JN-25
Encyclopedia
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 ambush 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.
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.
(IJN) during and slightly before 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 in the traffic which was actually broadcast. It was frequently revised during its lifetime, and each new version required a more or less fresh cryptanalytic
start. New code books were introduced from time to time and new superenciphering books were also introduced, sometimes simultaneously. 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 that edition of the JN-25 system which was sufficiently broken by late May 1942 to provide the forewarning which led to the U.S. victory at the Battle of Midway
.
The British
, Australia
ns, Dutch
and Americans
cooperated on attacks against JN-25 beginning well before the Pearl Harbor attack. The Japanese Navy was not engaged in significant battle operations until late 1941, so there was little traffic available with which to work. Before then, IJN discussions and orders could generally travel by more secure routes than encrypted 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, called OP-20-G
. It was centered at Pearl Harbor at the U.S. Navy's Combat Intelligence Unit (Station HYPO
, also known as COM 14), commanded 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 IBM
punched card
tabulating machine
(when available), a successful attack was mounted against the JN-25 edition which came into effect on 1 December 1941. Together they made considerable progress by early 1942. Cribs were used by exploiting common formalities in Japanese messages, such as "I have the honor to inform your excellency" and the use of formal, stylized titles (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 before or during the War. Purple traffic was diplomatic, not military, and in the period before the Pearl Harbor attack, the Japanese military, which controlled Japanese policy, did not trust the Foreign Office enough to tell it much. 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,
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.
, run by isolationist Col. Robert L 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.
World War II
World War II, or the Second World War , was a global conflict lasting from 1939 to 1945, involving most of the world's nations—including all of the great powers—eventually forming two opposing military alliances: the Allies and the Axis...
, 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 ambush at Midway
Battle of Midway
The Battle of Midway is widely regarded as the most important naval battle of the Pacific Campaign of World War II. Between 4 and 7 June 1942, approximately one month after the Battle of the Coral Sea and six months after Japan's attack on Pearl Harbor, the United States Navy decisively defeated...
and the shooting down of Isoroku Yamamoto
Isoroku Yamamoto
was a Japanese Naval Marshal General and the commander-in-chief of the Combined Fleet during World War II, a graduate of the Imperial Japanese Naval Academy and a student of Harvard University ....
in Operation Vengeance.
The Imperial Japanese Navy
Imperial Japanese Navy
The Imperial Japanese Navy was the navy of the Empire of Japan from 1869 until 1947, when it was dissolved following Japan's constitutional renunciation of the use of force as a means of settling international disputes...
(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 IWorld War I
World War I , which was predominantly called the World War or the Great War from its occurrence until 1939, and the First World War or World War I thereafter, was a major war centred in Europe that began on 28 July 1914 and lasted until 11 November 1918...
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
Red (cipher machine)
In the history of cryptography, 91-shiki injiki or Angōki Taipu-A , codenamed Red by the United States, was a diplomatic cryptographic machine used by the Japanese Foreign Office before and during World War II. A relatively simple device, it was quickly broken by western cryptographers...
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
Black-bag cryptanalysis
In cryptography, black-bag cryptanalysis is a euphemism for the acquisition of cryptographic secrets via burglary, or the covert installation of keystroke logging or trojan horse software/hardware on target computers or ancillary devices...
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.
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 NavyImperial Japanese Navy
The Imperial Japanese Navy was the navy of the Empire of Japan from 1869 until 1947, when it was dissolved following Japan's constitutional renunciation of the use of force as a means of settling international disputes...
(IJN) during and slightly before World War II
World War II
World War II, or the Second World War , was a global conflict lasting from 1939 to 1945, involving most of the world's nations—including all of the great powers—eventually forming two opposing military alliances: the Allies and the Axis...
(it was the 25th Japanese Navy system identified). Introduced in 1939 to replace Blue, it was an enciphered code
Code
A code is a rule for converting a piece of information into another form or representation , not necessarily of the same type....
