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Radio-proof your home, but only after taking legal advice

Nonsense stories seem to be a habit when the Infosec security conference comes to town, and "Spyguard" takes the prize this year: it's a "radio proof" paint, which has a transparent version for windows.

Nothing wrong with Spyguard. It's a product designed for critical security by the paranoid US Government, and all it does is attenuate radio waves, light, and infra-red radiation. Which means that technically, it is quite true to say that "Wireless hackers could be thwarted by a sheet of film that blocks WiFi and IR signals," as one report puts it - and equally, it is possibly true to claim that "MI5 is watching who buys it… "

But the metallised paint isn't magic: it blocks all radio frequency signals, and it won't distinguish between WiFi and GSM phone signals.

So if you paint this around the inside of your secret building, your mobile phone won't work inside it any more than your WiFi will leak out of it.

It just so happens that it is almost certainly illegal to screen mobile phone signals from any public place - as hotel keepers have discovered when they "jam" mobile phones in order to rip off their guests by making them dial through the switchboard.

But even if you get a radio-tight assurance from your lawyer that you are within your rights to shield a place with Spyguard, there are still one or two caveats.

First, wireless goes in all directions, and bounces around; so even one window left open, will "leak" any signals. Even the chimney or a ventillator might be enough of a gap and as for the airspace under and around a door, well, that can radiate microwave like a beacon.

But even if you can fix all that, you may still be vulnerable, because metallised paint can only attenuate the signal, not cut it off entirely. For complete blocking of wireless, you need a Faraday cage - continuous conductor.

Wire mesh will do it. Spyguard won't: have a look at the performance characteristics in the diagram:

 light atten IR UltraViolet SD1000 25 - 40 dB 30% 93% >99% SD2000 25 - 40 dB 30% 93% >99% SD3000 classified 30% classified >99%

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