Pearl Harbor: The Seeds and Fruits of Infamy - Ludwig von Mises ...

130 Pearl Harbor: The Seeds and Fruits of Infamy
As we have seen, the United States had an advantage over the
Japanese during the 1921–1922 Washington Naval Conference
on disarmament because it was reading the Japanese government’s
secret instructions to its representative. But the U.S. government
closed down its cryptographic agency in 1929, although the Navy
continued to maintain an intelligence offi ce, OP-20-G, which
operated after 1916 under Commander Laurence F. Saff ord.
And in 1930 the Army established its Signal Intelligence Service
(SIS), headed by William Friedman. By 1940, these two agencies
were deeply involved once more in analyzing and deciphering
Japanese codes.
At that time, the Japanese had many codes of varying complexities.
Each was intended for a diff erent purpose. Th e most
intricate were their diplomatic, consular, and naval codes. When
the Japanese were especially anxious to assure the security of a
message, they usually transmitted it in one of these codes. Th ey
considered their diplomatic code to be their most complex and
most indecipherable; they thought it was absolutely secure and
used it for their very most secret messages.
Japan’s Diplomatic Code
Th e intelligence experts in the Army’s SIS and the Navy’s
Op-20-G cooperated in the attempt to break the various Japanese
codes. Although the Japanese navy code long defi ed U.S. cryptographers,
they made considerable headway in breaking several
others, including the consular code. Th eir most spectacular success,
however, was with Japan’s diplomatic cipher.
After some 18 or 20 months of painstaking eff ort, the Army
and Navy experts fi nally succeeded in breaking this code. Th ey
even constructed a machine that could duplicate the operations
of the Japanese machine, including replicating the daily shuffl
ing and transpositional changes by which the Japanese hoped