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Lückenfüllerwalze
Programmable Enigma cipher wheel - wanted item

Towards the end of WWII, the German Army made several attempts to increase the cipher security of the Enigma machine. Well known examples of such attempts are the field-rewirable reflector UKW-D and the Enigma Uhr. A far better improvement however, was the Lückenfüllerwalze.
 
The Lückenfüllerwalze (Eng: gap-filling wheel) featured 26 user-configurable notches and allowed the number and position of the notches of each wheel to be changed frequently.

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 called Wahllückenwalze (selectable gap wheel). Its name is sometime erroneously written as Luckenfullerwalze (Without dots on the 'u').
  

The American Target Intelligence Committee (TICOM) [2] confiscated many cryptographic secrets, including the Lückenfüllerwalze immediately at the end of WWII and kept it under wraps for many years. If it had been produced in quantity and used in the field, it might have defeated the Allied code breakers at Bletchley Park.
 
At the Enigma Reunion 2009 at Bletchley Park in September 2009, we were able to make some detailed pictures of this extremely rare configurable notch wheel that is part of the collection of the NCM [1].

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 is called: regular 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 period.
 
Irregular stepping
The Lückenfüllerwalze however, allowed the number and position of the notches (gaps) to be changed in the field. If the number of notches was a carefully chosen relative prime (of 26) and the number of notches was different for each wheel, the cipher period would be greatly enhanced and the wheels would all step more often. As a result, the machine would be far less predictable. This is called: irregular stepping. Irregular stepping (with a fixed prime number of notches), was also a key feature of the Enigma G.

The inner (wire) core could be removed (see the images below) and inserted in any of 26 positions (Ringstellung). Production of the Lückenfüllerwalze was arranged by Heimsoeth und Rinke in Berlin, hence the manufacturer code jla on the rotors. All serial numbers were prefixed by Lf (Lückenfüller). The actual manufacturing took place at Ertel-Werk in Munich (see below).
 
Lückenfüllerwalze Courtesy NCM [1]
 
History and Development
During WWII, the Enigma frequently was the subject of (security) investigations by the Germans. Many suspected the system 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 [4]. So Menzer developed the Lückenfüllerwalze.

By February 1943, the Lückenfüllerwalze was ready for production by Heimsoeth & Rinke, but decisions were put off because the Enigma was still considered secure [4]. 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. At one of these meetings, the Lückenfüllerwalze was approved. A quantity of 12,000 units was ordered, and production at the Ertel Werk in Hohenaschau (near München) was nearly complete when the war ended.
 
Glossary
OKW   Oberkommand der Wehrmacht
German Army High Command.

OKW/Chi   Chiffrierdienst der OKW
Cryptologic Section 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, mainly in the field of cryptography, after WWII. (Wikipedia)

References
  1. NCM - National Cryptologic Museum (USA)

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

  3. Joseph A Meyer, De 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.

Further information

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