SIGABA / ECM Mark II
America’s most secure WWII cipher machine, designed by William Friedman and Frank Rowlett.
Its genius: 15 rotors in three banks. The 5 cipher rotors encrypt; the 5 control rotors and
5 index rotors determine which cipher rotors step—and by how much. This irregular,
unpredictable stepping defeated all known cryptanalytic attacks.
SIGABA traffic was never broken by any Axis power during WWII. The machine
remained classified until the early 2000s. Each machine weighed about 100 lbs and required
careful handling. Operators were ordered to destroy the device rather than let it be captured.
Over 10,000 units were produced between 1935 and 1947.