A fragment of an actual Purple machine found in Berlin at the end of WWII

In the history of cryptography, 97-shiki-obun In-ji-ki (九七式欧文印字機) ("System 97 Printing Machine for European Characters") or Angooki Taipu B (暗号機B型) ("Type B Cipher Machine"), codenamed PURPLE by the United States, was a diplomatic cryptographic machine used by the Japanese Foreign Office during, and just before, World War II. Purple was an electromechanical (stepping switch) cipher. The color 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 the CORAL and JADE stepping-switch systems. The PURPLE machine 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 attaches). All were designed by a Japanese Navy Captain. The information gained from decryptions was eventually code-named Magic within the US government.

Weaknesses

In operation, the encrypting machine accepted typewritten input (in Latin letters) and produced cyphertext output, and vice versa when decyphering messages. The result was a potentially excellent cryptosystem. In fact, operational errors, chiefly in key choice, made the system much less secure than it could have been. The Japanese believed it to be effectively unbreakable throughout, and somewhat after, the War. It was broken by a team from the US Army Signals Intelligence Service, then directed by William Friedman. The team was led by Frank Rowlett.

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 as Leo Rosen of SIS had chosen when building a "duplicate" in Washington in 1939 and 1940.

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

The German Enigma machine and PURPLE

The German Enigma machine was unrelated to PURPLE, though there have been published claims that PURPLE was merely an Enigma copy of some sort. In fact, the Purple machine was a Japanese development, one of a series designed by a Japanese Navy captain, though there seems to have been some assistance by at least one Polish officer prior to the 1930s. There is some evidence that the Germans shipped several military Enigma machines to Japan by submarine late in the War; several sources suggest that they never arrived.

Further reading

An account of the WWII 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.

References

• Freeman, Wes., Geoff Sullivan, and Frode Weierud, "PURPLE Revealed: Simulation and Computer-Aided Cryptanalysis of Angooki Taipu B", Cryptologia 27(1), January 2003. pp 1–43.

External links

• The PURPLE Machine (http://frode.home.cern.ch/frode/crypto/simula/purple/index.html) — information and a simulator (for Windows)
• PURPLE, CORAL, and JADE (http://home.ecn.ab.ca/~jsavard/crypto/ro020304.htm)