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Umkehrwalze D
Rewirable Enigma reflector

Umkehrwalze D, or UKW-D was a field-rewirable reflector for the Enigma-I cipher machine. It was introduced in January 1944 by the German Air Force (Luftwaffe) as an alternative to the existing reflectors UKW-B and UKW-C, in an attempt to improve the security of the Enigma machine. A special variant was available for use in the Naval M4 Enigma. It is also known as UKW Dora.
UKW-D was introduced by the Luftwaffe as part of a series of improvements that were intended to improve the cipher security of the Enigma. Another safety measure was the Enigma Uhr that was sometimes even used in combination with UKW-D (e.g. on the Luftwaffe Red key).

The new rewirable reflector was mounted in place of the existing UKW-B, and was rather cumbersome as it required part of the machine to be disassembled. The operators didn't like it and as a result, the wiring was changed only once every 10 days, as per updated key sheet.

Had UKW-D been used with all Enigma-I machines in the field, it would have posed a serious (and possibly fatal) threat to codebreakers at Bletchley Park. Luckily, UKW-D wasn't distributed widely and was therefore only used for specific – 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 provided long cribs and made breaking both Uhr and UKW-D very easy once the basic key had been broken.

UKW-D was also used in the Enigma KD, a variant of the (commercial) Enigma K that was used by the Mil Amt (Militärisches Amt, the successor of the Abwehr) from December 1944 onwards. It is little known that a special version of UKW-D was developed for the Kriegsmarine (Navy). The version of UKW-D shown above is such a rare Naval variant. It was developed for use with the four-wheel Enigma M4 and was supplied to units that had to communicate with the Air Force.

Another attempt to improve the cipher strength of the Enigma, was the development of the so-called 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...

Modified Key sheet
After UKW-D was introduced, the key sheets of the Luftwaffe were modified to include the wire pairs to be connected on the UKW. Below is an example of such a key sheet. Is has number 2744 which can be dated to August or September 1944. It contains the Enigma settings for a full month starting at the bottom. All key sheets were made for 31 days regardsless the length of the month. Unused days were simply ignored or used for emergency messages.

The UKW-D settings are given in the 4th column, with the heading: an der Umkehrwalze (at the reflector). The settings are printed sideways, as they were used for approx. 10 days. The sheet below was found in the Bletchley Park archives and was kindly supplied by Tony Sale [6]. It is also shown in Phil Marks's articles on UKW-D [1]. Click the image below to take a closer look.

These key sheets were used both by users of UKW-D and users of UKW-B. Many mistakes were made when exchanging messages between UKW-D and non-UKW-D users. Furthermore, the table above shows the bad key-making habits of the Luftwaffe. If features the non-crashing wheel order (i.e. a wheel was never used in the same position two days in succession) and the non-consecutive Stecker rule (never pair two consecutive letters together, e.g. CD or KL). Such mistakes were a great help to the codebreakers as they reduced the total number of possibilities.

The rightmost four columns of the table are used for the Kenngruppen (discriminants). Each discriminant consists of three letters (trigram) that were send as clear text in the preamble of the message to identify the key in use. The first two columns were used with UKW-B, while the last two indicated that UKW-D was used. Again this was a cause of numerous mistakes and confusion and occasionally British intercept stations overheared arguments between German operators. Needles to say that such conversations were often very helpful to the code breakers as well.
Opening UKW-D
Opening an UKW-D can be a bit tricky. The inner core is held in place by two spring-loaded balls, one at either side of the device, 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.
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, such a paint brush or ballpoint, to push the inner core out of the outer shell. That's all there is to it. The inner part should come out easily as shown in the images below. Remove the inner part entirely if you want to alter the wiring.

Please note that the inner core has an index pin that should mate with a hole in the outer shell. This index pin is present to ensure that the wire core can only be fitted one way around.

Also note that the outer shell has another index hole (on one of its curved mounting stubs). This hole should mate with a pin inside the Enigma, to ensure that the UKW is mounted with the right side up and keeps it in place during normal use of the machine.

Inside UKW-D are a series of small single-pin plugs - much like small banana-type plugs - and a bundle of textile-encapsulated wires. There are 12 patch cables (rather than 13), 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.
Around the circumference 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 have been omitted.

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 same contact but run clockwise.

The German notation (i.e. the index ring on UKW-D) was, of course, used on the key sheets.

The table below shows the mapping between BP notation (A-Z) and the physical markings on the UKW-D itself. 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. To add to the confusion, Turing (in his Traitise) started numbering from the contact at the dead top.

Contact numbering
Although the various numbering schemes of an UKW-D are well-described by Phil Marks in his excellent series about UKW-D in Cryptologia in 2001 [1][2][3], there is often confusion on how to interpret the data. For this reason, we will use the diagram below to explain this. In this diagram we are looking at the contacts of the UKW-D from the main axle (i.e. the right of the machine).

The official Enigma-numbering starts with the contact at the dead top, moving clockwise over the surface of the UKW. This numbering is shown in red (1-26). In the official BP-notation however, each wheel was canted forward by one position, as this is what would be visible through the window in the top lid of the machine. This notation was also adopted by the US cryptanalysts. It is shown in the above diagram in black (A-Z). The official German notation, as engraved on the circumference of the UKW, is shown in blue. This notation was used in the German code books and moves counter-clockwise, starting with the contact just in front of the topmost one.

UKW-D and Enigma M4
UKW-D was primarily used by the German Air Force, the Luftwaffe. It is little known however, that it was also issued to Naval units that needed to communicate with Luftwaffe commands. The latter is described in Philip Marks' excellent series of Cryptologia articles about UKW-D [1][2][3].
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 represented no more than a standard UKW with 26 different settings.

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, engraved just below the word 'Oben', is visible through the leftmost window of the lid.

UKW-D and Enigma K
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 [4].
Apparently, the Enigma KD was also used by the Swedish Intelligence Service, as the FRA has one in its private collection. Exactly how and why this Enigma KD came into their posession is currently unknown, but it might be a left-over from WWII.

At the Enigma Reunion 2009 at Bletchley Park (UK), the Enigma KD was on public display at the stand of the Swedish Radio Intelligence Agency FRA [5]. The full wiring of this machine is known.

 More information

  1. Philips Marks, Umkehrwalze D: Enigma's rewirable reflector - Part 1
    Cryptologia Volume XXV, Number 2, April 2001.

  2. Philips Marks, Umkehrwalze D: Enigma's rewirable reflector - Part 2
    Cryptologia Volume XXV, Number 3, July 2001.

  3. Philips Marks, Umkehrwalze D: Enigma's rewirable reflector - Part 3
    Cryptologia Volume XXV, Number 4, October 2001.

  4. David Hamer, Geoff Sullivan and Frode Weierud
    Enigma Variations: An Extended Family of Machines

    Cryptologia, Volume XXII, Number 3, July 1998.

  5. Försvarets Radioanstalt (FRA), Sweden (National Defence Radio Establishment)
    Enigma KD from private collection of FRA. September 2009.

  6. Tony Sale, Luftwaffe key sheet 2744, August/September 1944
    Personal correspondence. October 2009.

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