At
the beginning of the Second World War, German U-boat
fleet commander Admiral Karl Dönitz insisted on creating
a large fleet of at least 300 U-boats as the most
important weapon of the Kriegmarine. They would be used
to enforce a blockade of England in case of war. Although
Dönitz was supported by his chief Admiral Raeder, Hitler
refused their proposal and assured them there would never
be a war with Britain. Battle ships would remain the main
weapon of the Kriegsmarine and Dönitz had only 57
U-boats.
However, after the invasion of Poland,
Britain declared the war to Germany on 3 September 1939,
and the Royal Navy had almost ten times more warships
than the Kriegsmarine. After the loss of several German
war ships and having the destruction of Allied convoys to
Britain as a new priority, the Kriegsmarine was forced to
adapt their naval tactics.
The production of U-boats increased
rapidly and Dönitz was finally able to apply his new
strategy, the Rudel Taktik or so-called Wolf-pack
tactics. The German Kriegsmarine proved to be very
successful with their Wolf-pack technique. U-boats hunted
individually for convoys. If a convoy was found, they
shadowed their prey and other U-boats were contacted.
Once all U-boats on the spot, they sunk the convoy with a
closely coordinated attack. This technique was so
devastating that it could have decided the outcome of war
in favor of Germany. The Allies lost their ships faster
then they could build new ones to replace them.
Communication was the key to the success of the
Wolf-packs, and the Kriegsmarine used Enigma to encipher
their messages.
Hunting the shark
The Allied Forces faced the impossible
task to get control over the vast Atlantic Ocean.
Surprisingly, the Kriegsmarine offered them the solution
to this problem. In Bletchley Park, England, a team of
codebreaker was breaking into the German Wehrmacht,
Luftwaffe and Kriegsmarine communications. All
information, retrieved from decrypted messages, was
codenamed 'Ultra'. Many different sections worked in
Bletchley Park on different codes of the German armed
forces. Each section was designated by the barracks or
Huts where they worked in. Hugh Alexander and Alan Turing
led the attack on the naval Enigma encryption in Hut 8.
At the start of war, the U-boat
communication network used the three-rotor Enigma M3 to
encipher their messages. The M3 was identical to the
Wehrmacht and Luftwaffe Enigma but the Kriegsmarine had a
set of eight rotors to choose from, instead of the normal
set of five. Initially, the main keys to encrypt Enigma
messages on U-boats and surface ships in the home waters
and the Atlantic were the Heimisch keys. This key,
designated Dolphin by Bletchley Park, was the most
important one, although many other key were operational
on different naval locations. Despite the British Navy
having recovered two of the three additional naval rotors
from U-33 early in 1940 and the third rotor in August of
the same year, Bletchley still could not break the
Dolphin keys.
To break Dolphin, they had to know the
daily changing rotor combinations that were used. One way
to find out these combinations was the use of cribs.
Cribs were little pieces of assumed text. Once the
codebreakers located a crib in an Enigma message (there
were techniques for that) the associations between the
letters of the encrypted text and its plain non-encrypted
version were entered in a Turing Bombe. This machine,
designed by Alan Turing and Gordon Welchman, contained a
large number of drums, each replicating the rotors of the
Enigma, and ran through all possible settings to find the
key settings that belong to the combination of a given
piece of cipher and plain text. Once these settings were
found they could decipher all messsages of that
particular day. Unfortunately, few cribs were available
the until summer of 1941.
Another important setting of the Enigma
machine that had to be found was the message key, which
was the initial rotor positions at the beginning of the
enciphering of a message. The Kriegmarine message
procedures were far more complex than the procedures that
were used by the Wehrmacht and Luftwaffe. The
Kriegsmarine messages keys were selected from special
code books and different message key systems and code
books existed for normal messages, Kurzsignalen
(short-signals) and weather reports. Except for some
brief periods, Blechtley was unable to get grip on the
complex naval keys. More about the message procedures on this page.
Dolphin broken
A breakthrough occurred when several code books
were secretly captured. On 9 may 1941, a Kurzsignal
(short-signal) code book was recovered during the
boarding of Kapitänleutnant Lemp's U-110. This code book
proved to be important to develop new methods to attack
Dolphin. However, attacking and boarding U-boats was a
risky operation, especially because the operation had to
be kept secret. Harry Hinsley, one of the Bletchley
codebreakers, discovered that the same code books that
were used on heavily armed U-boats were also being used
aboard unprotected German trawlers, operating as weather
ships for the Kriegsmarine.
On 7 May 1941 HMS Somali boarded the
weather ship München south east of Iceland. Weather ship
Lauenburg was attacked and boarded on 28 June in the
Arctic Ocean by HMS Nigeria, together with HMS Tartar,
HMS Bedouin and HMS Jupiter. Both the München and
Lauenburg were sunk after a boarding party recovered the
Wetterkurzschlussel (weather short-signal) code books.
