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This refers to electronic eavesdropping and most information about this is classified
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Large classified computer centers are surrounded by a (earthed) conducting box so it all electronic signals are trapped inside
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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
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One classifies systems by their control zone which measures distance from device up to which one can detect signals
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One wants a small control zone
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Internet and phone system rely on principle of mass confusion. There are so many messages that it is impossible to detect the sensitive ones!
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