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Menzer Enigma
Enigma variable notch wheel
- wanted item
The Lückenfüllerwalze (English: gap-filling wheel), 1
is a user-configurable cipher rotor for the
Enigma cipher machine, introduced towards the end of
World War II (WWII) but never deployed in the field.
Like other innovations, such as Enigma Uhr and UKW-D,
its goal was to improve Enigma security.
It was intended to be used in combination with the field-rewirable reflector
UKW-D, and was invented by
Regierungsoberinspektor Fritz Menzer
of the German Cipher Bureau (OKW/Chi).
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Three Lückenfüllerwalzen (Lf) would be used in a machine.
Each Lf has 26 configurable turn-over notches, allowing
the number and position of the notches to
be changed frequently and easily.
The Lückenfüllerwalze was planned to be used in combination
with UKW-D, but like UKW-D and the
Enigma Uhr, it came too late and could not be distributed
effectively among the users in the field.
The Lückenfüllerwalze was also known as the
Wahllückenwalze (selectable gap wheel).
Its name is often erroneously written as Luckenfullerwalze
— without dieresis (¨) over the 'u'.
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Right at the end of WWII,
the American Target Intelligence Committee (TICOM)
confiscated many cryptographic secrets, including
the Lückenfüllerwalze,
and kept it under wraps for many years [2].
Had the Germans been able to produce the Lückenfüllerwalze in
quantity and distribute to all Enigma users in the field, it might
have defeated the Allied codebreakers
at Bletchley Park [6].
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In September 2009, at the
Enigma Reunion 2009
at Bletchley Park, we were able to make
detailed pictures of a genuine Lückenfüllerwalze
— from the National Cryptologic Museum (NCM) collection — brought
in by the director of the NCM [1].
As becomes clear from this picture,
the rotor has the same dimensions as (and is
compatible with) a standard Enigma rotor.
Most standard rotors only have a single triangular gap (notch),
which causes the rotor to its left to make a single step after one
full revolution.
This behaviour is known as regular stepping or Enigma stepping.
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The three extra Naval rotors (VI, VII and VIII) each have two
such gaps — commonly known as notches —
but these appeared not be very effective as their
number (2) is not a relative prime of 26 and they are located
exactly opposite each other, which effectively halves the cipher
period.
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Also known as Variable Notch Rotor [6].
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Lückenfüllerwalze courtesy NCM [1]
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A military Enigma machine, such as the Enigma I
and Enigma M4, features regular stepping, which means that,
when enciphering a message, the motion of the rotors is similar to the
motion of an odometer. In Enigma terminology, this is also known as
Enigma stepping. In addition to this, the stepping mechanism features
an anomaly, which - under certain conditions – causes the middle rotor to make an extra step on two successive key presses.
This is known as the double-stepping anomaly, and was described in detail
by David Hamer in 1997 [7].
The Enigma G on the other hand, features an irregular stepping
mechanism, first pioneered on the Zählwerk Enigma (A28).
Its rotors have multiple notches (different on each rotor), which cause a
more frequent and less predictable stepping motion.
Furthermore, its mechanism is cogwheel driven, as a result of which it
does not suffer from the double-stepping anomaly.
This priciple was not adopted for the military Enigma machines,
probably because it was too expensive.
The Lückenfüllerwalze brings the best of both worlds, as it adds
irregular stepping to a standard military Enigma. In fact it is potentially
stronger than the Enigma G, as the position of the notches can be
altered at will, which greatly increases the number of possibile settings.
It was mandatory however, that the number of notches was chosen carefully.
It had to be different on each rotor and had to be a relative prime,
as otherwise it would reduce the cipher period.
Furthermore, it does not fix the double-stepping anomaly, as the
stepping mechanism itself remains unaltered.
The inner wiring core of the Lückenfüllerwalze can be removed
and can be inserted in any of 26 possible positions (Ringstellung).
Production of the Lückenfüllerwalze was arranged by Heimsoeth und Rinke
in Berlin, hence the manufacturer code jla on the rotors,
but the actual production was carried out by
Ertel-Werk in München (Munich, Germany) [6].
