After 18 months of analysis, cryptanalysts from the Army's Signal Intelligence Service determined that telephone-switching units could be the basis for the Purple machine (pictured here is its cipher switch).
An analog machine was used by the U.S. Army to decipher a 14-part message telling the Japanese ambassador to break relations with the United States on December 7 at 1 p.m.
The U.S. Signal Intelligence Service, using the analog, had decrypted the message by 5 a.m. War was clearly imminent, according to the National Security Agency, but the message lacked details. Regardless, some believe that breaking Purple is the greatest feat in cryptologic history.
Photo by National Security Agency
Caption by Zoë Slocum
