SIGABA:

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Used in the closing days of the war, SIGABA was the only machine used by any participant of the war to remain totally unbroken for the duration of its use. The United States was fortunate enough to be the entity that possessed it.

Image Courtesy of the National Cryptologic Museum

The SIGABA was a rotor machine, much like the Enigma (The Navy called it ECM-Mark Two). However, it was better. The improvements:

• While the enigma only had three rotors, the SIGABA had 15.
• The rotors of the Enigma moved in a speedometer fashion. The SIGABA instead used an invention of Lt. Joseph N. Wenger, called the "stepping maze." This device made the movement of rotors much closer to random. At least one but not more than four rotors, moved with each letter!
• The SIGABA was kept under the strictest of security. Ten-thousand of them were made and used, but not one fell into Axis hands. The German military didn't even know its name, referring to it only as the "big" machine.

After World War II, the SIGABA remained in service until 1959, when it was retired for being too slow. The machines were systematically destroyed, and only a few still exist. The largest reserve belongs to the National Security Agency (NSA), which claims to own six.

It should be noted that this complicated system is extremely insecure in today's world. It only takes a matter of seconds for a modern supercomputer to break the SIGABA. (It would take the average desktop a very long time.)