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Lückenfüllerwalze
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 famous Enigma Uhr.
 
Another - far less well known - measure to make the Enigma safer, was the so-called Lückenfüllerwalze (gap-filling wheel). It 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 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).
  

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.

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.

The inner (wire) core could be removed (see the images below) and inserted in any of 26 positions (Ringstellung). The walze was made by Heimsoeth und Rinke (jla) and all serial numbers were prefixed by Lf (Lückenfüller).
 
Lückenfüllerwalze Courtesy NCM [1]
 
References
  1. NCM - National Cryptologic Museum (USA)

  2. TICOM - Target Intellicence Committee (TICOM)
    Brief explanation on Wikipedia

Further information


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Last changed: Thu,26 Aug 2010.18:38:13
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