United States Cryptologic History Cryptologic States United United States Cryptologic History Pearl Harbor Revisited: U.S. Navy Communications Intelligence 1924–1941 Pearl Harbor Revisited Harbor Pearl 2013 Series IV: World War II | Volume 6 n57370 Center for Cryptologic History This publication presents a historical perspective for informational and educational purposes, is the result of independent research, and does not necessarily reflect a position of NSA/CSS or any other U.S. government entity. This publication is distributed free by the National Security Agency. If you would like additional copies, please submit your request to: Center for Cryptologic History National Security Agency 9800 Savage Road, Suite 6886 Fort George G. Meade, MD 20755 Frederick D. Parker retired from NSA in 1984 after thirty-two years of service. Following his retirement, he worked as a reemployed annuitant and volunteer in the Center for Cryptologic His- tory. Mr. Parker served in the U.S. Marine Corps from 1943 to 1945 and from 1950 to 1952. He holds a B.S. from the Georgetown University School of Foreign Service. Cover: First Army photo of the bombing of Hawaii, 7 December 1941; the battleship USS Arizona in background is on fire and sinking. Signal Corps photo taken from Aeia Heights. Pearl Harbor Revisited: U.S. Navy Communications Intelligence 1924–1941 Frederick D. Parker Series IV: World War II | Volume 6 Third edition 2013 Contents Foreword ...................................................................... 5 Introduction .................................................................. 7 Earliest Efforts .............................................................. 8 Role of ONI ................................................................... 9 Early War Plans ............................................................. 11 Japanese Fleet Capabilities and Intentions ............................ 13 Cryptologic Challenge: Navy-Army Cooperation ................................................ 16 Bureaucracy Prevails ...................................................... 18 Status Quo in the Pacific ................................................. 18 A New Attempt at Cooperation .......................................... 19 Japanese Cryptography ................................................... 20 Recovering the “Blue Book” ............................................. 21 Introducing JN-25 .......................................................... 21 Turning Victory into Defeat .............................................. 22 Introducing “Purple” ...................................................... 23 Disarmament: Paying the Price .......................................... 24 Confusing Diplomacy ...................................................... 24 Struggling for Resources .................................................. 25 Planning for War ........................................................... 27 War Games .................................................................. 32 Pacific Buildup ............................................................. 33 National Emergency ....................................................... 33 Circumstances Favor Diplomatic Targets ............................... 34 Information Gaps .......................................................... 35 Cooperation with Allies ................................................... 35 The Move to Hawaii ....................................................... 36 Pearl Harbor Revisited Support to the Pacific Fleet .............................................. 37 Japanese Intentions Revealed ........................................... 38 Support to Asiatic Fleet ................................................... 38 Japanese Strategy ......................................................... 38 Japanese Navy ............................................................. 39 Diplomatic Messages ...................................................... 42 Warning Messages .......................................................... 43 Reaction ..................................................................... 44 COMINT after the Opening Attack ........................................ 46 Conclusion .................................................................. 47 Appendix A: Naval Messages Intercepted between 5 September and 4 December 1941 ................................... 49 Appendix B: Summary of Diplomatic Messages July–November 1941 ........................... 63 Appendix C: Highlights from COM-14 Daily COMINT Summary ...... 66 Notes ......................................................................... 75 Abbreviations Used ......................................................... 83 Notes on Sources ........................................................... 85 Bibliography ................................................................. 88 iv Foreword r. Parker’s monograph is the first in a series activity clearly signaled the creation of a massive car- treating the U.S. Navy’s communications rier strike force with the major naval objective some- intelligenceM (COMINT) efforts in the Pacific during where in the Pacific Ocean far distant from either World War II. A second volume, also by Mr. Parker Indochina or the Philippines. (A Priceless Advantage: U.S. Navy Communications This comparison of the COMINT record with Intelligence and the Battles of Coral Sea, Midway, and the published material covering the same period will the Aleutians), was recently published. benefit not only NSA but also the academic com- The series as a whole and this volume in particu- munity, which continues to pursue the history of the lar are unique in many ways but primarily because Second World War. Thus whether or not the results they represent a closely analyzed, comprehensive agree with the literature, particularly if they do not, examination of the COMINT record juxtaposed the effort to create an “official” COMINT history is with extensive research into the written history of more than justified. events. Mr. Parker’s work also includes research Mr. Parker’s perseverance, diligent research, and into the Japanese Navy messages which remained detailed analysis have made this a significant and untranslated until 1945 and undiscovered until now. unique contribution to U.S. COMINT history, U.S. These messages revealed the Japanese Navy plans military history, and U.S. history. for war with the United States, Great Britain, and the Netherlands and the preliminary exercises that Henry F. Schorreck occurred in the months prior to Pearl Harbor. This NSA Historian (Ret.) [1993] 5 5 Pearl Harbor Revisited Radiogram “AIRRAID ON PEARLHARBOR × THIS IS NO DRILL” from Commander in Chief of the Pacific Fleet (CINCPAC) to all ships in Hawaii area, December 7, 1941. 17th Naval District, Kodiak, Alaska, General Files, 1940-46, E 91, Dispatches, Pearl Harbor; Records of Naval Districts and Shore Establishments, 1784-1981; Record Group 181, National Archives 6 Introduction Introduction systems. Today most intelligence experts would call such a practice naive or ill advised. Yet a policy he aspect of the Pearl Harbor disas- requiring OP-20-G cryptanalysts to search primar- ter which is really surprising is that so ily for unique cryptographic features of codes and manyT people failed to do either the obvi- ciphers which might later be refined and employed ous or the sensible things. —Washington Star, by Navy cryptographers was not changed until 1942. 1 September 1945 Coupled with a reluctance to hire civilian trainees, This is the story of the U.S. Navy’s communica- this policy seriously delayed the training of enough tions intelligence (COMINT) effort between 1924 Navy cryptanalysts and linguists to deal with a work- and 1941. It traces the building of a program, under the load which increased exponentially both in complex- Director of Naval Communications (OP-20), which ity and volume after 1939. Ultimately, the resulting extracted both radio and traffic intelligence from for- shortage of cryptanalysts and Japanese linguists, eign military, commercial, and diplomatic commu- the problem of misplaced priorities, and interser- nications.1 It shows the development of a small but vice rivalry issues all contributed to misplacing the remarkable organization (OP-20-G, Section/Com- major focus of the Navy’s cryptanalytic and linguistic munications Security) which, by 1937, could clearly efforts on Japanese diplomatic messages. The unfor- see the military, political, and even the international tunate result of these circumstances was to postpone implications of effective cryptography and successful with fatal consequences an all-out effort on Japanese cryptanalysis at a time when radio communications Navy cryptosystems. were passing from infancy to childhood and Navy war This is not to minimize the value of the pre-Pearl 2 planning was restricted to tactical situations. It also Harbor efforts of Navy cryptanalysts and traffic ana- illustrates an organization plagued from its inception lysts. Even without the messages pertaining to the by shortages in money, manpower, and equipment; Japanese Pearl Harbor strike force, the magnitude of total absence of a secure, dedicated communications the information they produced pertaining to the Jap- system; little real support or tasking from higher com- anese 2nd, 3rd, and 4th Fleets and the Japanese 11th mand authorities; and major imbalances between col- Air Fleet was overwhelming. These intimate details lection and processing capabilities.