What is tempest radiation in electronics?
Read the following to find out:
Try to use a radio near a computer and the chances are you`ll hear strange buzzing humming and crackling noises. This is because every computer is also an unintentional radio transmitter and the funny noises are caused by the emissions known as "Tempest" radiation emawting from its cicuitry and in particular its screen.
Govermnent spooks have known since the Sixties that with the right equipment these faint signals can be picked up. If for example you have a sensitive document displayed on your computer spies lurking in the next roomcould intercept the radiation from the cathoderay tube in your monitor and reconstruct the onscreen image. They would then be able to read the document almost as easily as if they were looking over your shoulder.
As a result govemment agencies use specially shielded computers monitors and modems to block the emission of compromising radiation. In some cases whole rooms or even entire buildings are shielded. It`s an expensive business: shielded computers cost several times as much as unshielded ones and the US Department of Defence is thought to spend $1.5 bilhon a year on shielded equipment and testing. And technical information about Tempest originally the code name of a classified American research project has always been classified.
Now two Cambridge academics have lifted the lid on this shady subject. In a fascinating Paper Dr Ross Anderson and Markus Kuhn of the Cambridge University Computer Laboratory`s security group describe a series of Tempest experiments they carried out using a desktop computer a simple Tempest receiveir (essentially a modified television) and a cheap radio.
Their most significant finding is that Tempest shielding hardware is unnecesary in many cases. Instead they explain how emissions ftom monitors the most significant form of Tempest radiation can be dramatically reduced using nothing more than some clever screen fonts.
The Tempest emissions produced by monitors originate in the three electron guns that update the onscreen image dozens of times a second by scanning the screen`s phosphor coating a line at a time with a trio of flickering electron beams. Every time it switches from drawing dark screen dots (pixels) to drawing light dots each electron gun emits a pulse of telltale radiation. The more switching is involved the more radiation is emitted.
As a result a blank screen filled with a uniform colour producesthe least amount of radiation while a fine checkerboard pattern for which the electron guns must switch on and off for alternate pixels produces the most. A screenful of text is more like the latter than the former which is why it is possible to reconstruct it from its Tempest emissions.
Anderson and Kuhn`s software shielding technique which they have christened `Soft Tempest` involves smoothing the edges of the characters using a carefully chosen mathematical recipe. The onscreen appearance of the characters is almost unaffected
but their smooth grey edges mean the electron guns switch
on and off more gently. This dramatically (but not com
pletely) reduces the amount of radiation emitted and makes a document displayed on screen harder to to read by anyone tuning in to its Tempest emissions. In all but the highestsecurity environments this technique could provide dramatic cost savings compared with hardware shielding.
But rather than just hiding your secrets fmm your enemy you may want to feed him disinformation. So Anderson and Kuhn then go on to explain how Tempest enniissions can be deliberately subverted. To do this they take advantage of the fact that a fine blackandwhite checkerboard pattem looks like a uniform grey when displayed on a monitor but generates high levels of Tempest radiation. By embedding the text you wish your enemy to see as barelyvisible variations in the brightness of the checkerboard and then faintly superimposing your own text over the top in colour it is possible to mislead your enemy as to what is on your screen.
Finally and most intriguingly Anderson and Kuha`s paper also
outlines a technique in which careful variation of the brightness of just a
few screen pixels can he used to generate Tempest emissions that are
deliberately easy to detect. In effect the monitor is used as a covert radio
Their suggestion is that software packages would use the technique to transmit their serial numbers in encoded forin as a repeating radio signal. These transmissions could then be picked up by "software detector vans".
"If the van receives 20 signals from the same copy of Microsoft Word from a company that has only licensed five copies then probable cause for a search warrant has been established" say the authors who applied for a patent for the scheme in December.
But despite losing $1 bilhon a year throuth piracy Microsoft has rejected Anderson and Kuhn`s Tempestbased protection system.
Anderson and Kuhn`s paper can be found at: