans or the Japanese, he was reporting to the Allies much about German military preparations against the forthcoming D-Day invasion of Western Europe. All that time, the Americans and the British were reading every report the Japanese ambassador was sending in Purple to Tokyo.

The decrypted Purple traffic, and Japanese messages generally, was the subject of acrimonious hearings in Congress post-WW-II in connection with an attempt to decide who, if anyone, had allowed the attack at Pearl Harbor to happen and who therefore should be blamed. It was during those hearings that the Japanese learned, for the first time, that the Purple cipher machine had indeed been broken.^{[citation needed]}

[edit] Further reading

An account of the WW-II cryptographic struggle is Battle of Wits, by S. Budiansky, which is not too overwhelmingly long or technical. Combined Fleet Decoded by J. Prados has, in somewhat dispersed form, a complementary and fuller account of Japanese cryptography specifically, much of it from sources on the Japanese side. Both are recent enough to reflect much of the release of information that had been kept secret since the war.

[edit] References

1. ^ Clark, R.W. (1977). The Man who broke Purple. London: Weidenfeld and Nicolson, 103-112. ISBN 0-297-77279-1. 
2. ^ Langer, Howard (1999). World War II: An Encyclopedia of Quotations. Greenwood Publishing Group, 198. Retrieved on 2008-02-11. 
3. ^ Kahn, David (1996). The Codebreakers: The Comprehensive History of Secret Communication from Ancient Times to the Internet. Scribner.  Text from excerpt of first chapter on WNYC website

[edit] External links

• The Purple Machine — information and a simulator (for Windows)
• Purple, Coral, and Jade