While eavesdropping on the EM emanations of computers is an effective passive attack, a potential attacker can take a more proactive approach to deny you use of your computer or even destroy it. These attacks attempt to overload a computer's circuits with excessive radio frequency energy. Short bursts can corrupt data in a computer and cause it to crash or lose needed information. This type of attack is carried out by individuals with High Energy Radio Frequency (HERF) guns. HERF guns contain a high wattage signal generator and highly directional antenna. Because of the directional capabilities of this weapon, specific devices can be made to fail, possibly confusing any attempt to troubleshoot the perceived problem. More devastating than a HERF gun is an electromagnetic pulse (EMP) device. This type of device, an EMP transformer, "is designed to cause the electronic circuits, (transistors, chips, ICs, RAM etc.) to internally explode ... disabling the entire computer" (Schwartau and Death 1991, 22). In addition, a potential attacker could also "erase the contents of RAM chips, ROM, EPROM, and hard disks, floppies and tape back ups" (Schwartau and Death 1991, 22). Unfortunately, the countermeasures to these threats are not as well known as TEMPEST criteria are. It seems that the manufacturing process and the results of the weapons are still regarded as government secrets, with little substantial information released to the public. However, it should be noted that most U.S. defense contractors creating electronic components for field use are capable of building microelectronic level components which withstand EMPs and HERF attacks up to a certain intensity. Any attempt at supplying possible countermeasures to these weapons would be unsubstantiated conjecture.
More information on these types of weapons can be found at:
Information Warfare Weapons
Glossary of Information Warfare Terms
Electromagnetic Environmental Effects
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