70 Years Ago: COMINT

When Japan attacked Pearl Harbor, they achieved surprise by effective use of several techniques:
1. Failure to declare war in advance of the attack;
2. Radio silence during transit;
3. Use of couriers for planning instead of radio;
4. Deceptive radio transmissions (spoofing);
5. Cryptosecurity measures, including changing codes just before the operation.

Items 2 - 5 fall in the category of communications security.

Japan had been a target of US Navy Signals Intelligence (SIGINT) since WWI. A small cadre of specialists intercepted Japanese high frequency (HF) radio communications  transmitted in a specialized Japanese Morse code for Japanese Kana characters. By 1941, the US Navy had about twenty intercept stations sharing their take with OP-20-G at the headquarters on Nebraska Avenue in Washington, DC. OP-20-G directed the effort to exploit these signals, (since the intercepted signals were mostly radio communications, the specialty was known as communications intelligence, or COMINT). The intercepted signals were analyzed by traffic analysts, who reviewed the patterns of communication and extracted an electronic order of battle or EOB.

Other experts reviewed the intercepts to determine how they had been coded and encrypted, and to identify any vulnerabilities that might allow the messages to be decrypted, or "broken." The first step was to determine if the message was a code or a cipher. Ciphers could be attacked using mathematical techniques. Codes were a bit more complicated.

To break either a code or a cipher required vast quantities of message traffic for analysis. Unfortunately for COMINT purposes, the Japanese Navy had used telephone, telegraph or courier for communications in their home waters. As a result, by December, 1941, US Navy cryptanalysts were only able to break about 10% of Japan's operating code known as JN-25.

After Pearl Harbor, though, as Japan's military invaded the Philippines, Borneo, Hong Kong, Singapore, the Dutch East Indies and elsewhere, they were forced to transmit vast quantities of radio traffic. By March 7, US Navy COMINT was able to break enough traffic for Admiral Nimitz to send the carriers Lexington and Yorktown to attack Japanese forces invading Salamaua and Lae on the north coast of New Guinea.

By early May, Nimitz was able to position Lexington and Yorktown in the Coral Sea and provide them with excellent information about Japanese forces and plans to invade Port Moresby on the south coast of New Guinea. This effectively halted Japanese advances in the Southwest Pacific.

By late May, 1942, COMINT provided Nimitz and the US Carrier task forces with complete information about Japanese plans to attack and invade Midway.

On May 26, 1942, Halsey's TF-16 with carriers Enterprise and Hornet steamed into Pearl Harbor, having missed the action in the Coral Sea by a day. After refueling and reprovisioning, Nimitz will send them back to sea under command of Raymond Spruance to take position to oppose Admiral Nagumo's four carriers approaching Midway.

The following day, USS Yorktown, heavily damaged at the battle of Coral Sea, limped into Pearl Harbor and went immediately into drydock. It was estimated that repairs would take three months. Yorktown was given three days.

Nimitz needed all three carriers, plus the additional Army, Navy and Marine Corps aircraft he had already sent to Midway as reinforcements.

He knew from COMINT what he was up against.

The attack on Midway was scheduled for June 4, 1942.