SOFTWARE that allows a computer to receive radio signals could make spying on
other computers all too simple, according to two scientists at the University of
Cambridge. Such are the dangers that they are patenting countermeasures that
computer manufacturers can take to foil any electronic eavesdroppers.
Spies can already read documents written on computers by intercepting the
radio-frequency emissions from their electronics, but the tuning and antenna
equipment needed to do this is not available off-the-shelf and is very
expensive. But a new breed of 鈥渟oftware radio鈥�, designed to let computers tune
in to radio signals in any waveband, promises to make this type of eavesdropping
simple and cheap. A PC circuit board with a plug-in aerial does all the tuning
under software control and has a digital signal processor chip to cut noise.
鈥淓quipment to do this [spying] would now cost at least 拢30 000, but in
five years it will cost less than 拢1000, and it鈥檚 hackers who will be
writing the software,鈥� predicts Markus Kuhn, a research student who has filed
the patent with Cambridge cryptographic expert Ross Anderson
(see interview this issue, p 48).
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The late Peter Wright, who worked for British intelligence, was the first to
blow the whistle on electronic eavesdropping. His 1986 book, Spycatcher
, revealed how he had spied on messages sent by the French during Britain鈥檚
negotiations to join the European Economic Community鈥攅lectromagnetic
emissions from the input of the French encoding machine allowed plain text to be
received and read.
Insulating computers in metal jackets to prevent these telltale emissions is
difficult, expensive and ugly. Modern offices want stylish PCs鈥攂ut their
plastic cabinets emit radiation. So, in patent application GB 2 333 883, Kuhn
and Anderson detail how PC makers can foil spies without fitting PCs with metal
enclosures.
In a conventional PC, the magnetic heads of the hard disc drive rest over the
data tracks that were last accessed. The drive keeps spinning, the heads keep
reading and the readout amplifier keeps repeating the data鈥攚hich provides
a perfect signal for an eavesdropper鈥檚 tuner to lock onto. The inventors say the
answer is to load software into the PC that ensures that the drive heads are
always 鈥減arked鈥� over a safe area of the disc which contains no data.
But the monitor also transmits signals, which depend on the text displayed.
An ordinary TV receiver can display on-screen documents on a remote screen,
which can then be video-recorded and transcribed at the spy鈥檚 leisure. So
Anderson and Kuhn suggest using a text font with softened edges. This limits
high-frequency emissions鈥攔adiation which beams farthest afield from the
computer. Sharp-edged fonts need fast signal 鈥渞ise times鈥�, which demand
high-frequency harmonics.
And keyboards are also troublesome. They rely on a scanning signal, which
radiates the pattern of keys being pressed. So the patent suggests using a
random number generator to continually distort the scanning signal.
