Pearl Harbor Revisited: United States Navy Communications Intelligence, 1924-1941

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3 Series IV: World War II | Volume 6
Center for Cryptologic History
n57370
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-
M
treating the U.S. Navy’s communications rier strike force with the major naval objective some-
intelligence (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-
T
ter which is really surprising is that so ily for unique cryptographic features of codes and
many 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 policy seriously delayed the training of enough
This is the story of the U.S. Navy’s communica-
Navy cryptanalysts and linguists to deal with a work-
tions intelligence (COMINT) effort between 1924
load which increased exponentially both in complex-
and 1941. It traces the building of a program, under the
ity and volume after 1939. Ultimately, the resulting
Director of Naval Communications (OP-20), which
shortage of cryptanalysts and Japanese linguists,
extracted both radio and traffic intelligence from for-
the problem of misplaced priorities, and interser-
eign military, commercial, and diplomatic commu-
vice rivalry issues all contributed to misplacing the
nications.1 It shows the development of a small but
major focus of the Navy’s cryptanalytic and linguistic
remarkable organization (OP-20-G, Section/Com-
efforts on Japanese diplomatic messages. The unfor-
munications Security) which, by 1937, could clearly
tunate result of these circumstances was to postpone
see the military, political, and even the international
with fatal consequences an all-out effort on Japanese
implications of effective cryptography and successful
Navy cryptosystems.
cryptanalysis at a time when radio communications
were passing from infancy to childhood and Navy war This is not to minimize the value of the pre-Pearl
planning was restricted to tactical situations.2 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. It explains how, in concerning Japanese intentions, however, were not
1941, as a result of these problems, compounded by based on messages but on analytic judgments drawn
the stresses and exigencies of the time, the effort mis- from analysis of Japanese Navy communications
placed its focus from Japanese Navy traffic to Japa- procedures, patterns, and practices. As suspect quan-
nese diplomatic messages. Had Navy cryptanalysts tities from a suspect source, they were not accept-
been ordered to concentrate on the Japanese naval ed by the very commanders in whose service they
messages rather than Japanese diplomatic traffic, the had been developed. The lack of confidence in such
United States would have had a much clearer picture intelligence made traffic intelligence from the Pacific
of the Japanese military buildup and, with the warn- during the last half of 1941 more an elaborate rumor
ing provided by these messages, might have avoided than trustworthy source material. Commanders at
the disaster of Pearl Harbor. the theater level and in Washington, through lack of
early training or insight, were not prepared to exploit
This story also records what today must be
the intelligence provided by this source, particularly
ranked as an intensely important interlude when
when the messages themselves could not be read.
the Navy radio/traffic intelligence program delib-
erately avoided the underlying intelligence of inter- In addition to outlining the development of the
cepted traffic while exploiting foreign cryptographic Navy’s cryptanalytic attack against Japanese cryp-
7
Pearl Harbor Revisited
tographic systems, this review also examines other From 1919 to 1923 the Navy seemed to rely
interesting episodes overseas and in Washington, almost entirely on the Cipher Bureau. In 1923 the
which included two attempts, one unsuccessful, to Navy apparently felt that the Cipher Bureau had pro-
coordinate Navy and Army COMINT activities, duced neither the desired cryptographic improve-
efforts to improve fleet communications, and the ments nor the necessary insights into the activities
lessons learned and then forgotten about Japanese of foreign navies, in particular Japan’s growing fleet.
naval communications from Japanese Fleet maneu- Accordingly, using shipboard communicators, the
vers of the l930s. Coordination and cooperation U.S. Navy in 1923 began an ad hoc effort to listen
between the U.S. Navy COMINT Center on Cor- to foreign radio traffic, which its earlier work had
regidor and the British Far East Combined Bureau shown to be potentially vulnerable to penetration
in Singapore are briefly described. and exploitation.
In January 1924, Commander Ridley McClean,
Director of Naval Communication (DNC) estab-
Earliest Efforts
lished a research desk within the Code and Signal
Section with a complement of one officer, Lieuten-
he origins of the U.S. Navy’s COMINT
T ant Laurance F. Safford, and one civilian, Agnes
effort prior to 1924 are not entirely clear.
Meyer, both of whom were cryptanalysts/cryptog-
However, the Navy established a Code and Sig-
raphers. Safford and Meyer conducted research into
nal Section possibly with COMINT interests
foreign cryptography, organized training in collec-
as early as 28 July 1916. This small organization
tion and cryptanalysis, developed cryptographic
initially worked against German ciphers during
systems for naval communications, and arranged
World War I. It also tested the security of U.S.
with the Commander in Chief, Asiatic Fleet (CIN-
Navy ship-to-ship and ship-to-shore communi-
CAF), and certain naval district commanders to
cations during training maneuvers. During the
obtain copies of radio intercept of foreign messages.
maneuvers of 1917, for example, personnel from
The primary goal of the two was to develop cryp-
the section were involved in an overt attempt to
tographic systems for the U.S. Navy which would
intercept and exploit U.S. Navy communications
avoid the weaknesses observed in the cryptographic
in order to demonstrate their accessibility to for-
techniques employed by foreign governments.
eign intelligence efforts.3
Before tackling the problems of penetration
For some unknown reason, these initiatives
and exploitation on a regular basis, the Navy had
apparently ended with the World War in 1918. At
to construct an organization which could routinely
that time the Navy voluntarily consolidated its war-
intercept and process foreign cryptographic systems.
time efforts with those of the War, Justice, State,
Beginning as a totally decentralized effort loosely
and Postal Censorship Departments, forming a
managed from Washington, DC, collection and
single U.S. Cipher Bureau under the War Depart-
local exploitation of plain text was controlled by fleet
ment. Commanded by Captain Herbert O. Yardley,
and naval district commanders, while Washington
assisted by Captain John W. Manly, the consolidated
retained control of the cryptanalytic capability.
bureau consisted of thirteen cryptographers, twelve
of whom were women, and an administrator. It was Early in 1924, Commander McClean and
supported by eleven student officers, eight stenogra- the Chief of Naval Operations (CNO), Admi-
phers, and fifteen clerks. The Navy was represented ral Edward W. Eberle, encouraged the CINCAF,
by Chief Yeoman H. E. Burt.4 Admiral Thomas Washington, to expand radio
8