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Active July 1, 1922 - July 10, 1946
Allegiance  United States
Branch  United States Navy
Type Code and Signal Section
Role Signals intelligence
Garrison/HQ Navy Department building, Washington, D.C.
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.

The classes were conducted by the Room 2426 crew, and the radiotelegraph operators became known as the "On-The-Roof Gang". By June 1940, OP-20-G included 147 officers, enlisted men, and civilians, linked into a network of radio listening posts as far-flung as the Army's.

OP-20-G did some work on Japanese diplomatic codes, but the organization's primary focus was on Japanese military codes. The US Navy first got a handle on Japanese naval codes in 1922, when Navy agents broke into the Japanese consulate in New York City, cracked the safe, took photographs of pages of a Japanese navy codebook, and left, having put everything back as they had found it.

Before the war, the Navy cipher bureau operated out of three main bases:

The US Army Signal Intelligence Service (SIS) and OP-20-G were badly hobbled by bureaucracy. The groups had become rivals, competing with each other to provide their intelligence data, codenamed "MAGIC", to high officials. Eventually in 1940, SIS and OP-20-G came to agreement to provide MAGIC on alternating days, and try to draw up guidelines for which team handled what traffic. Complicating matters was that the Coast Guard, the FBI, and even the FCC also had radio-intercept operations.

The result was that much of the MAGIC was unused. There was no efficient process for assessing and organizing the intelligence, or getting it to proper end users.

Attack on Pearl Harbor[edit]

In the dark hours of the morning of 7 December 1941, the U.S. Navy communications intercept station at Fort Ward on Bainbridge Island, Washington, picked up a radio message being sent by the Japanese government to the Japanese embassy in Washington, D.C. It was the last in a series of 14 messages that had been sent over the previous 18 hours.

The messages were decrypted by a PURPLE analogue machine at OP-20-G and passed to the SIS for translation from Japanese, early on the morning of December 7. Army Colonel Rufus S. Bratton and Navy Lieutenant Commander Alvin Kramer independently inspected the decrypts.

They both became alarmed. The decrypts instructed the Japanese ambassador to Washington to inform the US Secretary of State, Cordell Hull, at 1:00 PM Washington time that negotiations between the United States and Japan were ended. The embassy was then to destroy their cipher machines. This sounded like war, and although the message said nothing about any specific military action, Kramer also realized that the sun would be rising over the expanses of the central and western Pacific by that time. The two men both tried to get in touch with Army Chief of Staff General George C. Marshall.

After some agonizing delays, Marshall got the decrypts and methodically examined them. He realized their importance and sent a warning to field commanders, including Major General Walter Short, the Army commander in Hawaii. However, Marshall was reluctant to use the telephone because he knew that telephone scramblers weren't very secure and sent it by less direct channels. Due to various constraints and bumblings, Short got the message many hours after the Japanese bombs had smashed the US Navy's fleet at anchor in Pearl Harbor.

After Pearl Harbor[edit]

As Japanese advances in the Philippines, the possibility of an invasion of Hawaii, and the increasing demand for intelligence, OP-20-G undertook two courses of action:

  • the staff and services of CAST were progressively transferred to a newly formed US-Australian-British station, FRUMEL in Melbourne, Australia.[1]
  • another signals intelligence center, known as NEGAT was formed in Washington, using elements of OP-20-G headquarters.[1]

Section Evolution[edit]

  • (July 1922-March 1935) Code and Signal Section (Op-20-G), Division of Naval Communications (DNC), OCNO (July 1922-March 1935).
  • (March 1935-March 1939) Communications Security Group (Op-20-G), DNC, OCNO
  • (March 1939-September 1939) Radio Intelligence Section (Op-20-G), DNC, OCNO
  • (October 1939-February 1942) Communications Security Section (Op-20-G), DNC, OCNO
  • (February 1942-October 1942) Radio Intelligence Section (Op-20-G), DNC, OCNO
  • (October 1942-July 1946) Communications Intelligence Organization (Op-20-G), DNC, OCNO
  • July 10, 1946 All Naval communications intelligence elements were collectively designated "Communications Supplementary Activities" of the 20th Division of the Office of Naval Communications, Section 2, (Op-20-2)

See also[edit]


Further reading[edit]