Radio revolutionized naval warfare by allowing a measure of command and control at great distances. However, this came at a price in security. Because radio is a broadcast, receivable by anyone, it can broadcast information to enemy as well as friend.
There are two significant aspects to this. First, it is possible for an enemy to analyze the pattern of broadcasts and detect enemy preparations. This is known as traffic analysis and it was practiced successfully by both sides in the Pacific War.
The second aspect is cryptography and cryptanalysis. Radio messages are encoded to prevent an enemy from discerning their contents (cryptography), but coded messages can be analyzed, the code broken, and the contents revealed (cryptanalysis). Both sides in the Pacific War were able to break at least some of the other’s codes, but the Allies were much more successful at cryptanalysis than the Japanese. Whereas Japanese code breakthroughs were usually into less important codes, such as weather ship codes, the Allies were able to break into every Japanese naval code except the flag officer's code. In particular, American cryptanalysts broke successive versions of the main Japanese naval operational code, which they dubbed JN-25.
Allied code breaking was one of the decisive capabilities of the war. It contributed directly to victory at Midway and was helpful in almost every other major operation of the war (the Guadalcanal campaign being a prominent exception.) Allied success at reading the Japanese merchant shipping code often put American submarines in the right places at the right times, and Allied success at reading the Japanese submarine code greatly reduced the effectiveness of Japanese submarines.
The Allies took great pains to conceal the source of their superb intelligence. However, the Japanese missed important clues that their codes had been broken. The most glaring was a report in the fanatically anti-Roosevelt Chicago Tribune that the Navy had known the Japanese battle plan before Midway. This glaring security breach was somehow missed by the Japanese. The reporter responsible was not prosecuted because this would only have drawn attention to the incident. However, the Navy officer responsible for the leak saw his career come to an abrupt end.
The most significant code penetration was of the main Japanese operational code, JN-25. The American naval intelligence unit at Pearl Harbor (HYPO), under the leadership of Lieutenant Commander Joseph Rochefort, had correctly surmised the nature of JN-25 by the time war broke out. A Japanese communications technician would prepare a message for transmission by first looking up the five-digit code number for each phrase or syllable in the message from a basic codebook. He would then select a random location in a second addend codebook containing over 30,000 random five-digit numbers. The sequence of random numbers from the second book would be added to the sequence of code numbers in the message (ignoring all carries in the addition) and transmitted along with the start location in the addend book. The communications technician responsible for decoding the message at the other end would reverse the process, subtracting the random addends from the coded message (ignoring all borrows) to reveal the original code numbers, which were then looked up in the basic codebook to reveal the original message.
Breaking the system required reconstructing both the basic codebook and the addend codebook. This was done largely by inspired guesswork, aided by the primitive computing machines available in 1941. (The U.S. Navy became an important customer of IBM as war approached.) Certain stereotypical phrases, such as "Long live the Emperor!", appeared in many messages, and by finding these phrases the code breakers could recover addends. As more and more addends were recovered, the code breakers could interpolate the gaps in the messages, filling out both the basic and the addend codebooks. But this was a tedious process that relied on having a very large volume of intercepted messages to work with. Little progress had been made prior to the attack on Pearl Harbor, but the enormous volume of messages intercepted after war broke out, combined with the highly predictable contents of certain messages, allowed the code breakers to reconstruct about 15% of the code books by the time of the Midway operation.
The code breaking process was made easier by Japanese complacence. The Japanese seemed to feel that their language was so difficult for Westerners to master, and their code system so cleverly constructed, that it was invulnerable to penetration by the Allies. The Japanese did periodically issue new addend books, but these were rapidly reconstructed because of the Japanese habit of continuing to send messages in the old code while the new code was distributed across the vast reaches of the Pacific. A message sent in both the old and new codes immediately gave the code breakers part of the new addend book. Another weakness of the system was that all the five-digit numbers were chosen such that the sum of the digits was divisible by three; this made it easier for the intended recipient to detect garbles, but it was also a weakness that was exploited by the code breakers.
References
The Pacific War Online Encyclopedia (c) 2007-2008 by Kent G. Budge. Index