Hackers can identify keystrokes using the electromagnetic radiation emitted when a key is pressed - and thia can be picked up through an office wall, security researchers have warned.

While other researchers have touted this theory, Sylvain Pasini and Martin Vuagnoux, both doctorate students with the Security and Cryptography Laboratory at the Ecole Polytechnique Fédérale de Lausanne in Switzerland, believe theirs is the first set of experiments showing such spying is feasible. They explained that manufacturers were not making keyboards snoop-proof because of cost pressures.

Keyboards "are not safe to transmit sensitive information", they wrote on the school's website. "No doubt that our attacks can be significantly improved since we used relatively inexpensive equipment."

The researchers tested 11 different wired keyboard models produced between 2001 and 2008, including some with USB connectors and keyboards embedded in laptops. All were vulnerable to one of four surveillance methods.

Two videos posted show two different experiments, both of which accurately picked up the typed text.

The first video shows a white Logitech keyboard with a PS/2 connector that was attached to a laptop for power. It was monitored with a simple wire cable about 1m away. After typing 'trust no one' on the keyboard, the same phrase is returned on the researchers' monitoring equipment.

In a second video, a larger antenna picked up keystrokes through an office wall. All together, the various experiments show they could monitor keystrokes from as far as 20m away.

Vuagnoux and Pasini have written a paper that's currently in peer review detailing the technique. It will be released soon at an upcoming conference, they wrote.

Efforts to reach Vuagnoux and Pasini were unsuccessful.