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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).

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 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].

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.

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.

  1. Also known as Variable Notch Rotor [6].

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Lückenfüllerwalze courtesy NCM [1]

Irregular stepping
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'.

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.

  1. In addition to this, Menzer also developed the SG-41 (Hitlermüle).
  2. 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].

  • 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
  • Development
  • Production
  • Compatibility
    Enigma I, M3, M4, A28, K
  • Contacts
  • Notches
OKW   Oberkommand der Wehrmacht
German Army High Command.
OKW/Chi   Chiffrierdienst der OKW
Cryptologic Section (Cipher Bureau) of the OKW, the German Army High Command.
TICOM   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)
  1. NCM - National Cryptologic Museum (USA)
    Retrieved October 2009.

  2. Wikipedia, TICOM
    Target Intellicence Committee (US).

  3. 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.

  4. 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.

  5. 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.

  6. History and Modern Cryptanalysis of Enigma's Pluggable Reflector
    Olaf Ostwald and Frode Weierud. Cryptologia, January 2016.
    Obtained from

  7. Actions involved in the 'double stepping' of the middle rotor 1
    David Hamer. Cryptologia, January 1997, Volume XX, Number 1.
  1. 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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