The attack program takes control of the physical prongs on general-purpose input/output circuits and vibrates them at a frequency of the researchers' choosing, which can be audible or not. The vibrations can be picked up with an AM radio antenna a short distance away.

For decades, spy agencies and researchers have sought arcane ways of extracting information from keyboards and the like, successfully capturing light, heat and other emanations that allow the receivers to reconstruct content.

The new makeshift transmitting antenna, dubbed "Funtenna" by lead researcher Ang Cui of Red Balloon Security, adds another potential channel that likewise be would be hard to detect because no traffic logs would catch data leaving the premises.

Cui showed the system in action for a few reporters ahead of his talk Wednesday at the annual security conference Black Hat in Las Vegas.