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Holzwurm
Vertical stick with RF bug

Holzwurm (woodworm) is a covert listening device (bug) with a built-in 31217-1 transmitter, made in the early 1980s in the former DDR (East-Germany), as an improved version of Stopfen and was used by the Ministerium für Staatssicherheit (MfS) – the repressive Stasi – for bugging hotel room access doors. It has a tapered threaded brass section at the bottom for quick fitting.

The image on the right shows the complete bug, which consists of two pieces: a tick part – which consists of a brass weight and a grey PVC pipe – and a thin part that is fitted inside the thick part.

The tapered brass weight is threaded, so that it can be screwed into a pre-drilled 25 mm hole at the bottom of the wooden framework of a door, by means of a special tool that fits two holes. At the bottom centre is a 1 mm hole, behind which the microphone is located. Two sharp nails – located at either side of the microphone – allow a thin wooden disc to be fitted as a disguise [A].
  
Stick transmitter in action

The thinner part is a black plastic stick that holds the actual 31217 transmitter and the batteries. It also contains the wire antenna and ensures that it is kept straight up. This was a standard Stasi solution that was available in several variants as shown below. It fits snugly inside the thicker grey outer tube, and connects to the microphone by means of a 3-pin DIN connector at the end.

Microphone (right) with removed 31217 transmitter (left) Stick transmitter in action Heavy weight with microphone Bottom side of the weight/microphone Connector between transmitter and microphone (3-pin DIN) Removable tip at the end of the battery compartment Weight/microphone compared to the size of a hand 31217-1 transmitter embedded in the inner stick
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Microphone (right) with removed 31217 transmitter (left)
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Stick transmitter in action
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Heavy weight with microphone
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Bottom side of the weight/microphone
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Connector between transmitter and microphone (3-pin DIN)
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Removable tip at the end of the battery compartment
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Weight/microphone compared to the size of a hand
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31217-1 transmitter embedded in the inner stick

Features
The diagram below shows the two individual parts of the bug. At the left is the inner stick that has a diameter of 18 mm and is made of black plastic. At the top is a red plastic cap with a bionet fitting, that gives access to the battery compartment. It accepts two common 1.5V AA-size pen­light batteries. About 2/3rd from the top is the actual 31217-1 bug (white). Its antenna is located in the battery compartment, providing an operational range of ~ 150 metres. At the bottom is a 3-pin DIN male plug, by which the inner stick is connected to the microphone of the outer stick.


The outer stick has a diameter of 23 mm and is made of grey PVC. At the bottom is a heavy brass section with metric thread, allowing to be screwed into a pre-drilled hole. At the center of the bottom is a 1 mm hole behind which a miniature dynamic microphone is located. At the top of the brass weight is a 3-pin DIN socket that mates with the DIN plug at the end of the inner stick.

31217-1 transmitter
At the heart of the Holzwurm is a medium-range miniature 31217-1 transmitter, developed in the DDR (East-Germany) in the mid-1970s at ITU, especially for the Stasi. It measures just 32 x 16 x 7 mm and weights less than 25 grams.

The device is housed in a silver-plated copper enclosure with a thin white PVC 'jacket', and operates in Band V (940-980 MHz).

 More information

  
31217-1 transmitter (bug)

Other sticks
Although the outer tube of the Holzwurm bug was made especially for building into doors, the inner tube was a more or less standard container for the existing 31217-1 bugs at the time, as demonstrated by the image on the right.

The image was taken from an original Stasi document in which the technology was described that was available at the time. [3]. The second one from the right is nearly identical to the inner tube of the Holzwurm bug.

 View the original page
 View the complete document

  
Different versions of the sticks that were available [B].

Interior
The diagram below shows a cross-section of the Holzwurm. At the left are the batteries. At the right is the brass part with the microphone. Approximately at the center is the 31217 transmitter with its antenna floating in the battery compartment. To the right of the transmitter are the 3-pin male and female DIN connectors by which the microphone is connected to the transmitter.

Cross-section of the bug

The brass part has a tapered shape that is threaded towards the bottom. The bottom surface as a diameter of 26 mm, and has a 1 mm hole at the centre, behind which the microphone is located.

Bottom of the stick at 2:1 scale

The two nails are 17.5 mm apart and are held in place by two 2 mm headless screws. They allow the bug to be covered by a thin wooded disc so that it look like a wooden dowel. The two large holes are 14 mm deep and have a diameter of 4.3 mm, to allow the bug to be screwed into the wooden frame of a door, by means of a special tool with two 4 mm pins, spaced at 17.5 mm.

