A covert listening device, more commonly known as a bug, is usually a combination of a miniature radio transmitter with a microphone. The use of bugs, called bugging, is a common technique in espionage and, increasingly, in police investigations.
Most bugs use a radio transmitter, but there are many other options for carrying a signal: radio frequencies may be sent through the main wiring of a building and picked up outside; transmissions from a cordless phone can be monitored; and it is possible to pick up the data from poorly configured wireless computer networks or tune in to the radio emissions of a computer monitor.
Bugs come in all shapes and sizes. The original purpose of bugs was to relay sound, but today the miniaturisation of electronics has progressed so far that even commercially-available bugs designed to carry TV signals are usually the size of a cigarette packet. Professional bugs can fit into pens, calculators and other commonplace items. Some are only the size of small shirt buttons, although the power and operational life of the smallest bugs is very short.
The development of modern 'wireless' technology has presented new security concerns. To be 'wireless' a device must transmit information, either by radio waves or infrared light, and this potentially makes all the information sent via that link available to others. Radio waves are the easiest to intercept, but even infrared transmissions can be picked up through a window. Some wireless devices, such as wireless computer networks, do encrypt transmissions, but the standard forms of encryption are weak. Such devices, whether wireless keyboards or wireless telephones, should not be used in any environment where sensitive information is handled.
Most bugs emit radio waves. The standard counter-measure for bugs is therefore to 'sweep' for them with a receiver, looking for the radio emissions. Professional sweeping devices are very expensive. Low-tech sweeping devices are available through amateur electrical magazines, or they may be built from circuit designs on the Internet. But sweeping is not foolproof. Advanced bugs can be remotely operated to switch on and off, and some even rapidly switch frequencies according to a predetermined pattern in order to make location with sweepers more difficult. A bug that has run out of power may not show up during a sweep, which means that the sweeper will not be alerted to the surveillance.
Those bugs that do not emit radio waves are very difficult to detect. Radio-based bugs are a technical solution to a problem - remotely listening to people's conversations - but a simpler option is simply to record the conversation on a normal recording machine. There are a number of options for this:
* Pocket sized devices, either worn or carried in baggage, linked to a small microphone which is usually mounted on the surface to pick up the audio. Digital devices such as minidisc recorders or the latest palm-sized camcorders produce very high quality recordings and are conveniently small.
* Larger recording devices hidden in the room, for example above suspended ceilings. These are popular in workplaces for monitoring staff.
* Ultra-directional microphones, or parabolic microphones. These are like the microphones seen on camcorders, or carried by sound technicians. They are constructed to receive signals only from one direction. The most high-tech directional microphones can eavesdrop on conversations from a hundred metres away or more. Microphone arrays can be used as well.
* Laser microphones. These are very expensive and highly technical to operate. A laser beam is bounced off a window, or off any object near to the conversation monitored. Any object which can resonate/vibrate (for example, a picture on a wall) will do so in response to the pressure waves created by noises present in a room. The electronics detect the minute difference in the distance travelled by the light to pick up this resonance and reproduce the sound causing it.
* Some equipment that is not a threat on itself may exhibit microphonics.
* Mobile phones can be used as bugs as well.
* The adversary can use a trojan horse to acquire access to microphones connected to a computer.
If a microphone is hidden in a room it is almost impossible to detect, as it has no radio emission. Very sensitive equipment could be used to look for magnetic fields, or for the characteristic electrical 'noise' emitted by the computerised technology in digital tape recorders; however, if the place being monitored has many computers, photocopiers or other pieces of electrical equipment installed, it may become very difficult. Older analog equipment is even more difficult to detect.
Another method is using very sensitive infrared cameras to detect waste heat of a bug, or different thermal conductivity of a place where it is hidden after briefly chilling the surface of the object with eg. liquid nitrogen.
Bugging devices in EU headquarters
Electronic bugging devices were found in March 2003 at offices used by French and German delegations at the European Union headquarters in Brussels. Devices were also discovered at offices used by other delegations. The discovery of the telephone tapping systems was first reported by Le Figaro newspaper, which blamed the US.