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Posts Tagged ‘JN-25’

On March 13, 1942, U.S. cryptanalysts wedged the first cracks into Japan’s JN-25 code system.  As we well know, this bright spot falls into that dark, 6-month period for the U.S. armed forces between the disaster at Pearl Harbor and its first victory at Midway.

The advent of radio had really transformed radio communications for the world’s navies, allowing messages to be sent instantly over long distances.  Unfortunately, anyone with a receiver and the proper frequency could hear the message, and if one knew the language, well…secrets didn’t stay secrets very long.  So out came the codes, and they increased in complexity rapidly as each previous version was cracked by the enemy.

During World War II, Japan used numerous different coding systems.  There was one for the army, a Flag Officers Code (that the U.S. never cracked), and numerous others.  But JN-25 (as it was called by the U.S.) was the biggie, as it was used by the Japanese Navy…hence the “JN”.

This system consisted of a codebook with nearly 30,000 entries.  On top of that book was a “superenciphering” 300-page additive book, with each page containing 100 random five-digit sequences.  This created a sort of two-tiered encryption, which proved to be a tough nut to crack, indeed…even with the use of a very rudimentary computer (the IBM ECM Mark III).

But U.S. cryptanalysts were aided in their jobs by the Japanese themselves.  First, the five-digit sequences in the additive book were not used just once, but repeatedly, which gave codebreakers a hook on which to grab.  Second, Japanese command formality meant that phrases like “I have the honor to inform your Excellency” were used many times, as were nicknames for various commanders.  This repetition is anathema to encryption, because repeated patterns are the first things for which codebreakers look, and even using a different five-digit superencryption key couldn’t hide those pattern phrases for very long.

So rather that having to decipher a massive code system, it really became an exercise in collecting enough Japanese messages and putting enough smart people to work finding those repeated patterns.  Add in two parts patience and two parts persistence, and stir until enough of JN-25 was cracked to begin reading messages.

Within a month, the U.S. Navy had enough information to try a little test.  “Island AF” kept coming up in messages and the Navy suspected it was a reference to Midway.  So they told the guys on Midway to transmit that they were low on water.  Sure enough, a coded Japanese message was intercepted days later reporting Island AF low on water.

Game over.  Check and mate.

The Japanese knew their systems could be hacked, so they changed them periodically.  But their initial arrogance (caused by their incredible successes) meant they didn’t change them as much as they should have.  And they never really altered the basic structure of their messaging.  So U.S. codebreakers would simply look for the phrases and nicknames, which largely gave them “the key to the candy store.

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The Japanese Navy in May of 1942 was still largely unbloodied.  And it was not because they had simply avoided battle.  Rather, they had pretty much stomped any enemy that had dared oppose them.  Even the “setback” in the Coral Sea couldn’t really be looked on as a defeat.  After all, while the Japanese had lost a carrier and some airplanes, they were still the overwhelming power in the Pacific.  And the Americans had lost the USS Lexington, leaving them with just three aircraft carriers (a fourth, the USS Saratoga, had just finished repairs but was in San Diego being re-outfitted).

And now the focus was on Island AF.  You wondering where that is?  Island AF is actually Midway Island in Japanese code-speak.  The Japanese used an assortment of coding systems for their various organizations, but JN-25 was the one used by the Imperial Japanese Navy.  It was the most important and also the most secure of all Japanese ciphers.  And therefore, it was the biggest target of U.S. codebreakers.

And U.S. codebreakers had cracked much of it.  In fact, “Island AF” was part of the U.S. scheme as well.  The messages deciphered by U.S. intelligence mentioned “Island AF” on numerous occasions, and it was suspected the reference was to Midway.  So the U.S. Navy phoned the guys on Midway and told them to send an uncoded message back stating they were low on fresh water.  It was done and, not long after, the U.S. intercepted and deciphered a Japanese message about “Island AF” being short of water.  The U.S. had their answer.  From this they learned that Midway was the subject of intense Japanese interest.

But the Japanese were also aware that their codes could be compromised, so they periodically changed them.  They had done so in the weeks leading up to attack on Pearl Harbor, which greatly assisted in the success of that operation.  And now, six months had passed and another major operation (Midway) was looming.  It was time for another change.

But it’s possible that Japanese success over the first half of 1942 bred a little over-confidence in the Navy.  On May 25, 1942, the Japanese significantly changed their JN-25 coding structure and ciphers, which meant U.S. codebreakers had to start their jobs all over again.  But the change occurred right after they had broadcast their full operating plan for the attacks on Midway.  And so the plans (or at least parts of it) fell right into the hands of the U.S. Navy.

For the Japanese, the change from JN-25b to JN-25c meant they would achieve surprise when they occupied Guadalcanal in July of 1942.  But their attempt to take Midway would end in disaster.

Recommended Reading: Incredible Victory – The Battle of Midway

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