After we ran a story about the need for TV licenses in the UK and the policing thereof, we got a fair number of emails from readers asking us whether we knew about Tempest as a possible method of monitoring what people are doing on their PCs.

Well, no we didn't and neither did the TV Licensing Agency, but we do now. For the benefit of those who were similarly ignorant - including the TVLA - we thought we'd produce a short and sweet synopsis of the technology.

The underlying principle is fairly simple: any electrical device generates an EM field. A PC generates such a field, which if intercepted and properly processed can reveal information about what was on the screen. TEMPEST protects electronic equipment, preventing the emission of the compromising emmanations.

Possession of equipment capable of doing the intercepting and analysis is not allowed in the US, under pain of very-serious-looking-men-in-suits-and-dark-glasses-arriving-at-your-house-and-taking-you-away.

Disappointingly, the TVLA had not even heard of TEMPEST (much like us then) and currently employs extremely low-tech and stultifyingly dull methods to track down license evaders.

It says that it has no plans to research Tempest or any other similar methods of tracking what people are watching on their PCs.

Despite the opportunity for some real secret agent style spying, the TVLA keeps track of who hasn't paid by checking lists of names off against each other. One list comes from TV sellers who are obliged to tell the TVLA when anyone buys a TV, and the other is a list of people who have paid up. Names on the former that fail to show up on the later get double-checked.

Related link

A Google search on TEMPEST will bring up a lot of links. Here's one.

Related Story

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