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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.
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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).
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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.
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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.
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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).
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Lückenfüllerwalze Courtesy NCM
[1]
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