Behind the Victory: Midway and JN25
By: Stephen P. Zammit
In the Pacific Theatre during the Second World War, Japanese forces were relentless in their attacks and
victories over the Allied forces in South East Asia and in the Western Pacific. The string of victories from
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December 7 1941 to May 20 1942 included – the attack on Pearl Harbor, the capture of Thailand,
Malaya, Wake Island, Guam Island, the Philippines, Hong Kong, Singapore, Burma, Dutch Indonesia and
the invasion New Guinea. Japanese naval and air assets roamed the Pacific attacking Allied bases in
Australia, Ceylon, east Africa, and even the west coast of North America at will. But, the sea battles of
Coral Sea and more specifically at Midway, in May / June 1942 put a halt to the wave of Japanese
victories.
What happened at Coral Sea and Midway? Why did
American Navy stop the Japanese advance at these critical
points – Were American aircraft and ships better than
Japanese equipment? - No, production of new classes of
ships and aircraft would not arrive in the Pacific until 1943.
Did American forces have more men, aircraft and ships?
Again no, the tide of the American industrial strength would
not be felt in the Pacific until 1943. Was it better leadership?
Admiral Chester W. Nimitz, who would superbly command
U.S naval forces in the central Pacific from the early days of
1942 until the end of the war, was equally matched by Admiral Isoroku Yamamoto overall commander of
Japanese forces during the battles of Coral Sea and Midway.
The difference was Allied (American, Australian, British, and Dutch) intelligence and cryptanalyst who cooperated in the breaking of the Japanese naval code JN-25. JN-25 a military forces code system (the
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25 Japanese Naval system identified) was the most used and secured communication system used by
Japanese naval forces at the beginning of World War Two. Introduced in 1939, JN-25 code replaced the
earlier code “BLUE” (as named for the blue binding that the intercepted code messages were placed in);
JN-25 an enciphered code, producing five numeral groups in the traffic which was actually broadcast,
was frequently revised, with new code books being created on a regular basis.
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Before December 7 , 1941 interception of JN-25 transmissions was limited, thus Allied cryptanalysts had
little material or opportunity to decode this new code. But, from December 1941 to May 1942 the
increased radio traffic between Japanese units allowed U.S. Navy’s signal intelligence cryptanalysts in
OP-20-G to crack the current version of JN-25 just in time for Coral Sea and Midway.
Commander Joseph J. Rochefort (1900-1976) commander of Station Hypo (Hawaii) with assistance from
the teams at Station Cast (Cavite, Philippines) and the British Far East Combined Bureau these elements
of Allied cryptanalyst were able to crack the JN-25 code. Rochefort’s team in Hawaii was also able to
convince Admiral Nimitz that the next major attack in the central Pacific would be the Midway Island
chain. This was done by having an unencrypted emergency (false) message sent to from Midway – that
AF’s need for assistance in the repair of its water desalination system (with AF being – Midway’s
CSCSS Featured Briefing: Behind the Victory / Midway and JN25 © 2012 Center for Strategic Cyberspace + Security Science Centre for Strategic Cyberspace + Security Science / CSCSS codename) to Pearl Harbour. The Japanese listening posts intercepted the message and reported the
fact in encrypted messages to naval command. Rochefort’s team intercepted these messages, and with
the ability to read the JN-25 code confirmed that Midway was the next target of the Imperial Japanese
Navy. An addition benefit to this trickery was that in follow up messages the Allied cryptanalyst were able
to intercept and decode the timing of the attack – early June 1942.
The advantage of superior cryptanalysts, code breakers, scientists, technology, and knowledge in the
field of intelligence provided the information which enabled the Allies to defeat the Japanese at these two
critical battles. Nimitz positioned his outnumbered forces strategically where he could ambush and defeat
the Imperial Japanese forces at Midway. The decimation of Japan’s aircraft carrier fleet and senior naval
aviation personnel would be a serious problem for the Japanese throughout the remainder of the Pacific
war. For the Allies the Battle of Midway was a deceive victory and the turning point in the Pacific, a victory
that may have not happened without the Allied code breakers.
In relation to information and information warfare, JN25 and the cryptanalysts group involved within the
U.S. Navy; this is a prime example of information manipulation, interception, cryptography and
misdirection to achieve a strategic victory. Ultimately turning the tide in the Pacific.
In terms of cyber and information warfare, old is new, and our contemporary information age it is an
ongoing continuance.
CSCSS Featured Briefing: Behind the Victory / Midway and JN25 © 2012 Center for Strategic Cyberspace + Security Science Centre for Strategic Cyberspace + Security Science / CSCSS