, producing five numeral groups in the traffic which was actually broadcast. It was frequently revised during its lifetime, and each new version required a more or less fresh cryptanalytic
Cryptanalysis
Cryptanalysis is the study of methods for obtaining the meaning of encrypted information, without access to the secret information that is normally required to do so. Typically, this involves knowing how the system works and finding a secret key...
start. New code books were introduced from time to time and new superenciphering books were also introduced, sometimes simultaneously. In particular, JN-25 was significantly changed on 1 December 1940, and again on 4 December 1941, just before the attack on Pearl Harbor
Attack on Pearl Harbor
The attack on Pearl Harbor was a surprise military strike conducted by the Imperial Japanese Navy against the United States naval base at Pearl Harbor, Hawaii, on the morning of December 7, 1941...
. It was that edition of the JN-25 system which was sufficiently broken by late May 1942 to provide the forewarning which led to the U.S. victory at the Battle of Midway
Battle of Midway
The Battle of Midway is widely regarded as the most important naval battle of the Pacific Campaign of World War II. Between 4 and 7 June 1942, approximately one month after the Battle of the Coral Sea and six months after Japan's attack on Pearl Harbor, the United States Navy decisively defeated...
.
The British
United Kingdom
The United Kingdom of Great Britain and Northern IrelandIn the United Kingdom and Dependencies, other languages have been officially recognised as legitimate autochthonous languages under the European Charter for Regional or Minority Languages...
, Australia
Australia
Australia , officially the Commonwealth of Australia, is a country in the Southern Hemisphere comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands in the Indian and Pacific Oceans. It is the world's sixth-largest country by total area...
ns, Dutch
Netherlands
The Netherlands is a constituent country of the Kingdom of the Netherlands, located mainly in North-West Europe and with several islands in the Caribbean. Mainland Netherlands borders the North Sea to the north and west, Belgium to the south, and Germany to the east, and shares maritime borders...
and Americans
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...
cooperated on attacks against JN-25 beginning well before the Pearl Harbor attack. The Japanese Navy was not engaged in significant battle operations until late 1941, so there was little traffic available with which to work. Before then, IJN discussions and orders could generally travel by more secure routes than encrypted 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.
Washington, D.C.
Washington, D.C., formally the District of Columbia and commonly referred to as Washington, "the District", or simply D.C., is the capital of the United States. On July 16, 1790, the United States Congress approved the creation of a permanent national capital as permitted by the U.S. Constitution....
by the U.S. Navy
United States Navy
The United States Navy is the naval warfare service branch of the United States Armed Forces and one of the seven uniformed services of the United States. The U.S. Navy is the largest in the world; its battle fleet tonnage is greater than that of the next 13 largest navies combined. The U.S...
's signals intelligence command, called OP-20-G
OP-20-G
OP-20-G or "Office of Chief Of Naval Operations , 20th Division of the Office of Naval Communications, G Section / Communications Security", was the US Navy's signals intelligence and cryptanalysis group during World War II. Its mission was to intercept, decrypt, and analyze naval communications...
. It was centered at Pearl Harbor at the U.S. Navy's Combat Intelligence Unit (Station HYPO
Station HYPO
Station HYPO, also known as Fleet Radio Unit Pacific was the United States Navy signals monitoring and cryptographic intelligence unit in Hawaii during World War II. It was one of two major Allied signals intelligence units, called Fleet Radio Units in the Pacific theaters, along with FRUMEL in...
, also known as COM 14), commanded by Commander Joseph Rochefort
Joseph Rochefort
Joseph John Rochefort was an American Naval officer and cryptanalyst. His contributions and those of his team were pivotal to victory in the Pacific War....
. With the assistance of Station CAST
Station CAST
Station CAST was the United States Navy signals monitoring and cryptographic intelligence fleet radio unit at Cavite Navy Yard in the Philippines, until Cavite was captured by the Japanese forces in 1942, during World War II. It was an important part of the Allied intelligence effort, addressing...
