OP-20-G
| OP-20-G | |
|---|---|
| Active | July 1, 1922 - July 10, 1946 |
| Allegiance |
 United States
|
| Branch |
 United States Navy
|
| Type | Code and Signal Section |
| Role |
Signals intelligence Cryptanalysis |
| Garrison/HQ | Navy Department building, Washington, D.C. |
| Commanders | |
| Notable commanders |
Captain Laurance Safford Captain Joseph Rochefort Captain John R. Redman |
OP-20-G or "Office of Chief Of Naval Operations (OPNAV), 20th Division of the Office of Naval Communications, G Section / Communications Security", was the U.S. Navy's signals intelligence and cryptanalysis group during World War II. Its mission was to intercept, decrypt, and analyze naval communications from Japanese, German, and Italian navies. In addition OP-20-G also copied diplomatic messages of many foreign governments. The majority of the sections effort was directed towards Japan and included breaking the early Japanese “Blue” book fleet code. This was made possible by intercept and High Frequency Direction Finder (HFDF) sites in the Pacific, Atlantic, and continental U.S., as well as a Japanese telegraphic code school for radio operators in Washington, D.C.
The Code and Signal Section was formally made a part of the Division of Naval Communications (DNC), as Op-20-G, on July 1, 1922. In January 1924, a 34-year-old U.S. Navy lieutenant named Laurance F. Safford was assigned to expand OP-20-G's domain to radio interception. He worked out of Room 2646, on the top floor of the Navy Department building in Washington, D.C..
Japan was of course a prime target for radio interception and cryptanalysis, but there was the problem of finding personnel who could speak Japanese. The Navy had a number of officers who had served in a diplomatic capacity in Japan and could speak Japanese fluently, but there was a shortage of radiotelegraph operators who could read Japanese Wabun code communications sent in kana. Fortunately, a number of US Navy and Marine radiotelegraph operators operating in the Pacific had formed an informal group in 1923 to compare notes on Japanese kana transmissions. Four of these men became instructors in the art of reading kana transmissions when the Navy began conducting classes in the subject in 1928.
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