General Heinz Guderian with an Enigma machine in a half-track being used as a mobile command center during the Battle of France, 1940
The main German cipher machine, derived from a Dutch
invention that failed in several commercial models in the late 1920s. Various
models of increasing complexity were used by the Heer, Luftwaffe, Kriegsmarine,
and in diplomatic traffic. It was also used by the Reichsbahn (German
railways). The Italian Navy used a derivative machine, the C38M. Polish
intelligence partially broke Enigma ciphers in 1932. By 1939 the Poles had a
foothold understanding of the original Dutch machine and therefore were able to
rig replicas of its German descendants. The French also made headway from 1938.
Polish intelligence Enigma replicas, and dearly acquired knowledge of German
ciphers, were supplied by the Poles to the Western Allies in July 1939. The
French and Poles passed additional information to the British in 1940. The
British broke the naval code for the Italian C38M in September 1940, a year
before that cipher was withdrawn. That greatly aided the Royal Navy in the
Mediterranean naval campaign in 1940-1941. Naval Enigma rotors were recovered
from a sunken minelayer U-boat off Scotland in February 1940. That told British
intelligence that all German ships and U-boats carried them. Thereafter high
priority was assigned to capture of U-boats and other enemy craft. German
trawlers off Norway proved especially vulnerable: capture of Enigma code books
or rotors from two trawlers led to breaking of the Kriegsmarine code. In May
1941, U-110’s Enigma machine was captured intact along with all code books.
That and such capture or recovery successes were kept at the highest level of
secrecy, including by deceit of captured U-boat crews or separate incarceration
from other German prisoners.
The British built “bombes”-machines that mimicked
and thus helped work out Enigma’s rotor sequences. There were never enough
bombes to meet the demand of the code breakers at Bletchley Park, plus all the
armed services and Britain’s clamoring allies. If the British had been more
willing to provide technical information to the Americans-which they did not
for mostly valid security reasons-it is conceivable that many more bombes would
have been made much earlier. That was certainly Admiral Ernest King’s firm
view, but in fairness King was not the most cooperative ally either. U. S.
intelligence decided to make their own bombes in September 1942, with the first
poor quality models available in May 1943. By the end of the year, 75 better
quality bombes had been manufactured in the United States, greatly increasing
code breaking capacity. It was still an infernal problem to decode: the two
inner settings of the German naval cipher were set by officers only every two
days, while naval cipher clerks changed the two outer settings every 24 hours.
Enigma operators then chose three of the machine’s eight rotors, each of which
had 26 point positions. All that provided 160 trillion potential combinations.
On the receiving end, each U-boat had two nets of six frequencies each
(“Diana” and “Hubertus”). And yet, Bletchley Park broke
into the cipher.
The Kriegsmarine added a fourth rotor to its ciphers in
January 1942, creating a prolonged “information blackout” that
reduced enemy ability to detect wolf packs and divert convoys around them. The
British made it a top priority to capture another machine from a U-boat or
weather ship. U-559 was forced to the surface on October 30, 1942, by a
sustained depth charge attack by five destroyers and destroyer escorts. Its
documents were recovered, but the machine went down with the scuttled
submarine. Still, it became clear that German operators were not fully
utilizing the fourth rotor. An American ASW Support Group captured U-505 off
Cape Verde in June 1944. The haul of Enigma material was enormous. It was also
current and forward looking to new naval codes. Deciphering signals was greatly
aided by COLOSSUS I, the first electronic computer put together by the
brilliance of Alan Turing and engineers at Bletchley Park and elsewhere. It
made processing and reading German ciphers faster than ever, often close to
“real time.” COLOSSUS II came online in June 1944. A measure of how
Enigma proved vulnerable to stiff-minded German overconfidence is the
remarkable fact that the source of most intercepted signals, Admiral Karl
Dönitz, went to his deathbed in 1980 convinced that no enemy ever read his
Enigma ciphers.
“Secret writing machine.” Siemens & Halske T52 A German cipher machine that turned patterned holes in paper ribbons into transmittable radio pulses, or back into readable messages. Its 10-rotor system made the code-breaking task of British intelligence at Bletchley Park extremely difficult. The British did not break the Geheimschreiber until they developed the COLOSSUS I and II mechanical computers by mid-1944. When the Western Allies did break the code, they gleaned much information of high value, for the Wehrmacht used Geheimschreiber machines for its top-level headquarters’ communications.
“Station X.” The site of, and usual shorthand
reference for, the British Code and Cypher School founded in 1919 and located
about 80 miles north of London. During World War II it housed the critical
code-breaking operation run by MI6. It employed some of the most brilliant
British minds of the century-notably Alan Turing, inventor of the fi rst
computer-as well as cryptanalysis specialists from Allied countries such as
France, Poland, and the United States. The Americans actually took a long time
to arrive and longer to be fully integrated: the first U. S. team did not reach
Bletchley Park until April 25, 1943. Work at Bletchley Park was
compartmentalized by “hut,” with groups in different huts listening
to various of the hundreds of Luftwaffe, Kriegsmarine, or Wehrmacht codes.
