The side of Van Eck phreaking that most people have heard of is TEMPEST. TEMPEST has been incorrectly attributed as an acronym on many occasions, with some rather tortuous creations cropping up, for example, Telecommunications Electronics Material Protected from Emanating Spurious Transmissions or Transient ElectroMagnetic Pulse Emission STandard.
TEMPEST is, in fact, a codeword and as such has no particular meaning; it is capitalized using standard convention for codewords. As with anything that has its roots in the back rooms of secret organizations, the exact origin is somewhat shaded in mystery being attributed both to the American NSA and the British CESG at GCHQ.4 Early references to TEMPEST can be found as far back as 1953—acoustic tests over a telephone line were run against the Whirlwind 1 computer at MIT being used to determine the state of program execution.
Since then both the British and American governments have devoted significant time and resources setting standards that secure systems have to comply with in relation to their emissions. These standards are collectively known as TEMPEST.
There are four main methods to reduce the security risk of RF emissions.
Screening Surrounding a system with sufficient screening to prevent RF emissions from leaking out will obviously prevent anyone from eavesdropping. This could be done with a thick metal case, which has holes in it sufficiently small with respect to the wavelength to prevent wavelengths from escaping. Obviously, in the case of a screen, you need a fairly large hole at the front to allow viewing, so this solution isn't as straightforward as it might seem. The next stage is to encase the entire room that the machine is in, operator and all, in something called a Faraday Cage. This cage is a container made from a conducting metal or conducting mesh smaller than the wavelength of the radiation you want to keep from escaping. It absorbs the radiation and dissipates it over the mesh, rather than simply diminishing its effects through its thickness.
Filtering A cable extending beyond a controllable boundary (for example, a telephone line to the outside world) can act as a conduit for spurious signals. In this instance, you should place filters to allow only legitimate data through.
Digitizing Converting a signal from analog to high frequency digital makes it much harder to intercept the resulting emanations clearly. Couple this with any form of encryption on the wire and it becomes nearly impossible.
4 NSA stands for National Security Agency and CESG for Communications Electronic Security Group, at the UK Government Communications Headquarters (GCHQ)—just in case you didn't know.
Nonelectrical Transmission Mediums Fiber optics have no detectable RF emanations, so they are ideal for high security communications.
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