How can a landline telephone be tapped

Tapping into phone calls is relatively easy

Telephone calls in the fixed network or with a mobile phone can be tapped relatively easily from a technical point of view, as long as they are not encrypted in a complex manner. Most switchboards in telecommunications companies around the world are equipped with so-called LI interfaces ("Lawful Interception"). Authorities and secret services can access this according to the respective legal and political framework. "In this way, even several services can eavesdrop simultaneously without seeing who else is on the line," said Andy Müller-Maguhn, spokesman for the Chaos Computer Club and co-owner of a company that manufactures encryption telephones.

When wiretapping in the fixed network, "bugs" are used less often because they only affect certain devices or rooms. It is more effective to tap into fiber optic lines or multiplex connections in order to filter out telephone calls based on the connection identifier or certain voice characteristics. Secret services proceed in a similar way when listening to satellite connections.

IMSI catchers make the cell phone look like a real radio cell

When making calls with a normal cell phone, eavesdroppers use a weak point in the technical processes in the GSM cellular system (global system for mobile communication). For example, the radio network can unequivocally recognize a mobile phone using the "International Mobile Subscriber Identity" (IMSI) and log it into the network. However, the cell phone is not able to verify the authenticity of a radio cell. Secret services and others therefore use so-called IMSI catchers, which usually consist of a laptop and a radio antenna. They wedge themselves between the cell phone and the actual cellular network. The "trapped" mobile phone is fooled into a real radio cell.

However, an IMSI catcher must always be close to the person being monitored. If larger areas such as the UN quarters in New York are to be spied on, eavesdroppers would presumably attack the connections between surrounding cell phone stations or tap the LI interface. Yesterday, the former British Minister for Development Aid, Clare Short, said that UN Secretary General Kofi Annan had been wiretapped by her country's secret service. Other UN employees have also been spied on.