Magic (cryptography) - PURPLE Traffic

PURPLE Traffic

The Japanese Foreign Office used a cipher machine to encrypt its diplomatic messages. The machine was called "PURPLE" by U.S. cryptographers. A message was typed into the machine, which enciphered and sent it to an identical machine. The receiving machine could decipher the message only if set to the correct settings, or keys. American cryptographers were able to build a machine that could decrypt these messages.

The PURPLE machine itself was first used by Japan in 1940. U.S. and British cryptographers had broken some PURPLE traffic well before the attack on Pearl Harbor. However, the PURPLE machines were used only by the Foreign Office to carry diplomatic traffic to its embassies. The Japanese Navy used a completely different crypto-system, known as JN-25.

U.S. analysts discovered no hint in PURPLE of the impending Japanese attack on Pearl Harbor; nor could they, as the Japanese were very careful not to discuss their plan in Foreign Office communications. In fact, no detailed information about the planned attack was even available to the Japanese Foreign Office, as that agency was regarded by the military, particularly its more nationalist members, as insufficiently "reliable". U.S. access to private Japanese diplomatic communications (even the most secret ones) was less useful than it might otherwise have been because policy in prewar Japan was controlled largely by military groups (e.g., in Manchuria and elsewhere in China), not by the Foreign Office. The Foreign Office itself deliberately withheld from its embassies and consulates much of the information it did have, so the ability to read PURPLE messages was less than definitive regarding Japanese tactical or strategic military intentions.

U.S. cryptographers had decrypted and translated the 14-part Japanese diplomatic message breaking off ongoing negotiations with the U.S. at 1 p.m. Washington time on 7 December 1941, even before the Japanese Embassy in Washington could do so. As a result of the deciphering and typing difficulties at the embassy, the note was delivered late to American Secretary of State Cordell Hull. When the two Japanese diplomats finally delivered the note, Hull had to pretend to be reading it for the first time, even though he already knew about the attack on Pearl Harbor.

Throughout the war, the Allies routinely read both German and Japanese cryptography. The Japanese Ambassador to Germany, General Hiroshi Ōshima, often sent priceless German military information to Tokyo. This information was routinely intercepted and read by Roosevelt, Churchill and Eisenhower. According to Lowman, "The Japanese considered the PURPLE system absolutely unbreakable.... Most went to their graves refusing to believe the had been broken by analytic means.... They believed someone had betrayed their system."