Purple (cipher Machine) - Weaknesses and Cryptanalysis

Weaknesses and Cryptanalysis

In operation, the enciphering machine accepted typewritten input (in the Roman alphabet) 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 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 unbreakable throughout the war, and even for some time 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 Konstantin Umansky, 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. 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 uniselector - a standard element used in large quantities in automatic telephone exchanges in countries like the United States, Canada, the UK and Japan, which had extensive dial-telephone systems.

Apparently, all 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. American occupation troops in Japan in 1945-52 searched for any remaining units.

The Purple machine itself was first used by Japan in June 1938, but U.S. and British cryptanalysts had broken some of its messages well before the attack on Pearl Harbor. U.S. cryptanalysts decrypted and translated Japan's 14-part message to its Washington Embassy (ominously) breaking off negotiations with the United States at 1 p.m. Washington time on 7 December 1941, before the Japanese Embassy in Washington had done so. Decryption and typing difficulties at the Embassy, coupled with ignorance of the importance of it being on time, were major reasons the "Nomura note" was delivered late.