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Covert infrared telephony - wanted item

ALD-3K was a device for routing an analogue PSTN telehone line over an infrared (IR) optical communication channel, developed around 1986 by INT in East-Berlin (DDR), for the repressive East-German intelligence service Ministerium für Staatssicherheit (MfS), better known as the Stasi. The device allowed the Stasi to covertly use unmonitored telephone lines in West-Germany [1][A].

The system consisted of two stations: a remote one in the target area (A) and a local base station in East-Berlin (B). Station (A) consisted of a telephone transfer unit (German: Über­leitungs­einrichtung) and the modular FINOW-I optical transceiver shown in the image on the right. It was connected to a telephone line of the West-German Public Switched Telephone Network.

Station (B) consisted of a more powerful infrared transceiver, such as the JO-4.02, an inter­face and a telephone set. It could be used by the Stasi for incoming as well as outgoing telephone calls.
FINOW-I receiver (left) and transmitter (right)

The device was developed by Institut für Nachrichtentechnik (INT) in East-Berlin, as part of its research project Anwendung von Lumineszenzdioden (ALD) — the practical application of Light Emitting Diodes (LEDs) [B]. INT was responsible for the development of of the ALD-3K devices and the telephone interfaces, but the actual production was done by the OTS of the Stasi [3].

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During the Cold War, the telephone lines between East-Germany and West-Germany were closely monitored by the intelligence services at both sides of the Iron Curtain. In West-Germany this was done by the Bundesnachrichtendienst (BND), whilst in East-Germany this was the responsibility of the Ministerium für Staatssicherheit (MfS) — the Stasi. As part of their assignment, the Stasi had intelligence gathering agents in West-Germany, in particular in the West-German part of Berlin.

To allow an agent to pass information to Stasi headquarters without crossing the border, a variety of technical solutions were used, including the use of optical transmission equipment, in German known as Lichtsprechgeräte. Although these devices provide virtually undetectable voice circuits, they require a direct line-of-sight (LOS) and have a limited range, typically between 2 and 8 km.

By using the ALD-3K, the Stasi could directly 'tap' into an existing analogue telephone line in West-Berlin, and only had to bridge the short distance across the Berlin Wall. Agents in West-Germany could then call in from virtually anywhere in the world and deliver their messages to the Stasi without being noticed by the West-German authorities. In the same vein, the Stasi could use the unmonitored telephone line to make outgoing calls directly to its agents in the free West.

  1. ALD-3K description (German) 1
    MfS, VVS-o052, B 2/86. 1986. 9 pages.

  2. ALD final report (German) 1
    Dipl.-Ing. E. Kube, Abschlußbericht.
    INT, July 1968 — March 1969.
  1. Document obtained from BStU [2] and kindly supplied by Detlev Vreisleben [1].

  1. Detlev Vreisleben, Personal correspondence
    November 2021.

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

  3. Dipl.-Ing. Erhard Kube, personal correspondence with Detlev Vreisleben
    Dresden, 15 October 2008.
  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 obtained from BStU [2] and kindly supplied by Detlev Vreisleben [1].

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© Crypto Museum. Created: Saturday 11 December 2021. Last changed: Saturday, 11 December 2021 - 12:20 CET.
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