Electrical devices emit electromagnetic waves. With devices that process information (e.g.
computers, monitors, network switching elements, printers), these emissions can also contain
the information currently being processed. Emissions carrying such information are referred
to as compromising interference signals. An attacker who is in a neighbouring building or
even in a vehicle parked in the vicinity, for instance, may try to receive these signals and
reconstruct the processed information based on them. The confidentiality of the information
is therefore questionable. A possible objective of such an attack is industrial espionage.
The limit values of the Electromagnetic Compatibility Act (EMVG) are generally not low
enough to prevent someone from intercepting the compromising interference signals. If this
risk cannot be accepted, additional safeguards must therefore be taken as a general rule to
prevent this.
Compromising emanations are not limited to electromagnetic waves. Sound waves, for
example when using printers or keyboards, may also be used under certain circumstances to
obtain useful information.
It must also be noted that compromising interference signals are also caused or increased by
external manipulation of devices in certain cases. If, for example, a device is irradiated with
electromagnetic waves, it may happen that the reflected waves contain confidential
information.