170 TURNING THE TIDE 1943 –194 4
Both sides stood to gain a
huge advantage if they could
intercept and decrypt enemy radio
transmissions. However, gathering
signals intelligence (SIGINT) was a
complex and laborious process,
made harder by machine-encoding
devices such as Enigma (Germany),
Typex (Britain), and Purple (Japan).The code war
In fall 1939, the British set
up a specialist cryptography
department, Ultra, at Bletchley Park,
Buckinghamshire, staffed by civilian
and military experts. By April 1941,
these cryptographers, using electro-
mechanic decoding devices called “bombes,” could read
transmissions from the German Luftwaffe, followed by variants
of the code used by other German services.
This information was strategically vital, and included Rommel’s
intentions before El Alamein (see pp.76–77), German U-boat locations
in the North Atlantic (see pp.64–65), and German deployments in
Normandy before D-Day (see pp.186–189). However, the intelligence
had to be used sparingly to prevent the Axis powers realizing the
Enigma code had been broken. Elsewhere, the work of US code-
breakers (MAGIC) on Japanese diplomatic codes and the JN-25 naval
code yielded valuable intelligence in the Pacific theater, while German
SIGINT on the Soviets was poor due to effective Soviet coding.ENIGMA MACHINE
CODE-
BREAKING
Both the Axis powers and the Allies devoted significant
resources to breaking the ciphers used by governments
and armed forces to conceal their communications. The
Allies, in particular, achieved great success in this field.
The German Enigma
machine used a typewriter
key attached to a series of
rotating wheels (rotors),
internal wiring, and a
plugboard, which produced
billions of variants for each
letter depressed. In 1938,
the Polish intelligence
service provided
information on the Enigma’s
construction, which proved
key to cracking the code.△ Converting into code
German soldiers encipher a message
using an Enigma machine. Errors and
shortcuts made by operators in a hurry
often provided a way into the code.US_170-171_F_Code_breaking.indd 170 04/03/19 10:47 AM