The story about Communications Intelligence (COMINT) and World War II’s Battle of Midway is now well known. The U.S. Navy COMINT system in early 1942 had intercepted detailed information about a major impending Japanese operation against the Americans, scheduled for June. Navy cryptanalysts had solved enough of the Japanese enciphered code system, known to them as JN-25, to provide Admiral Chester Nimitz with detailed intelligence about the enemy naval forces coming against him.
The only problem was that in the decryptions of Japanese Navy communications, the target of their attack was referred to only as “AF.”
Some Navy intelligence personnel speculated that AF meant Samoa, Hawaii again, or even the west coast of the United States. The COMINT analysts and Nimitz’s intelligence officer believed the target actually was Midway Island. However, Nimitz could not commit America’s limited forces on a hunch.
In a conversation between the chief of the cryptanalytic unit at Pearl Harbor, Joseph Rochefort, and a veteran analyst, Jasper Holmes, a brilliant idea emerged. The U.S. Navy would have their garrison on Midway send a fake message in the clear, claiming problems with their water supply. This message would undoubtedly be picked up by Japanese COMINT stations, which might then confirm the assumption about the identity of “AF”.
This ploy worked.
On May 22, 1942, the Fleet Radio Unit Melbourne (FRUMEL, the U.S. Navy’s COMINT station in Australia) published this translation of a Japanese message broadcast to KIMIHI, the radio designation for Japanese Naval Intelligence headquarters in Tokyo, thus confirming AF was Midway:
“Refer this unit’s report dated 19th, at the present time we have only enough water for two weeks. Please supply us immediately.”
The Battle of Midway in early June was a stellar American naval victory and the Japanese Navy never recovered from its defeat there.