The Breaking of the JN-25 Code

Next to 911, Pearle Harbor may have been the most dramatic shaking of our nation’s sense of security in our history.  While that attack eventually backfired on Japan because it mobilized the American people to defeat the axis powers, it did accomplish the goal of instilling a great deal of concern that Japan would be able to dominate the war effort and eventually defeat us because they were able to pull off such a stunning blow as bombing our navy while in port.

The effort to fight back against Japan throughout the balance of the war was one of our country’s most difficult tasks and yet it was one that eventually proved to be eventually successful.  The fact that Japan was able to surprise us at Pearle Harbor created a tremendous need to vastly improve our abilities to spy on our enemies and get the jump on them when they are planning destruction against us.  While our intelligence communities had been working diligently even before the onset of war to crack codes used by foreign diplomats, the need to get inside the secret communications of Japan as we entered a period of open warfare with them was nothing short of crucial.

The code system that was the primary means of security for the Japanese military communications became known as JN-25.  The name of the code was not cryptic as JN simply stood for “Japanese Navy”.  But JN-25 was unique because it was a separate code the Japanese used for military intelligence so the need to crack this code became a high priority for our military spy efforts to help us defeat the Japanese in World War II.

A top secret but high priority effort by the intelligence community of the United States.  The project was given the code name “Magic” and the team that worked night and day to crack JN-25 included 738 highly trained naval staff headquartered at Pearl Harbor.  The commander in charge of the operation was Commodore John Rochefort who was uniquely qualified for the job with a perfect fluency in Japanese and an unstoppable stubbornness that he would stop at nothing to break the Japanese code.

The most brilliant mathematical and technical wizardry of the time was used to break this complex code.  The team even used a rudimentary computer known as the IBM ECM Mark III to analyze and decipher the massive 33,000 word code that was used by the Japanese to protect their military secrets. The team was able to exploit some weaknesses in the way the Japanese communicated that would eventually prove to be their undoing.  Despite the use of code, the Japanese often used flag phrase such as “I have the honor to inform your Excellency” and nicknames for commanders and key military personnel that were of tremendous help to the American team working to crack the JN-25 code.

Another flaw in the Japanese security plan came from their arrogance in thinking the code could not be cracked so they rarely altered the code itself.  By providing the American intelligence network with a reliable code, patterns could be discovered which lead to the deciphering of the JM-25.  Once the code was broken, our military had access to the most secure communications of the Japanese Navy which lead to victory after victory against this impressive military force including their defeat at the battle of Midway.  The breaking of JN-25, therefore, stands as one of the most important espionage efforts in World War II and led to the saving of thousands of American and allied lives and our ultimate victory against Japan and the axis forces.

1 thought on “The Breaking of the JN-25 Code”

  1. 1) You spelt Pearl wrong in the very first sentence.

    2) The man’s name was Joseph John Rochefort and went by “Joe”, and at the time of the Battle of Midway he was a Commander in the United States Navy, not a Commodore. He retired as a Captain.

    3) The code was JN-25, not JM-25.

    I’d strongly recommend reading “The Battle of Midway” by Craig L. Symonds. It’s pretty much the definitive work on the subject.


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