Tempest and Control Zones
This refers to electronic eavesdropping and most information about this is classified
Large classified computer centers are surrounded by a (earthed) conducting box so it all electronic signals are trapped inside
Of course NSA (National Security Agency) tries to protect nation’s security by exploiting inter alia ability to intercept and understand communication between unfriendly folks
One classifies systems by their control zone which measures distance from device up to which one can detect signals
- One wants a small control zone
Internet and phone system rely on principle of mass confusion. There are so many messages that it is impossible to detect the sensitive ones!