Japanese naval codes
In 1923, American agents seized a Japanese naval attaché's luggage in Washington D.C. They removed a set of codebooks bound in red covers from his briefcase. This operation provided the first major intelligence on Imperial Japanese Navy communications. Agnes Driscoll spent three years analyzing the material she received. She managed to crack the additive portion of what became known as Red Book code. The system relied on two separate books for encryption and transmission. One book contained the actual codes while another held random numbers added before sending messages. Knowledge gained from this effort helped cryptanalysts later break Blue Book code. The success proved that even diplomatic security could be compromised by physical theft.
Codebreakers named their target JN-25 after it was the twenty-fifth system identified by Western analysts. John Tiltman worked with Alan Turing at GCSB to understand its structure by 1941. They determined the code used five-digit groups translated from words via a codebook. A second book added random numbers to these digits before transmission. Knowing the method did not allow them to read any single message initially. By April 1942, American teams had deciphered about one in every five words within traffic. Commander Joseph Rochefort led Station HYPO at Pearl Harbor during this critical period. His unit struggled against heavy volumes of encrypted Japanese radio traffic. British and Australian agencies provided punched card machines manufactured by International Business Machines. These tools helped process the massive amount of data generated daily. Common phrases like I have the honor to inform your excellency served as known plaintext attacks. Such cribs allowed analysts to align patterns across thousands of intercepted messages.
Decryption efforts produced a specific intelligence report stating AF is short of water. This phrase confirmed that the main target of the Japanese Fleet was Midway Island. The information came from exploitation of JN-20 inter-island cipher systems. Periodic changes to JN-25 had temporarily blacked out U.S. decryption capabilities earlier in the year. Analysts relied on JN-20 to fill those gaps in understanding enemy movements. The resulting intelligence enabled an ambush operation by American forces in June 1942. A new version of JN-25 called JN-25b went into effect just before the attack on Pearl Harbor. It remained vulnerable enough for Allied teams to make progress by early 1942. The victory at Midway demonstrated how decrypted communications could alter naval strategy. Without this knowledge, the outcome of the battle might have been very different for both sides.
Admiral Isoroku Yamamoto planned a tour of forward bases in April 1943. His itinerary appeared within traffic encrypted by JN-25d code. Codebreakers recovered large portions of the additive book used for encryption only two weeks after its introduction. This breakthrough provided specific details about Yamamoto's flight schedule and route. United States forces used this intelligence to shoot down his plane during Operation Vengeance. The event occurred one year after the Battle of Midway. New versions of codes like JN-25c were issued from May 1942 onward. These updates included changes to super-enciphering books but left vulnerabilities exposed. The ability to read messages containing fleet dispositions proved decisive again. Japanese leadership failed to realize their codes had been compromised until too late.
A whale factory ship named Nisshin Maru No. 2 visited San Francisco on the 28th of May 1941. U.S. Customs Service Agent George Muller and Commander R.P. McCullough boarded her vessel. They seized codebooks without informing Office of Naval Intelligence officials. Copies were made clumsily before returning the originals to the ship. The Japanese quickly realized JN-39 was compromised and replaced it with JN-40. British cryptanalysts John MacInnes and Brian Townend worked at Kilindini in Kenya. They discovered an error by the Japanese that revealed a fractionating transposition cipher structure. By November 1942, they could read all previous traffic and break each message as received. Enemy shipping including troop convoys became trackable for Allied attack forces. Over two weeks they broke previously impenetrable systems known as JN-167 and JN-152. These merchant marine codes exposed critical supply lines across the Pacific theater.
British, Australian, Dutch and American agencies co-operated on breaking JN-25 well before Pearl Harbor. Station CAST operated jointly under Lieutenant Rudolph Fabian and Lieutenant John Lietwiler in the Philippines. The Far East Combined Bureau in Singapore provided additional support resources. Traffic analysis proved far more useful than direct decryption during early stages of conflict. Publicly available accounts differ regarding how much of JN-25 was broken before December 1941. Most credible sources agree no more than ten percent was readable at the time of the attack. IJN discussions often traveled via courier or direct delivery rather than broadcast methods initially. Once naval warfare began in late 1941, cryptographic depth increased dramatically. This surge in volume gave analysts enough material to succeed substantially against subsequent versions. Collaboration between nations allowed them to pool resources and share findings effectively.
In June 1942 the Chicago Tribune published an article implying U.S. Navy knew about Midway attacks beforehand. Reporter Stanley Johnston received Nimitz's executive order from Commander Morton T. Seligman. Col. Robert R. McCormick ran the newspaper and held isolationist views. The government considered prosecuting the publication under the Espionage Act of 1917 but dropped charges. A grand jury investigation generated further publicity without resulting in prosecution. Walter Winchell claimed the incident tossed security out of the window. Japanese forces changed codebooks starting with new JN-25c issued two months later. They believed Allies had obtained codebooks or additive tables through traffic analysis. Changes indicated reluctance to accept that Westerners could break their codes. Security breaches forced rapid adjustments to communication protocols across the fleet.
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Common questions
When did American agents seize Japanese naval codebooks in Washington D.C?
American agents seized a set of codebooks bound in red covers from a Japanese naval attaché's luggage in 1923. This operation provided the first major intelligence on Imperial Japanese Navy communications and enabled Agnes Driscoll to crack the additive portion of Red Book code over three years.
What was the specific intelligence report that confirmed Midway Island as the main target?
Decryption efforts produced an intelligence report stating AF is short of water which confirmed that the main target of the Japanese Fleet was Midway Island. This information came from exploitation of JN-20 inter-island cipher systems during June 1942.
How did United States forces shoot down Admiral Isoroku Yamamoto's plane?
Codebreakers recovered large portions of the additive book used for encryption only two weeks after its introduction in April 1943. This breakthrough provided specific details about Yamamoto's flight schedule and route allowing United States forces to execute Operation Vengeance one year after the Battle of Midway.
Which agencies co-operated on breaking JN-25 before Pearl Harbor?
British, Australian, Dutch and American agencies co-operated on breaking JN-25 well before Pearl Harbor through organizations like Station CAST and the Far East Combined Bureau in Singapore. Most credible sources agree no more than ten percent was readable at the time of the attack in December 1941.
When did the Chicago Tribune publish an article implying U.S. Navy knew about Midway attacks beforehand?
In June 1942 the Chicago Tribune published an article implying U.S. Navy knew about Midway attacks beforehand based on information received by reporter Stanley Johnston. The government considered prosecuting the publication under the Espionage Act of 1917 but dropped charges following a grand jury investigation.