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 pm
The US Signal Intelligence Service, using the analog, had decrypted the message by 5 am. 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.
Credit: National Security Agency
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