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During WWII, several attempts were made to make Enigma traffic more
secure. In January 1944, a field-rewirable reflector, called UKW-D,
was introduced by the Luftwaffe (German Air Force).
It allowed the wiring of the UKW (reflector) to be changed in the
field. The Luftwaffe codebooks were adapted to include the
settings for UKW-D.
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Since removing and disassembling UKW-D was rather cumbersome,
the wiring was changed only once every 10 days.
It was sometimes used in conjunction wit the Enigma Uhr,
another device to make the Enigma more secure, for example on
the Luftwaffe-key 'Red'.
It is little known that an UKW-D was also developed for the
Kriegsmarine (Navy).
The version of UKW-D shown here is such a rare Naval variant.
It was developed for use with the Enigma M4
and was supplied to units that had to communicate with the Air Force.
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If UKW-D had been used with all Enigma machines in the field,
it would have posed a real (and perhaps fatal) threat to
Bletchley Park.
Luckily, UKW-D wasn't distributed widely and was difficult to
operate. It was therefore only used for important messages.
Like with the Uhr, the Germans had to be able to exchange
messages with both UKW-D and non-UKW-D users,
so they mixed messages with and without the devices,
using the same basic key.
This made breaking both Uhr and UKW-D very easy once
the basic key had been broken.
Another attempt to improve the cipher strength of the Enigma,
was the development of the Lückenfüllerwalze
that was intended to be introduced towards the end of the war,
together with UKW-D, for all Enigma traffic. Luckily, it came too late...
UKW-D was also used in the Enigma KD
a variant of the (commercial) Enigma K,
used by the Mil Amt
(Militärisches Amt, the successor of the Abwehr)
from December 1944 onwards.
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Inside UKW-D are a series of small single-pin plugs and a bundle of textile-encapsulated wires.
There are 12 patch cables, each with a plug at either end. They are used to connect the letters in pairs, just like
in a standard UKW. Due to space constraints (there are two mounting screws) one pair is connected
permanently.
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Around the circumfere of the inner core are 24 letters of the international alphabet (not 26).
Each letter A-Z corresponds to a single socket of UKW-D. The letters J
and Y are missing.
Please note that the letters on the index ring do not match
BP's notation.
On the ring, it starts with the contact just in front of the topmost
one (A) running counter clockwise,
whilst BP starts at the topmost contact, running clockwise.
The German notation (i.e. the index ring on UKW-D) was, of course, used on the key sheets.
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BP contacts (A-Z) are mapped to the German UKW-D contacts in the following order:
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| Bletchley Park Notation |
ABCDEFGHIJKLMNOPQRSTUVWXYZ |
| UKW-D index ring |
A-ZXWVUTSRQPON-MLKIHGFEDCB |
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The missing contacts (i.e. the position of the spring-loaded ball bearings) are marked with a '-' sign.
So, in BP notation, the letter pair BO is wired permanently.
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Opening an UKW-D can be a bit tricky. The inner core is held in place by two spring-loaded
balls that protrude a small hole in the outer shell. Some people seem to think that you
have to press both balls inwards, e.g. with a screwdriver, in order to removed the inner
core. Over time, we've seen many UKW-D units with scratches around this area.
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Removing the core is, in fact, very easy and should not cause physical damage to the UKW.
Simply use the rear end of a non-metal object, e.g. a paint brush, to push the inner core
out of the outer shell. That's all.
The inner part should come out easily as shown in the 3rd picture below. Remove the inner
part entirely if you want to alter the wiring.
Please note that the inner part has an index pin that should mate with a hole in the outer
shell. This is done to ensure that the wire core can only be fitted in one way.
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Also note that the outer shell has another index hole (on one of its mounting stubs).
This hole should mate with a pin inside the Enigma, to ensure that the UKW is mounted
the right side up (see the rightmost image below).
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Using UKW-D in an Enigma M4
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UKW-D was primarily used by the German Air Force, the Luftwaffe.
It was, however, also issued to Naval units that needed to communicate
with Luftwaffe commands.
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The UKW-D shown here was built especially for use in the Naval
Enigma M4.
The outer shell is made of zinc plated messing. As zinc easily oxidates
in a moist environment, it was used to protect the inner contacts of UKW-D.
When used inside an M4, it replaces the existing (thin) UKW-B and
Zusatzwalze Beta. This way, the machine was compatible
with a 3-wheel (service) Enigma that was also using UKW-D.
There was no point in retaining the Zusatzwalze,
as together with UKW-B it was in fact an UKW with 26 different settings.
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The images below show how UKW-D is mounted inside the M4.
Engraved in the body of UKW-D is the text Oben (top) which means
that it has to be mounted with that side facing upwards.
Inside the Enigma is an index pin that should mate with a hole in the
body of UKW-D to prevent it from being mounted in the wrong orientation.
Once UKW-D is in place and the cover is closed, the red letter D
is visible through the leftmost window.
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In December 1944, a new Enigma machine came into service at the German
Secret Intelligence Service Mil Amt (the successor of the Abwehr).
It was used on the Berlin - Madrid - Lisbon link and appeared to be
an Enigma KD; a rewired commercial Enigma (K) with UKW-D.
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Apparently, the Enigma KD was also used by the Swedish Intelligence Service,
as they do have one in their own private collection.
Exactly how and why this Enigma KD came into their posession is currently
unknown.
At the Enigma Reunion 2009 we were able
to see the Enigma KD for the very first time. The wiring is still unknown.
 More information
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- Philips Marks, Umkehrwalze D: Enigma's rewirable reflector - Part 1
Cryptologia Volume XXV, Number 2, April 2001.
- Philips Marks, Umkehrwalze D: Enigma's rewirable reflector - Part 2
Cryptologia Volume XXV, Number 3, July 2001.
- Philips Marks, Umkehrwalze D: Enigma's rewirable reflector - Part 3
Cryptologia Volume XXV, Number 4, October 2001.
- David Hamer, Geoff Sullivan and Frode Weierud
Enigma Variations: An Extended Family of Machines
Cryptologia, Volume XXII, Number 3, July 1998.
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