All electrical and electronic equipment produces electromagnetic radiation (electrical emanations). The electromagnetic signals propagate through space (radiated emissions) and along electrically conductive pathways (conducted emissions). If these emissions are intercepted and analysed they could reveal that the signals contain information rather than merely ‘noise’ and thus the protectively marked data could be revealed. A receiving device can intercept these signals without being detected even when located some distance away, access to the original equipment is not required.
Electromagnetic testing for commercial items, for instance as part of the CE compliance procedure, examines both the power of the emissions (signal strength) to ensure that it will not adversely affect other products in the locality and the tested items susceptibility to electromagnetic radiation generated from other sources to ensure it will not itself be adversely affected.
TEMPEST is concerned with only the emitted radiation (both radiated and conducted) and whether the signal is ‘information bearing’. TEMPEST is defined as the study of the unintentional emission of protectively marked data from an equipment or system.