Code makers and breakers of WWII

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 analogue machine was used by the US Army to decipher a 14-part message telling the Japanese ambassador to break relations with the US on 7 December at 1pm.

The US Signal Intelligence Service, using the analogue, had decrypted the message by 5am. 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