Radio interception program began 1924

United States Naval Intelligence OP-20-G, the Code and Signal Section of the Division of Naval Communications, began its radio interception program in 1924, focusing on Japanese transmissions.

It came to the fore of military intelligence after decoding a series of 14 Japanese messages on Dec. 6, 1941, coordinating the attack on Pearl Harbor. Bureaucratic delays and misfires kept the advance notice from reaching troops on the ground before the bombing had done its damage to the Navy fleet.

The Washington branch of OP-20-G used the ECM Mark II rotor machine, also known as SIGABA, to encipher and decipher radio transmissions. The teletype-style machine used a system that randomized the action of the rotors to form its codes, designed on the same principles as Britain's Enigma code machine. They also worked in conjunction with British intelligence to develop the Combined Cypher Machine (CCM), which employed by ECM Mark II and the British Typex Machine's technologies.

OP-20-G upgraded to a high-speed bombe machine in 1942 and 1943, with 16 drum-operated four-rotor Enigma style randomizers. A total of 121 Naval bombes were built; the last of them is on display at the U.S. National Cryptologic Museum.

Intelligence gleaned from encoded transmissions, relayed as quickly as possible to the front, played decisive roles in the Battle of the Coral Sea and the Battle of Midway, as well as the downing of a Japanese plane carrying Fleet Admiral Isuroku Yamamoto in 1943, among other Allied victories.