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Stealth wallpaper keeps company secrets safe

A type of wallpaper that prevents Wi-Fi signals escaping from a building without blocking mobile phone signals has been developed by a British defence contractor. The technology is designed to stop outsiders gaining access to a secure network by using Wi-Fi networks casually set up by workers at the office.

It is the work of moments for an employee to connect a paperback-sized Wi-Fi base station to a company network. That person can then wander around the office with their laptop while remaining wirelessly connected to the internet.

But it is also the work of moments then for an outsider to breach that company's computer security using the Wi-Fi connection. Unless the Wi-Fi base station is protected by security measures that most amateur users would not bother to set up, it gives anyone up to 100 metres away the chance to bypass the corporate firewall and wirelessly hack straight into the network.

Until now, the only way to ensure people are not illicitly gaining access to company secrets has been to turn offices into a signal-proof "Faraday cage", by lining the walls with aluminium foil, and using glass that absorbs radio waves in the windows. This ensures all electromagnetic emissions are absorbed, but it also means that no one can use a cellphone in the building.

So the UK's telecoms regulator, Ofcom, has paid BAE Systems, formerly British Aerospace, to come up with an answer for firms who are becoming increasingly worried about the threat. BAE Systems has based its solution on the secret "stealth" technology that it uses to hide military radars. The covering, called Frequency Selective Surface (FSS) sheeting, is used to shroud radar antennas on warships or aircraft.

Copper coated

Solid metal antennas normally give a very strong reflection to enemy radar scanners. To hide them, FSS sheeting can be electrically set to allow through only the precise frequency the antenna wants to transmit and receive, while absorbing all other frequencies including those of the incoming radar.

BAE's anti-Wi-Fi wallpaper is made from a 0.1-millimetre-thick sheet of kapton, the same plastic used to make flexible printed circuit boards in lightweight portable gadgets like camcorders. The kapton is coated on each side with a thin film of copper.

On one side most of the copper is removed, leaving a grid of copper crosses. On the other side, matching crosses, turned through 45 degrees, are etched away ¬- leaving a film of copper with a grid of cross-shaped holes. BAE says that by carefully changing the size of the crosses and their spacing, the sheet can pass precisely defined frequencies, while blocking all others.

But they are not revealing how the military technology works except to say it is a little like an optical diffraction grating creating interference to destroy certain light frequencies. "We have developed formulae for this, which we aren't going to give away," says project leader Kevin Mitchell.

On or off

Ofcom engineers have confirmed to New Scientist that the wallpaper can block Wi-Fi at 2.4, 5 and 6 gigahertz, while letting through GSM and 3G cellphone signals, plus emergency service calls.

Better still, the filtering can be switched on or off if diodes are connected between the copper crosses. When a current is fed through the diodes, all frequencies are blocked. Switching them off "opens" the panel to let mobile and emergency signals through.

The wall covering can be mass produced at relatively low cost. A square metre will cost about £500: peanuts to big business.

BAE is now working on a transparent, ultra-thin version for windows. William Webb, Ofcom's R&D chief, says: "With this new technology, signals can be shared securely and go where they need to go, and no further."



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