JN-25 - The Japanese Fleet's Cryptographic System, a.k.a. 5 number
code (Sample).
JN stands for Japanese Navy, introduced 1 June 1939. This was a very simple
old-type code book system used by the American Army and Navy in 1898 and
abandoned in 1917 because it was insecure. Version A has a dictionary of
5,600 numbers, words and phrases, each given as a five figure number. These
were super-enciphered by addition to random numbers contained in a second
code book. The dictionary was only changed once before PH on Dec 1, 1940, to
a slightly larger version B but the random book was changed every 3 to 6
months- last on Aug 1. The Japanese blundered away the code when they
introduced JN25-B by continuing to use, for 2 months, random books that had
been previously solved by the Allies. That was the equivalent of handing
over the JN-25B codebook. It was child's play for the Navy group OP-20-G
(738 men whose primary responsibility was Japanese naval codes) to
reconstruct the exposed dictionary. We recovered the whole thing immediately
- in 1994 the NSA published that JN-255B was completely cracked in December
1940. In January 1941 the US gave Britain two JN-25B code books with keys
and techniques for deciphering. The entire Pearl Harbor scheme was laid out
in this code. The official US Navy statement on JN-25B is the NAVAL SECURITY
GROUP HISTORY TO WORLD WAR II prepared by Captain J. Holtwick in June 1971
who quotes Captain Safford, the chief of OP-20-G, on page 398: "By 1
December 1941 we had the code solved to a readable extent." Churchill
wrote "From the end of 1940 the Americans had pierced the vital
Japanese ciphers, and were decoding large numbers of their military
and diplomatic telegrams."(GRAND ALLIANCE p 598) Safford reported that
during 1941 "The Navy COMINT team did a thorough job on the Japanese
Navy with no help from the Army."(SRH-149) " ... many pattern
messages could be read practically entire with as few as 1500
meanings." (NSA).
In 1979 the NSA released 2,413 JN-25 orders of the 26,581 intercepted by
US between Sept 1 and Dec 4, 1941. The NSA says "We know now that they
contained important details concerning the existence, organization,
objective, and even the whereabouts of the Pearl Harbor Strike Force."
(Parker p 21) Of the over thousand radio messages sent by Tokyo to the
attack fleet, only 20 are in the National Archives. All messages to the
attack fleet were sent several times, at least one message was sent every
odd hour of the day and each had a special serial number. Starting in early
November 1941 when the attack fleet assembled and started receiving radio
messages, OP-20-G stayed open 24 hours a day and the "First Team"
of codebreakers worked on JN-25. In November and early December 1941,
OP-20-G spent 85 percent of its effort reading Japanese Navy traffic, 12
percent on Japanese diplomatic traffic and 3 percent on German naval codes.
FDR was personally briefed twice a day on JN-25 traffic by his aide, Captain
John Beardell, and demanded to see the original raw messages in English. The
US Government refuses to identify or declassify any pre-Dec 7, 1941 decrypts
of JN-25 on the basis of national security, a half-century after the war.
