Jun 3

Navy Codebreakers – Paving the Way for Midway

Thursday, June 3, 2010 12:01 AM

From Robert D. Ballard and Rick Archbold, “Return to Midway” (Toronto: Madison Books, 1999), 34.

“A modern personal computer could break Japan’s World War II naval code in a matter of minutes. In the spring of 1942, however, it took cryptanalysts in Australia, Washington, D.C., and Hawaii untold hours of intense effort to achieve the breakthrough that made an American victory at Midway possible.

The Japanese naval code, known as JN 25, consisted of approximately 45,000 five-digit numbers, each number representing a word or a phrase. Breaking this code, which was modified regularly, meant divining the meanings of enough of these numbers that a whole message could be decrypted by extrapolating the missing parts. According to one of the leading codebreakers involved, it was like putting together a jigsaw puzzle with most of its pieces always missing.

Leading the codebreaking effort was Station Hypo, the code name for the combat intelligence unit at Pearl Harbor under Commander Joseph Rochefort. In the dank, air-conditioned basement where the intelligence teams worked, Rochefort padded around in bedroom slippers with a bathrobe thrown over his seldom pressed uniform. Rochefort and his staff could work virtually nonstop, and often went for days on a few hours’ sleep. Many of Rochefort’s men were bandsmen from the battleship CALIFORNIA, damaged at Pearl Harbor. Rochefort thought their musical skills might make them adept codebreakers in much the same way that Marine bandsmen used to serve as fire control technicians on ship — the ability to quickly read and play music made them excellent mathematical problem solvers.

By May 8, Rochefort knew that a major enemy operation, whose objective was sometimes called AF, was in the offing and that it would take place somewhere in the Central Pacific. Several days later, he was sure the target was Midway. His superiors in Washington weren’t convinced, so he and his team devised a test that would flush out the location of AF. The radio station on Midway dispatched an uncoded message falsely reporting that the water distillation plant on the island had broken down, causing a severe water shortage. Within 48 hours, Station Hypo decrypted a Japanese radio transmission alerting commanders that AF was short of water. And by May 27, when the Japanese finally got around to modifying the current version of JN 25, Rochefort had built up such a detailed picture of their plans that Lieutenant Commander Edwin Layton, Admiral Nimitz’s intelligence officer, was able to predict almost precisely when and where the enemy striking force would appear.”

As we prepare to commemorate the victory at the Battle of Midway, we should remember the people — officers and enlisted alike — who played such a critical role in what we know call “intelligence preparation of the battlefield.”