All serial numbers are prefixed 'Lf'.
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During WWII, the Enigma frequently was the subject of (security)
investigations by the Germans. Some suspected the cipher to contain
flaws in the design and thought that it might have been broken by the Allies. Cryptographic inventor Fritz Menzer
therefore developed several alternatives and improvements.
Menzer
was Regierungs-Oberinspector at OKW/Chi
(the Cryptologic Section of the
German Army High Command) [4].
In 1939, Menzer
developed Schlüsselgerät 39 (SG-39),
which was in fact an improved Enigma.
It consisted of an Enigma with the addition of three coupled Hagelin
pin-wheels in order to provide variable stepping of the rotors.
Because of constant delays in development and production, the SG-39 was not completed
until 1944, prompting Menzer
to develop the Lückenfüllerwalze [4]. 1
By February 1943, the Lückenfüllerwalze was ready for production by
Heimsoeth & Rinke, but decisions were put off because the Enigma
was still considered secure [3].
At various security conferences between November 1944 and January 1945,
conducted by General Gimmler, "worry was expressed over the fact that the
military [Enigma] machine had not been changed throughout the war", whilst it
was known by the Germans that the British used a 10-rotor
Typex machine. 2
At one of these meetings, the Lückenfüllerwalze was approved.
8,000 units were ordered, but this was later increased to
12,000 [6]. Heimsoeth und Rinke was the official contractor
— hence the manufacturer code jla on the rotors — but
the actual production was carried out by
Ertel Werk in Hohenaschau (near München).
The rotors were almost complete when the war ended.
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In addition to this, Menzer
also developed the SG-41 (Hitlermüle).
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The Germans had learned from documents in October 1942 that
British Naval units would use a Typex with 10 rotors for
inter-service working [3 p.9].
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Device Enigma cipher rotor with configurable notches Purpose Encrease Enigma cipher security Inventor Fritz Menzer Contractor Heimsoeth und Rinke (jla) Manufacturer Ertel Werk (bac) Country Germany Development 1942-1943 Production 1944-1945 Compatibility Enigma I, M3, M4, A28, K Contacts 26 Notches 1-26
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OKW
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Oberkommand der Wehrmacht
German Army High Command.
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OKW/Chi
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Chiffrierdienst der OKW
Cryptologic Section (Cipher Bureau) of the OKW, the German Army High Command.
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TICOM
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Target Intelligence Comittee
Cover name for the Anglo-American operation to find and seize German
intelligence assets immediately after WWII had ended, mainly in the field of industrial developments, communication and cryptography.
(Wikipedia)
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- NCM - National Cryptologic Museum (USA)
Retrieved October 2009.
- Wikipedia, TICOM
Target Intellicence Committee (US).
- Joseph A Meyer, Der Fall WICHER: German Knowledge of Polish Successes on ENIGMA
NSA Technical Journal, Spring 1975 - Vol. XX, No.2. p. 9.
TOP SECRET UMBRA.
Declassified and approved for release by NSA on 31 October 2007.
DOCID: 3838699.
- David P. Mowry, Regierungs-Oberinspector Fritz Menzer
Regierungs-Oberinspector Fritz Menzer: Cryptographic Inventor Extraordinaire.
TOP SECRET UMBRA.
Declassified and approved for release by NSA on 13 July 2005.
DOCID: 2757002.
- Army Security Agency, European Axis Signal Intelligence in World War II, Vol. 2
Notes on Geman High Level Cryptography and Cryptanalysis. 1 May 1946.
TOP SECRET CREAM.
Declassified and approved for release by NSA on 1 June 2009.
DOCID: 3560816.
- History and Modern Cryptanalysis of Enigma's Pluggable Reflector
Olaf Ostwald and Frode Weierud. Cryptologia, January 2016.
Obtained from cryptocellar.org.
- Actions involved in the 'double stepping' of the middle rotor 1
David Hamer.
Cryptologia, January 1997, Volume XX, Number 1.
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Reproduced here by kind permission from the author.
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© Crypto Museum. Created: Wednesday 09 September 2009. Last changed: Tuesday, 23 January 2024 - 11:08 CET.
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