Complete device compared to the size of a hand Partly disassembled Stock Removable tip at the end, giving access to the battery compartment. Brass weight with microphone and DIN socket Two white dots to align the inner stick The tools that were used for building Holzwurm into a door [B]. Document kindly provided by Detlev Vreisleben [1][2].
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Complete device compared to the size of a hand
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Partly disassembled Stock
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Removable tip at the end, giving access to the battery compartment.
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Brass weight with microphone and DIN socket
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Two white dots to align the inner stick
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The tools that were used for building Holzwurm into a door [B]. Document kindly provided by Detlev Vreisleben [1][2].

History
During the Cold War, hotel rooms were interesting targets for intelligence agencies like the Stasi. They were often used by business men, foreign agents and dissidents, and allowed the Stasi to gather useful intelligence by eavesdropping on the (compromising) conversations in the room.

It appeared that the main access door and its surrounding frame were ideally suited for hiding bugs, not least because they often had a hollow space inside, that provided room for the micro­phone, the transmitter, the antenna and plenty of batteries that could last for up to a full year.

The image on the right shows a typical hotel room door that is on display at the Stasi Museum in Berlin (Germany) [4]. Part of the panelling has been removed, so that we can see the typical internal carton honeycomb structure that gives the door its strength without making it heavy.
  
Bottom end of a door, showing the coneycomb structure and the bug

At the bottom of the door – here cut-off and shown at the top – a 25 mm hole is drilled through the wooden framework (with a special tool) deeply into the honeycomb structure of the door. In the above example, an extra hole is drilled next to the first one, for adding additional batteries.

The batteries and the transmitter were then put inside the door, after which the hole was closed with a wooden stub that held the microphone.

The image on the right shows a typical bug that was used by the Stasi from the mid-1970s on­wards for installing in doors. It consists of a 31217-1 transmitter, with a dynamic Knowles microphone hidden inside the wooden stub.

 More information

  
31217-976 bug with micophone in wooden dowel

Hotel room door
The diagram below illustrates how and where the Holzwurm was built into the door of, say, a hotel room. The door is first removed from its hinges, after which a 25 mm hole is drilled into the bottom, right through the horizontal part of the frame. The device is then screwed into this hole from the bottom — slightly too deep — using a special tool. The excess space below the bottom of the brass part, is then covered with a wooden disc so that it looks like an inconspicuous dowel.

Cross-section of a hotel room door with Holzwurm bug fitted at the bottom

At the centre of the disc is a 1 mm hole, which allows the sound in the room to pass the 1 mm hole in the bottom of the brass section, and reach the microphone inside . Note that it is also possible to build the Holzwurm into the top of the door, which is actually easier as it doesn't require the door to be removed, but it is discovered more easily during a visual inspection.

The tools that were used for building Holzwurm into a door [B]. Document kindly provided by Detlev Vreisleben [1][2].


Connections
  1. GND
    red
    +V (GND)
  2. 0V
    blue
    0V
  3. Input
    grey
    Audio
  4. ANT
    brown
    Antenna
Documentation
  1. Neuerervorschlag 'Holzwurm'
    New proposal for 'woodworm' (German).
    MfS, Abteilung 26. Potsdam, 23 June 1983. 1,2

  2. Mounting tool, wooden discs, transmitters and tools
    MfS, Abteiling 26, BV-Pdm-Nr.: 221186, selection 13 of pages. 1
  1. Document obtained from BStU [2] via Detlev Vreisleben [1].
  2. The year is uncertain as it is barely readable on the provided copy of the document.

References
  1. Detlev Vreisleben, 31217-1, technical description and operating instructions
    Personal correspondence, May - August 2018.

  2. Bundesbeauftragte für die Stasi-Unterlagen (BStU) 1
    Federal Commissioner for the Stasi-Records.

  3. MfS, Foto 3, Sendercontainer für Techniken Band V
    GVS-o018, BVfS Pdm-Nr.: 221186. Abt. 26-5. Page 338. 2

  4. Stasi Museum Berlin, hotel room door with radio bug
    Visited March 2019.
  1. Full name: Bundesbeauftragte für die Unterlagen des Staatssicherheitsdienstes der ehemaligen Deutschen Demokratischen Republik (DDR) — Federal Commissioner for the Records of the State Security Service of the former German Democratic Republic (GDR) — officially abbreviated to BStU.
  2. Document kindly supplied by BStU [2] via Detlev Vreisleben [1].

Further information
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© Crypto Museum. Created: Saturday 04 August 2018. Last changed: Saturday, 06 July 2019 - 07:45 CET.
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