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← 31217 Stasi Hulzwurm →
The wooden dowel has a tiny – barely visible – 1 mm hole at the centre,
which allows the sound in the room to pass to the
sensitive microphone that
is hidden inside it. As dowels are commonly used as part of a wooden
construction, it proved to be a very realistic and also effective disguise.
The device was supplied in a beige envelope
on which the serial number (138), and the frequency (976 MHz)
and the intended purpose was written.
Inside the envelope was also
a small datasheet.
It was also possible to build the device into the top surface of the door
— which was much easier as it didn't require the door to be removed from its
hinges —
but that made it easier to discover by means of a visual inspection.
It is known that the British MI5/MI6
often used this method
[4].
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Stopfen is the German word for Plug — in this case it refers to
a wooden plug that is used to close a hole.
As the official designator of the device is
currently unknown, we have nicknamed it 'Stopfen', based on a note
that was handwritten on the envelope
by a Stasi employee at the time of its release.
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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.
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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) [3]. 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.
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At the bottom of the door – here cut-off and shown at the top – a 28 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.
Although the above described solution was an effective and difficult to
discover eavesdropping method, it was difficult to ensure that the
transmitting antenne — which was basically a short piece of wire — was
pointing straight up in order to give the best possible range. This
problem was countered in 1983, with the introduction of its successor:
the stick-shaped Holzwurm bug.
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Stopfen is based on the standard-issue Stasi bug 31217-111
shown in the image on the right, which has a range of 50 - 100 metres
depending on its position and any obstacles.
Although it has a free-running oscillator, it is fairly stable when it is
left untouched. Because of its high frequency — it operates in the 940 - 980
MHz band — it was not easily discovered.
➤ More information
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In order to counter problems with the antenna of the Stopfen — which
couln't be held up straight properly — the bug was succeeded in 1983 by
the so-called Holzwurm (woodworm) transmitter
shown in the image on the right.
Holzwurm consists of a brass cylinder that contains the microphone,
plus a grey PVC pipe that holds the batteries and the wire antenna.
➤ More information
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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.
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© Crypto Museum. Created: Tuesday 02 July 2019. Last changed: Sunday, 21 November 2021 - 21:45 CET.
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