(also known as COM 16, jointly commanded by Lts Rudolph Fabian and John Lietwiler) in the Philippines
Philippines
The Philippines , officially known as the Republic of the Philippines , is a country in Southeast Asia in the western Pacific Ocean. To its north across the Luzon Strait lies Taiwan. West across the South China Sea sits Vietnam...
, and the British Far East Combined Bureau
Far East Combined Bureau
The Far East Combined Bureau, an outstation of the British Government Code and Cypher School, was set up in Hong Kong in March 1935, to monitor Japanese, and also Chinese and Russian intelligence and radio traffic...
in Singapore
Singapore
Singapore , officially the Republic of Singapore, is a Southeast Asian city-state off the southern tip of the Malay Peninsula, north of the equator. An island country made up of 63 islands, it is separated from Malaysia by the Straits of Johor to its north and from Indonesia's Riau Islands by the...
, and using an IBM
IBM
International Business Machines Corporation or IBM is an American multinational technology and consulting corporation headquartered in Armonk, New York, United States. IBM manufactures and sells computer hardware and software, and it offers infrastructure, hosting and consulting services in areas...
punched card
Punched card
A punched card, punch card, IBM card, or Hollerith card is a piece of stiff paper that contains digital information represented by the presence or absence of holes in predefined positions...
tabulating machine
Tabulating machine
The tabulating machine was an electrical device designed to assist in summarizing information and, later, accounting. Invented by Herman Hollerith, the machine was developed to help process data for the 1890 U.S. Census...
(when available), a successful attack was mounted against the JN-25 edition which came into effect on 1 December 1941. Together they made considerable progress by early 1942. Cribs were used by exploiting common formalities in Japanese messages, such as "I have the honor to inform your excellency" and the use of formal, stylized titles (see known plaintext attack).
The Purple cipher
Cipher
In cryptography, a cipher is an algorithm for performing encryption or decryption — a series of well-defined steps that can be followed as a procedure. An alternative, less common term is encipherment. In non-technical usage, a “cipher” is the same thing as a “code”; however, the concepts...
(also sometimes referred to as AN-1), used by the Japanese Foreign Office
Ministry of Foreign Affairs (Japan)
The is a cabinet level ministry of Japan responsible for the country's foreign relations.The ministry is due to the second term of the third article of the National Government Organization Act , and the Ministry of Foreign Affairs Establishment Act establishes the ministry...
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 before or during the War. Purple traffic was diplomatic, not military, and in the period before the Pearl Harbor attack, the Japanese military, which controlled Japanese policy, did not trust the Foreign Office enough to tell it much. 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 "maruJapanese ship naming conventions
Japanese ship naming conventions are different from those in the West. Japanese warships have never been named after people. Prior to World War II, Japanese ship naming conventions underwent several changes before being settled.- Merchant ships :...
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
United States Naval Districts
The naval district is a military and administrative command ashore, established for the purpose of decentralizing the U.S. Navy Department's functions with respect to the control of the coastwise sea communications and the shore activities outside the department proper, and for the further purpose...
(responsible for the area) boarded her and seized her codebooks, without informing Office of Naval Intelligence
Office of Naval Intelligence
The Office of Naval Intelligence was established in the United States Navy in 1882. ONI was established to "seek out and report" on the advancements in other nations' navies. Its headquarters are at the National Maritime Intelligence Center in Suitland, Maryland...
(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-25 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.1942 Chicago Tribune incident
In June 1942 the Chicago TribuneChicago Tribune
The Chicago Tribune is a major daily newspaper based in Chicago, Illinois, and the flagship publication of the Tribune Company. Formerly self-styled as the "World's Greatest Newspaper" , it remains the most read daily newspaper of the Chicago metropolitan area and the Great Lakes region and is...
, run by isolationist Col. Robert L 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
Espionage Act of 1917
The Espionage Act of 1917 is a United States federal law passed on June 15, 1917, shortly after the U.S. entry into World War I. It has been amended numerous times over the years. It was originally found in Title 50 of the U.S. Code but is now found under Title 18, Crime...
. For various reasons, including the desire not to bring more attention to the article, the charges were dropped.