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
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
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)
confiscated many cryptographic secrets, including
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 .
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
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
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Lückenfüllerwalze Courtesy NCM 
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) .
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 . 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 .
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.
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.
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© Crypto Museum. Created: Wednesday 09 September 2009. Last changed: Monday, 12 June 2017 - 20:07 CET.