Computer keystrokes can be snooped from afar by detecting the slight electromagnetic radiation emitted when a key is pressed, according to new research.

Other security experts have theorised keyboards were vulnerable to such detection, wrote Sylvain Pasini and Martin Vuagnoux, both doctorate students with the Security and Cryptography Laboratory at the Ecole Polytechnique Fédérale de Lausanne in Switzerland.

But Vuagnoux and Pasini believe theirs is the first set of experiments showing such spying is feasible. They blamed cost pressures on keyboard manufacturers for not making keyboards more snoop proof.

Keyboards "are not safe to transmit sensitive information," they wrote in an entry 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 1-metre wire cable about a metre 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 told, various experiments shows they could monitor keystrokes from as far as 20 metres 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.