Breaking the Enigma cipher
BOMBE was the name of an electro-mechanical machine, developed during WWII
by Alan Turing and Gordon Welchman, whilst working as codebreakers at
It was used to help breaking the German Enigma
codes and was (partly) based on the so-called BOMBA,
an earlier machine
developed by Polish mathematicians in 1938. From 1943 onwards, an improved
version of the 4-sheel BOMBE was built in the US by the US Navy and,
separately, by the US Army.
The Poles were the first to break the military variant of the Enigma in 1932.
Their success was based on pure mathematical analysis, information from a German
spy by the name of Hans-Thilo Schmidt (codename Asche) and a machine intercepted
in the Polish mail. They then bought a commercial Enigma machine and used the
gathered information to convert it into a military one.
Initially, the Germans used a very simple key management scheme,
which allowed the daily key to be recovered on a regular basis with
simple hand methods.
Between September 1938 and May 1940 however, all settings were changed daily,
making hand methods unpractible. Luckily, there was a major flaw in the German
keying procedures, as the randomly chosen message key was sent enciphered
twice at the beginning of each message.
This allowed the Polish mathematicians Marian Rejewski, Henryk Zygalski and
Jerzy Rózyki of section BS-4 of Biuro Szyfrów (Polish Cipher Bureau)
to develop a semi-automatic machine that was used to recover the key.
The machine was called the Bomba Kryptologiczna (Cryptologic Bomb)
and, according to Rejewski, it was able to recover the key-settings
in under two hours.
Although all Bomby were destroyed in 1939, just before the Germans invaded Poland,
Rejewski made a sketch many years later, which appears in The Secret War in 1978 .
In 1979, an improved version of this sketch appeared in a report by Marian Rejewski . It is shown on the right.
The machine is based on the principle that the 3-letter message key is sent twice at
the beginning of each message and that a particular plaintext letter, might yield the
same ciphertext 3 positions apart. E.g. AWB TWY. Such coincidences were
called Females. As both letter groups (AWB and TWY) originate from the same
plaintext, it is possible to search for that unique combination.
If enough females were found, the Bomba could be used to find the Ringstellung.
The initial wheel positions (Grundstelling) was already given in clear in the preamble of the
As the Germans used just 3 Enigma wheels at the time (I, II and III), there were
6 possible wheels orders that had to be investigated. This was done by running
6 Bomby in parallel. Each Bomba had 6 complete Enigma rotor-sets at its top
surface (1). These were connected in pairs. Each pair was used to solve one (of three)
Although the exact operation of the Bomba is still unknown, many have tried to
explain its principle by reconstructing a theoretical model. The best attempt so far
is described by David Link in Cryptologia in 2009 .
For the recovery of the Ringstellung, the configuation of the plug board
(Steckerbrett) does not have to be taken into account as it doesn't move
during the encipherment. All that matters is that the
same input letter produces the same result twice, three steps apart. In  Link
describes a simple method to solve the Steckers once the wheel order and the
Ringstellung is known.
Based on the information presented by the Poles, British mathematician Alan Turing
developed a machine that was capable of recovering the key settings even if the
Germans would drop the double encryption of the message key at the beginning of
each message. The machine was called Bombe
(later: Turing-Welchman Bombe) and was built by the British
Tabulating Machine Company (BTM) in Letchworth, Hertfordshire (UK) under supervision of
Harold (Doc) Keen .
The name was derived from Bomba, a similar machine developed by the Poles
shortly before the outbreak of WWII. It made use of the fact that the
indicator was sent twice at the start of each message, a major flaw in
the German cryptographic procedures.
Although the concept of the Bomba was known to
Turing had to use a different approach, as the Germans discovered their
weakness and gave up the doubly enciphered indicator, just before the start
of WWII. This change rendered the Bomba useless.
Turing designed the British Bombe in 1939. Compared to the Polish
Bomba, it used a completely different approach. It was based on
the assumption that a known (or guessed) plaintext, a so-called crib,
is present at a certain position in the message.
The Bombes were built by the British Tabulating Company (BTM, later ICL)
at Letchworth (UK) under the supervision of Harold 'Doc' Keen .
The first machine, called 'Victory', was delivered at Bletchley Park on 18
The Bombe was further enhanced with the so-called diagonal board,
an invention of fellow codebreaker Gordon Welchman, that greatly reduced
the number of steps needed for the codebreaking effort.
A second Bombe, with Welchman's diagonal board present,
was installed on 8 August 1940.
It was named 'Agnus Dei', later shortened to 'Agnes' or 'Aggie'.
The first machine (Victory) was later modified with a diagonal board as well.
During the course of the war, over 200 Turing-Welchmand Bombes were built.
To avoid the risk of losing them in case of a bomb attack,
they were spread between Bletchley Park
and its so-called Outstations in Wavendon, Adstock, Gayhurst,
Eastcote and Stanmore, where they were operated by WRNS, RAF-technicians and
civillian personnel .
Like the German Wehrmacht (Army) and Luftwaffe (Airforce),
the Kriegsmarine (Navy) used a three-wheel Enigma machine with
Steckerbrett (plug-board). The Navy called it the M3.
Unlike the Wehrmacht and the Luftwaffe however, the Navy used additional
codebooks for shortening their messages. The shorter their
radio broadcasts, the smaller the risk of Direction Finding (DF).
Because Admiral Karl Dönitz, head of the U-Boat section of the German
Navy had his doubts, three extra wheels were introduced (VI, VII and VIII),
exclusively for the Kriegsmarine. But that wasn't all he changed.
On 2 February 1942, completely out of the blue, the German Navy introduced
the Enigma M4, a four-wheel machine, exclusively
for U-Boat communication.
