Japanese naval codes The vulnerability of Japanese naval codes and ciphers was crucial to the conduct of World War II , and had an important influence on foreign relations between Japan and the west in the years leading up to the war as well . Every Japanese code was eventually broken , and the intelligence gathered made possible such operations as the victorious American ambush of the Japanese Navy at Midway and the shooting down of Isoroku Yamamoto in Operation Vengeance . The Imperial Japanese Navy ( IJN ) used many codes and ciphers . All of these cryptosystems were known differently by different organizations ; the names listed below are those given by Western cryptanalytic operations . Red code . This was a code book system used in World War I and after . It was so called because the American copies made of it were bound in red covers . It should not be confused with the RED cipher used by the diplomatic corps . This code consisted of two books . The first contained the code itself ; the second contained an additive cipher which was applied to the codes before transmission , with the starting point for the latter being embedded in the transmitted message . A copy of the code book was obtained in a " black bag " operation on the luggage of a Japanese naval attache in 1923 ; after three years of work Agnes Driscoll was able to break the additive portion of the code . Blue code . This was another code book system which succeeded the Red code . JN - 11 . The Fleet Auxiliary System , derived from the JN - 40 merchant - shipping code . JN - 25 . JN - 25 is the name given by codebreakers to the chief , and most secure , command and control communications scheme used by the Imperial Japanese Navy ( IJN ) during World War II ( it was the 25th Japanese Navy system identified ) . Introduced in 1939 to replace Blue , it was an enciphered code , producing five - numeral groups for transmission . New code books and / or new superenciphering books were introduced from time to time , each new version requiring a more or less fresh cryptanalytic attack . In particular , JN - 25 was significantly changed on 1 December 1940 , and again on 4 December 1941 , just before the attack on Pearl Harbor . It was the 1941 edition which was sufficiently broken by late May 1942 to provide the critical forewarning of the Japanese attack on Midway . British , Australian , Dutch and American workers were cooperating in attacks on JN - 25 well before Pearl Harbor , but because the Japanese Navy was not engaged in significant battle operations before then , there was little traffic available to act as raw material . Before then , IJN discussions and orders could generally travel by routes more secure than broadcast , such as courier or direct delivery by an IJN vessel . Publicly available accounts differ , but the most credible agree that the JN - 25 version in use before December 1941 was not more than perhaps 10 % broken at the time of the attack , and that primarily in stripping away its superencipherment . JN - 25 traffic increased immensely with the outbreak of naval warfare at the end of 1941 and provided the cryptographic " depth " needed to succeed in substantially breaking the existing and subsequent versions of JN - 25 . The American effort was directed from Washington , D . C . by the U . S . Navy ' s signals intelligence command , OP - 20 - G ; at Pearl Harbor it was centered at the Navy ' s Combat Intelligence Unit ( Station HYPO , also known as COM 14 ) , led by Commander Joseph Rochefort . With the assistance of Station CAST ( also known as COM 16 , jointly commanded by Lts Rudolph Fabian and John Lietwiler ) in the Philippines , and the British Far East Combined Bureau in Singapore , and using an punched card tabulating machine manufactured by International Business Machines , a successful attack was mounted against the December 1941 code edition . Together they made considerable progress by early 1942 . " Cribs " exploited common formalities in Japanese messages , such as " I have the honor to inform your excellency " ( see known plaintext attack ) . The Purple cipher ( also sometimes referred to as AN - 1 ) , used by the Japanese Foreign Office as its most secure system , had no cryptographic connection with any version of JN - 25 , or indeed with any of the encryption systems used by the Japanese military . Purple traffic was diplomatic , not military , and in the period before the Pearl Harbor attack the Japanese military , which effectively controlled Japanese policy , distrusted the Foreign Office and told it little . JN - 25 traffic , on the other hand , was limited to military matters , mostly IJN operational ones , from which strategic or tactical information could sometimes be inferred . Nevertheless , decrypted Purple traffic was very valuable , especially later in the war , JN - 39 . This was a naval code used by merchant ships ( commonly known as the " " maru " code " ) , broken in May 1940 . 28 May 1941 , when the whale factory " Nisshin Maru II " visited San Francisco , U . S . Customs Service Agent George Muller and Commander R . P . McCullough of the U . S . Navy ' s 12th Naval District ( responsible for the area ) boarded her and seized her codebooks , without informing Office of Naval Intelligence ( ONI ) . Copies were made , in a clumsy way , and the originals returned . The Japanese quickly realized JN - 39 was compromised , and replaced it with JN - 40 . JN - 40 . JN - 39 was replaced by JN - 40 , which was originally believed to be a code super - enciphered with a numerical additive in the same way as JN - 25 . However , in September 1942 , an error by the Japanese gave clues to the codebreakers at the FECB , Kilindini . In fact , the code was a fractionating transposition cipher based on a substitution table of 100 groups of two figures each followed by a columnar transposition . By November 1942 , they were able to read all previous traffic and break each message as they received it . Enemy shipping was thus trackable , enabling Allied submarines to successfully attack it . JN - 152 . A simple transposition and substitution cipher used for broadcasting navigation warnings . In 1942 the FECB at Kilindini broke JN - 152 and the previously inpenetratable JN - 167 , another merchant shipping cypher . JN - 167 . A merchant - shipping cipher ( see JN - 152 ) . 1942 " Chicago Tribune " incident . In June 1942 the " Chicago Tribune " , run by isolationist Col . Robert R . McCormick , published an article that implied that the United States had broken the Japanese codes . This was a serious breach of national security . The government at first wanted to prosecute the Tribune under the Espionage Act of 1917 . For various reasons , including the desire not to bring more attention to the article , the charges were dropped .