The U.S. Army's SIGABA, called the ECM (Electric Cipher Machine) in the Navy, was the only machine system used during World War II to remain completely unbroken by an enemy. The Germans referred to the U.S. machine, SIGABA, as the 'Big' machine. It utilized the same principle of rotating, removable, wired rotor wheels that the German Enigma employed. However, unlike the stepping motion of the Enigma, the SIGABA/ECM's motion appeared to be random. It wasn't, but it was so complicated, the German's never broke it, and the Japanese gave up trying. Frank Rowlett of the Army's Signal Intelligence Service developed the complicated stepping motion.