Invented by William F. Friedman in 1936 after he conceived the
idea for the C-325 in 1935. C-325 was also known as SIGFOY, and was
a response to the German Enigma machine. A patent was filed August
14,1944 but, for security reasons, the patent was not issued until
March 17, 1959. The machine contained three intermediate rotors and
one reversing rotor and powered by two flashlight batteries.
From 1944 to 1946, more than 1,100 units were distributed to
all posts of the U.S. State Department's Foreign Service. It was
found not to meet the operational requirements of the State
Department so its operational use was discontinued in 1946. Many of
its problems were attributed to faulty construction leading to many
encryption errors and the delicate bulbs and batteries which caused
problems in the field.