Signals were passed to code translators in Hut Three, which accepted its first
Americans only in January 1944. There were over 10,000 people working on or
otherwise supporting the extraordinarily complex and crucial work done at
Bletchley Park by 1945. All their extraordinary work was kept secret for
several decades after the war. Outposts of cryptanalysis tied to Bletchley Park
were also maintained overseas, such as the “Combined Bureau, Middle
East” in Cairo.
MAGIC
U.S. code for intercepts of Japanese diplomatic messages,
and some military communications. This body of information is sometimes
referred to as “the other ULTRA.” Cryptanalysis of the U.S. Army’s
Secret Intelligence Service (SIS) broke Japanese “PURPLE” machine
encryptions before the start of the Pacific War. The intercepts allowed
American intelligence officers to read exchanges between Tokyo and the Japanese
Embassy in Washington. While providing important insight into Japanese
political and foreign policy thinking and relations, MAGIC did not provide
operational or other “actionable” intelligence-mainly because
Japanese diplomats were not told about Army or Navy operations in advance. MAGIC
thus did not provide advance warning of the attacks on Pearl Harbor (December
7, 1941), the Philippines, or Hong Kong. MAGIC traffic from Japanese Embassy officials
in Berlin and European neutral capitals provided indirect intelligence on
German plans, including the build-up for BARBAROSSA in mid-1941. Useful
information was gleaned from 1943 to 1944 about some secret Wehrmacht weapons
research and about planned strategy and dispositions along the Atlantic Wall.
PURPLE
U.S. code name for the Japanese electronic cipher machine
that encrypted diplomatic messages. That cipher traffic was broken and read by
U. S. Army intelligence agents of the Secret Intelligence Service (SIS) by late
September 1940. The intercepts that resulted were code named MAGIC. The U. S.
gave a copy of their PURPLE decoder machine to the British, who then also read
Japanese diplomatic ciphers. The Japanese never knew that their diplomatic
traffic was read by the enemy. They revealed much of military value as a
result.
ULTRA
“Very Special Intelligence.” Code name for the
initially British system of interception and decryption of German signals
intelligence from 1940. ULTRA also intercepted and decrypted Italian signals.
Its intelligence was shared by the major Western Allies by formal agreement
from mid-1943. Although the relationship was uneasy at first, it proved one of
the major successes of the Anglo-American alliance by war’s end. The code term
“ULTRA” was later applied to Allied interception of Japanese signals
intelligence, though not to diplomatic or political intercepts. Vast amounts of
German signals were spewed out by Enigma machines and Geheimschreiber machines
used by a variety of German military, diplomatic, police, and intelligence
sources. ULTRA understanding of some intercepts-the Germans used nearly 200
code ciphers during the war, many of which were never penetrated-was greatly
aided by widespread and often sloppy enemy tradecraft, especially within the
Luftwaffe. For instance, Luftwaffe and other German operators often repeated
signals on the same topic at the same time, permitting content analysis to
identify certain key terms or coded locations, which provided clues to
penetrate deeper into the cipher. There was also much real heroism and risk
taken by Allied agents, and sheer mental sharpness and perseverance by code
breakers starting with Polish and French intelligence before the war.
Winston Churchill was a key supporter of British signals
breaking. He read ULTRA reports daily. British ULTRA decrypts aided defense
during the Battle of Britain in 1940, helped RAF Bomber Command carry out its
extended bomber offensive, and significantly aided British 8th Army win the
desert campaigns (1940-1943): intercepts revealed German logistics problems and
allowed the Royal Navy and RAF to further cripple supply. Probably the single
most critical contribution of ULTRA was to support Allied victory over the
U-boats in the Battle of the Atlantic (1939-1945). German historian Jürgen
Rohwer estimates that ULTRA intercepts reduced Allied shipping losses by 65
percent as early as the end of 1941. ULTRA intelligence was also key to
understanding to what degree deception operations succeeded or failed in land
campaigns, up to the level of directly influencing the operational and
strategic thinking of Adolf Hitler. Notable confirmation of deception success
came in the BARCLAY and related MINCEMEAT operations, and for a series of
critical deceptions called COCKADE. Unknown to the Western Allies, John
Cairncross was a Soviet double agent in place inside Bletchley Park and MI6. He
fed Moscow ULTRA intercepts that contributed directly to the Red Army’s success
at Kursk.
Such important successes made ULTRA one of the top secrets
of the war. ULTRA was so crucial that some operations that might have been
undertaken were not, out of fear of revealing to the Germans that Enigma codes
were compromised: ULTRA was just too strategically important to risk for any
one tactical or operational gain. ULTRA not only aided operations, it helped
shape Allied strategy at the highest levels of leadership. The secret of ULTRA
was kept by at least 20,000 people for over 30 years. It was not until the
1970s that the first quasi-official accounts were authorized, and not until
1988 that the British official history astonished the historiographical world
with rich detail that illuminated and altered understanding of many key events
of the war.
Suggested Reading: David Khan, Seizing the Enigma (1995). R. Lewin, The American Magic (1982). Ralph Bennett, Behind the Battle (1994); F. H. Hinsley, British Intelligence in the Second World War (1979-1990); Simon Singh, The Code Book (1999).