The Kriegsmarine presumed both ships lost at sea.
Meanwhile, the weather section in Hut
10 broke the manual cipher that was used for
meteorological reports within the Kriegsmarine. These
reports originated from the U-boats and were sent by them
as Wetterkurzsignalen (weather short-signals). The
U-boats first encoded these reports with the
WetterKurzschlussel before encrypting them with Enigma. A
Wetterkurzsignal consisted of a fixed number of letters,
each letter representing a specific weather condition or
value. However, having the decrypted meteorological
reports and knowing into which codes they were converted,
Bletchley could now guess the plain text letters in the
original with Enigma encrypted Wetterkurzsignal from the
U-boat. This well-thought detour provided good cribs to
attack Dolphin.
Sometimes, due to lack of cribs,
Bletchley Park used a technique called Gardening. British
bombers dropped or planted series of sea mines on well
determined places. German U-boats, spotting these mines,
transmitted contact messages. Messages about these mines
and about cleared sea-lines were often send by both
Enigma and a manual cipher called Werftschlussel
(shipyard key). After Bletchley Park broke the
Werftschlussel, they again had a plain text version and
Enigma enciphered version of that text, providing them
with new cribs for their Bombes. Bletchley Park was now
constantly breaking the U-boat messages and knew their
battle orders and locations. This enabled the Admiralty
to re-route Allied convoys. Things were looking good...
Triton Strikes Back
The dropping results made Admiral
Dönitz suspicious. Although reassured by the Abwehr,
German Foreign Intelligence, that Enigma was unbreakable,
he insisted on improving the security of Enigma. On 1
February 1942 the famous Enigma M4 model with four rotors
and new code books were introduced. The new and more
complicated Triton code, designated Shark by Bletchley
Park, caused a complete black-out. They could no longer
break the U-boat radio traffic and no longer had any idea
of the U-boat positions. On top of that, new
Wetterkurzschlussel code books were introduced one month
earlier. This deprived Bletchley Park completely of any
cribs. The Kriegsmarine called the springtime of 1942 Die
Glücklichen Zeiten or Happy Times, because of the
enormous shipload they sunk. Note that the propaganda
term Happy Times is relative, taken in account the
dangerous and extreme circumstances the U-boat crews were
operating in.
After ten nerve wrecking months of
heavy losses, Bletchley Park succeeded in breaking into
Shark. This was due to several different reasons. An
important event was the attack on Kapitänleutnant Hans
Heidtmann's U-559 by the British destroyer HMS Petard on
30 October 1942. After taking heavy fire from HMS Petard,
the sinking U-boat was boarded by three British sailors.
They managed to get the Enigma code books and the new
edition of the Wetterkurzschlussel. Two of them returned
once more to recover the Enigma M4, but went down with
the U-boat. They payed their bravery with their live, but
not without result. This mission proved to be a turning
point in breaking Shark. On 13 December 1942, more than
ten months after the start of the black-out, Bletchley
Park could finally inform the Admiralty again about
U-boats positions.
After breaking a substantial number of
messages, the codebreakers realized that the U-boat
weather reports, encoded with the WetterKurzschlussel,
were sent with the four-rotor Enigma in the less
complicated M3 mode (the fourth rotor in A position with
ring setting A). This was done to be compatible with the
three-rotor Enigma M3, used on weather ships. The Bombes
in Bletchley Park, developed to crack the three-rotor
Enigma, took more than 20 days to crack the four-rotor
Enigma key settings. However, a three-rotor key setting
could be retrieved in less than 24 hours. This discovery
was an enormous time profit. With the broken
meteorological reports from Hut 10 and the recovered
Wetterkurzschlussel they finally broke Triton
continuously. When a new edition of the
Wetterkurzschlussel came into service in March 1943, the
seized U-559 Wetterkurzschlussel became useless,
resulting in a new black-out. Fortunately, the
Kurzsignalheft code book, also recovered from U-559,
provided new ways to find cribs in U-boat short-signals
and enabled the codebreakers to re-enter shark after nine
days. Except for some brief periods, the codebreakers
never lost Shark again.
Kurzsignale fail
The U-boats used the Kurzsignalheft
(short-signal book) to encipher contact messages. The
Kriegsmarine converted default tactical expressions with
a code table, called Kurzsignalheft, before enciphering
them with Enigma. A contact with a convoy could for
example be converted into UGKU, an enemy airplane into
HKJL, or a meeting point for refueling into KLUG. More
about these short-signals on Kurzsignale on
German U-boats. The use of
Kurzsignale seemed to be a clever approach. It was harder
for Allied Signal Intelligence to trace these short
messages with HDFD (High Frequency Direction Finding or
Huffduff). Moreover, attempts to decipher these short
messages didn't give any readable sentences, and
approaching the correct key did not reveal pieces of
normal sentences, helping to find the key settings. Also,
the conversion of text into four letter codes shortened
the cipher text. Less cipher text also provides less
statistics to the code breakers. Nothing but
advantages...they believed.
