In 1985, Wim Van Eck, a Dutch researcher, published a paper in Computers & Security entitled "Electromagnetic Radiation from Video Display Units: An Eavesdropping Risk?" In this paper, he details how the electromagnetic emanations from a display device can be intercepted to give a representation of what is being displayed on the screen.
Although the security issues of intentional radio frequency (RF) emissions are common knowledge, such as those from a wireless network, the unintended ones can also cause security leaks. The physical principles are exactly the same: passing an electrical current down an antenna creates electromagnetic radiation. The only difference is that with intentional RF emissions the antenna is a deliberate, separate piece of equipment, specifically and optimally designed to emit at a specific frequency and wavelength.
In the Van Eck scenario, the antenna is created in the coils that are used to align the electron beam scanning inside a CRT (monitor). These magnetic coils direct the scanning electron beam toward the correct color of phosphor coating on the glass. The high frequency modulation of these electromagnetic coils emits an RF stream very similar to a standard terrestrial television broadcast signal, only lacking the synchronization data. If this synchronization is applied from an external source, reconstructing the original image is elementary.
The original paper makes for fascinating reading and provides a great deal more depth on the subject, complete with experimental verification and all of the equations and waveforms that any self-respecting physics geek needs.
Van Eck demonstrated the principle on CRT screens, although many different areas of a computer involve electrical currents passing down wires, such as wired keyboards, VGA or DVI cables, external drives, network connections, and so on. Each of these will create some form of RF emission; it is only a matter of tuning the receiving equipment to pick it up.
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