Researchers at the University of Michigan have developed the system and say it should protect data even if files have been left open on screen.

A radio transceiver installed inside the laptop's casing is programmed to identify its owner by means of a small transmitter worn like a wristwatch. This lets the laptop know how far away its master is. Whenever separated by a set distance, automatic encryption of data is triggered.

"And as soon as the user comes back within radio range, the computer will begin unlocking the computer so that it is ready to resume work when the user sits down," says researcher Brian Noble.

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