Enigma variable notch wheel
- wanted item
The Lückenfüllerwalze (English: gap-filling wheel), 1
is a user-configurable cipher wheel for the
Enigma cipher machine, introduced towards the end of
World War II (WWII) but never deployed in the field.
It was planned to be used in combination with the field-rewirable reflector
Like other innovations, such as Enigma Uhr and UKW-D,
its goal was to improve Enigma security.
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'.
Right at the end of WWII,
the American Target Intelligence Committee (TICOM)
confiscated many cryptographic secrets, including
and kept it under wraps for many years .
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 .
In September 2009, at the
Enigma Reunion 2009
at Bletchley Park, we were able to make
detailed pictures of a Lückenfüllerwalze, brought
in by the director of the National Cryptologic Museum (NCM) .
It is part of the NCM collection.
As becomes clear from this picture,
the wheel has the same dimensions as (and is
compatible with) a standard Enigma wheel.
Most standard wheels only have a single triangular gap.
Once the wheel has made a full revolution,
the wheel to the left of it makes a single step.
This motion is known as regular or Enigma stepping.
The three extra Naval wheels (VI, VII and VIII) each have two
such gaps (often called 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
Also known as: Variable Notch Wheel .
Lückenfüllerwalze courtesy NCM 
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 wheels 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 causes the middle rotor to make two successive steps
under certain conditions. This is known as the double-stepping anomaly .
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 different on each wheel and had to be a relative prime number,
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) .
All serial numbers are prefixed 'Lf'.
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) .
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 . 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 .
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 . Heimsoeth und Rinke was the official manufacuturer
— 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 nearly complete when the war ended.
Menzer also developed the SG-41 (Hitlermüle).
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].
Oberkommand der Wehrmacht
German Army High Command.
Chiffrierdienst der OKW
Cryptologic Section of the OKW, the German Army High Command.
Target Intelligence Comittee
Cover name for the Anglo-American operation to find and seize German
intelligence assets, mainly in the field of communication and
cryptography, after WWII.
- 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.
- 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.
- 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.
- 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
Cryptologia, January 1997, Volume XX, Number 1.
Reproduced here by kind permission from the author.
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© Crypto Museum. Created: Wednesday 09 September 2009. Last changed: Tuesday, 06 September 2022 - 09:10 CET.