At the same time they changed their codebooks, leaving the codebreakers
at Bletchley Park in the dark.
The Enigma M4 had an extra rotor, called the Zusatzwalze (extra wheel),
inserted between the 3rd wheel and the Umkehrwalze (reflector).
As the Bombe was built specifically for breaking three-wheel Enigma traffic,
it was not suitable for attacking the new M4 machine.
Apparently, Dönitz, had managed to fulfill the logistic nightmare of
replacing all existing M3 machines by the new M4.
But as the code was changed overnight, and the rest of the
Navy was still using the 3-wheel machine, Turing figured that the machines
were somehow compatible.
He turned out to be right. When the 4th wheel was set at 'A', the machine
would behave like the old M3. This was done, for example, when communicating
with the Naval Weather Service. Furthermore, the 4th wheel never moved during
encypherment, making it effectivelty a selector between 26 different
reflectors (UKW). Turing soon managed to recover the full
wiring of the extra wheel, but was in desperate need for new codebooks
in order to read the U-Boat traffic.
In the event it took until 30 October 1942, nearly 9 months after the
introduction of the M4, before new codebooks were captured.
In the meantime, some 3-wheel Bombes had been adapted for attacking
4-wheel Enigma traffic and orders were given to Doc Keen at BTM for
the development of an enhanced Bombe.
Although the British 4-wheel Bombe worked as expected, it faced many problems.
At this stage of the war, Great Britain had shortages of nearly everyhing.
As a result, BTM was unable to deliver enough machines in time and the
machines that were delivered often had contact problems.
Since their involvement in the war in 1942, the Americans had been pushing
the Brits to share their knowledge about the Bombe and allow them to copy
its design. Finally, in late 1942, when the British 4-wheel Bombe was facing
problems and the daily losses in the Battle of the Atlantic were accumulating,
the Brits finally gave in and allowed the US to build its own Bombe.
The US-Bombe was built by the National Cash Registers (NCR)
in Dayton (USA), where it was developed by Joe Desch.
Initially, the US Navy wanted him to build a fully electronic
machine, but Desch found this to be impractible,
as it would require the machine
to have more than 70,000 electronic valves (tubes).
Towards the end of 1942, Desch proposed a less elegant but far more
realistic approch: an electro-mechanical machine, just like the British
Bombe, but much faster and more reliable.
The US Navy immediately approved the project.
Desch set out to build the machine and by mid-1943, he had
the first prototypes running.
Although the initial design faced reliability problems,
he managed to improve it considerably. In the end,
the US Bombe turned out to be very reliable, fast and effective.
By December 1943, 120 machines were installed. For the remainder of the war,
the US took care of breaking the majority of German Naval Enigma
traffic and in particular the messages of the dreaded German U-Boats.
By the end of the war, the US Bombe would have saved thousands of lives,
but had destroyed that of Joe Desch, who had suffered a nervous breakdown
and would never really recover from that.
Deborah Anderson, Desch's daughter, has dedicated
a website to the work
of her father and his collegues at NCR .
After the war, all US Bombes were destroyed, except for one unit which
is now on permanent display at the National Cryptologic Museum (NCM)
at Fort Meade (USA) .
Rebuilding the British Bombe
In the UK, all British Bombes were destroyed or dismantled once the war was over.
As far as we know, not a single machine has survived.
This has triggered a group of enthusiasts - led by John Harper - to start
a rebuilding project in the mid-1990s.
The aim of the project was to recreate a fully functional replica of a
war-time Bombe machine, which was completed in 2007 .
It is now on public display at the
Bletchley Park Museum and is part of the permanent
exhibition in B-Block. The Bombe is being demonstrated on a regular basis
and is currently being used to do real WWII codebreaking work.
A far more detailed description of the Bombe Rebuild Project is
available on John Harper's website
The image on the right was taken at the
2009 Enigma Reunion
at Bletchley Park, where the Bombe was demonstrated by John Harper.
It shows the Bombe in action when trying to break a real war-time
Please note that different versions of the Bombe exist. Furthermore,
a number of attachments and 'special versions' were built for specific
tasks and experiments. The version reconstructed by the Bombe Rebuild
team is the later 3-wheel 36-Enigma version, with high-speed Siemens-type
sense relays. During the war, 69 of such machines were built.
It is featured in the video below.
This video shows the reconstructed Bombe in action. It was
made during the Enigma Reunion at Bletchley Park in 2009, when project
manager John Harper was giving live demonstrations. Some of the moving
parts inside the Bombe are clearly visible. The video also shows the
Checking Machine and a British Typex cipher machine
that was converted to act as an Enigma machine.
It is sometimes thought that Enigma was broken with help from
the first valve-based electronic digital computer,
that was developed at Bletchley Park during WWII.
It is even claimed in some books and TV documentaries.
This claim is not correct however, as
Colossus was used to break the
far more advanced Lorenz SZ-40/42 cipher machine,
that was used by Hitler's High Command.
- Brian Johnson, The Secret War
- Marian Rejewski,
Mathematical foundations of the German Enigma cipher machine solutions (Polish)
Rejewski's report of 1979.
- David Link, Resurrecting Bomba Kryptologiczna: Archaeology of Algorithmic Artefacts, I
Cryptologia, Volume 33, Issue 2, April 2009, pp. 166-182.
- John Keen, Harold (Doc) Keen
- John Harper, The Bombe Rebuild Project
Website, showing the progress and the various stages of the project.
- Milton Keynes Heritage Association, Outstations from the Park
- Dayton Codebreakers, US Navy Cryptanalytic Bombe
Website by Deborah Anderson.
- National Cryptologic Museum (NCM)
Based in Fort Meade (USA) near the NSA building.
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