Unfortunately for the Germans, the use
of Kurzsignale resulted into recognizable patterns in the
Enigma messages. A convoy, nearing a U-boat, would
probably evoke a contact message. An airplane, spotting a
U-boat, would result in a airplane contact message. In
Bletchley Park, tactical information was linked to
positions, obtained by HFDF and reconnaissance reports,
to find out what type of message was sent by that
particular U-boat. In combination with the recovered
Kurzsignalheft code book, Bletchley Park was able to
predict the content of the enciphered messages, thus
providing them again with crucial cribs to feed into
their Bombes. Meanwhile, new Bombes were developed to
deal with the four-rotor Enigma. By June 1943, the first
four-rotor Bombes came into action, and by the end of
1943 another fifty four-rotor Bombes went operational at
OP-20-G, the American Naval codebreakers. In the fall of
1943, Shark messages were generally broken within 24
hours.
U-boats down
The 'Ultra' information was extremely
effective in the strategically very important North
Atlantic Ocean. After the initial hard times, Bletchley
Park broke the Enigma messages on a daily base. The tide
of U-boat war was turned. Except for some brief periods,
the entire German communication system was intercepted by
a large number of listening stations called Y-stations,
and the codes broken in Bletchley Park, with over 7000
employees at its peak. With the positions of the U-boats
unveiled, Allied ships were simply re-routed to avoid
fatal confrontations with the U-boats, and an active hunt
for the U-boats begun. The elite weapon of the
Kriegsmarine became decimated, resulting in heavy losses
among the U-boat crews. An estimated 700 U-boats and
30,000 crewman were lost at sea. The German command
related these losses to new detection techniques like the
ASDIC sonar system, U-boat detection planes, and
destroyers escorting convoys. This was partially a
correct assumption, but they never suspected
cryptanalysis of their Enigma encrypted radio traffic.
The Enigma machine provided without a doubt a for those
days unbreakable encryption. However, unsafe procedures
and tactical mistakes turned the Enigma machine into the
Achilles heel of the German war machine. Germany kept on
using Enigma without any suspicion in all parts of their
forces, which resulted in catastrophic consequences for
Nazi Germany.
Reality vs Fiction
Some books and movies, like Jonathan
Mostow's "U-571" (fiction 2000) wrongly suggest
the codebreakers were helped by captured Enigma machines.
British and French Intelligence already received
three-rotor Enigma replicas from Polish Secret Service,
as early as 1939. In reality, the four-rotor Enigma and
its internal wiring was obtained by cryptanalysis on
intercepted messages and captured code books. The
breakthrough in Shark was established without possessing
an actual M4 Enigma. The secret and infamous U-boat and
weather ship boardings were performed by the British
Navy, and their primary goal was to capture the
Kurzsignale, Wetterkurzschlussel and Enigma code books,
and not to seize an Enigma machine. Although
"Enigma" (fiction 2002), from Michael Apted
with Dougray Scott and Kate Winslet is also historical
incorrect, it gives a better technical view on the
attempts to break into the Shark codes and how they
worked in Bletchley Park.
At
the beginning of the Second World War, German U-boat
fleet commander Admiral Karl Dönitz insisted on creating
a large fleet of at least 300 U-boats as the most
important weapon of the Kriegmarine. They would be used
to enforce a blockade of England in case of war. Although
Dönitz was supported by his chief Admiral Raeder, Hitler
refused their proposal and assured them there would never
be a war with Britain. Battle ships would remain the main
weapon of the Kriegsmarine and Dönitz had only 57
U-boats.
A breakthrough occurred when several code books
were secretly captured. On 9 may 1941, a Kurzsignal
(short-signal) code book was recovered during the
boarding of Kapitänleutnant Lemp's U-110. This code book
proved to be important to develop new methods to attack
Dolphin. However, attacking and boarding U-boats was a
risky operation, especially because the operation had to
be kept secret. Harry Hinsley, one of the Bletchley
codebreakers, discovered that the same code books that
were used on heavily armed U-boats were also being used
aboard unprotected German trawlers, operating as weather
ships for the Kriegsmarine.
The U-boats used the Kurzsignalheft
(short-signal book) to encipher contact messages. The
Kriegsmarine converted default tactical expressions with
a code table, called Kurzsignalheft, before enciphering
them with Enigma. A contact with a convoy could for
example be converted into UGKU, an enemy airplane into
HKJL, or a meeting point for refueling into KLUG. More
about these short-signals on