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. The Allies also broke successive versions of the Japanese diplomatic machine code, dubbed Purple.
The U.S. Navy used the ECM (Electronic Cipher
Machine) Mark II, which resembled the German
Enigma or Japanese Purple. It used three banks of five rotors each and
had on the order of 100,000,000,000,000 possible settings. Operators
were strictly admonished to avoid the kinds of procedural errors that
allowed the Allies to penetrate Axis machine codes, and it is claimed
that the ECM Mark II was never penetrated.
The Japanese did have important successes against Chinese codes. The Owada Receiving
Station monitored Chinese and American
military traffic from 1936 on. In July 1937 the station successfully
intercepted and decoded Chinese defense plans for 27 Army,
and it evaluated the results of Japanese air strikes during
the Shanghai Incident later than
year by intercepting and decoding Chinese damage reports.
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 deduced 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 representing 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 (known as cribs) 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.
Both ULTRA (breaking of Japanese operational codes) and MAGIC
(breaking of Japanese diplomatic codes) remained largely classified
postwar. The fact that Japanese diplomatic codes were being read at the
time of Pearl Harbor became public knowledge in the postwar
Congressional investigations of that disaster. A small number of MAGIC
messages from later in the war were released in the 1950s, but the
first relatively complete set of MAGIC digests were not released until
1978. Even these had many sections whited out. A complete set of
uncensored MAGIC digests was finally published in 1995, revealing
(among other things) that MAGIC was reading the diplomatic
traffic of a large number of neutral countries, and even a few Allied
powers (such as Nationalist China
and the Free French.) ULTRA did not
begin to be revealed until the 1970s.
The fact that ULTRA and MAGIC remained secret for so long has had an
enormous influence on postwar historians. The greatest single flaw in
Morison's otherwise excellent History
of United States Naval Operations of World War II is that it was
written when ULTRA was still highly classified, and Morison recites the
official cover story for every important ULTRA-based intelligence coup
in the war. A useful companion while reading Morison today is Prados' Combined Fleet Decoded, which gives
the ULTRA background for many important command decisions during the
war.
The gradual disclosure of MAGIC has had its effect on historiography
of the Japanese surrender, as
documented by Frank (1999). Early and
fragmentary MAGIC disclosures, indicating that the Japanese were
seeking peace terms through the Russians,
have been seized upon by critics of Truman's
decision to use nuclear weapons
as evidence that the Japanese were on the verge of surrendering and the
bombings were unjustified.
Frank has argued that the complete MAGIC record released in 1995
reveals a much more complex picture that is much less supportive of
this interpretation. For example, the ULTRA decrypts reveal a uniform
picture of determination on the part of the military to continue the
war.
References
San
Francisco Maritime National Park Association (accessed 2010-4-19)
The Pacific War Online Encyclopedia © 2007-2010 by Kent G. Budge. Index
