die@cpoint.UUCP (David I. Emery) (01/23/89)
A year or so ago I saw a demo on TV (on ABC's Nightline I think) of
what was refered to as a simple device that could be built with $25
worth of parts that could recover an image of the text on a CRT display
belonging a nearby PC or terminal from its spurious electromagnetic radiation.
The demo, done in London to circumvent strong US government opposition
to demoing this technology on commercial TV, showed a somewhat noisy but
quite readable image of text on the screen of a PC about 25 feet away from
the loop antenna used to pick the signal up.
Does anyone know what the $25 circuit consists of ?
Can it really be built for $25 ?
Some thoughts
It is quite clear that the video circuitry in a monitor radiates
broadband pulses with a spectral peaks at the ringing frequencies of the video
circuitry (eg. the wires to the tube ring like a spark transmitter when
excited by the hf energy contained in the bright/dark and dark/bright
transistions). And it is also clear that a monitor radiates lots of
energy at harmonics of its sweep frequency (try using a VLF receiver
near a TV).
To me the obvious thing to do is to recover phase lock to the
horizontal sweep by locking on to some harmonic, and divide that reference
down to generate frame/field timing. The data might be recovered by
generating a sample pulse delayed by a variable digital time delay circuit
from the phase locked timing reference and using that pulse to sample
the output of an envelope detector following a very broadband hf to vhf
amplifier driven by the pickup loop antenna. By averaging thousands of these
pixel wide samples and then slowly sweeping the time delay by one pixel at a
time one could very slowly reconstruct the image. Naturally this could be
done much faster (but not for $25) by using a modified frame grabber as
the correlator. Another technique involves video frequency analog ccd
shift registers in a correlator.
Is there a better way of doing this I am missing ?
--
David I. Emery Clearpoint Research Corp.
99 South Street, Hopkinton Ma. 01748 1-508-435-2000
{decvax, cybvax0, mirror}!frog!cpoint!die hughes@math.Berkeley.EDU (eric hughes) (01/24/89)
In article <2010@cpoint.UUCP>, die@cpoint (David I. Emery) writes: > > A year or so ago I saw a demo on TV (on ABC's Nightline I think) of >what was refered to as a simple device that could be built with $25 >worth of parts that could recover an image of the text on a CRT display >belonging a nearby PC or terminal from its spurious electromagnetic radiation. > > The demo, done in London to circumvent strong US government opposition >to demoing this technology on commercial TV, showed a somewhat noisy but >quite readable image of text on the screen of a PC about 25 feet away from >the loop antenna used to pick the signal up. > > Does anyone know what the $25 circuit consists of ? >Can it really be built for $25 ? This very topic is the cover story in the Winter 1988 issue of _Abacus_ (vol 5 no 2). The title of the article is "The Tempest over Leaking Computers." The author is Harold Joseph Highland. Allow me to quote a paragraph from the article: "Anyone can put together a primitive eavesdropping unit for somewhat over $100. All you need is a portable television set with a voltage-controlled oscillator (VCO) tuner (one with no detents or stops between the channels and no fine tuning control), a dipole antenna with a 75-ohm balum (a balance transformer to match the impedance of the antenna with that of a television set's cable), and a 75-ohm cable to connect the set to the antenna's balum. The unit will have a limited range, between 20 and 40 feet, and the reception may be poor because the picture will be unsteady. If extensions are used with the vertical and horizontal hold controls for finer tuning, picture quality will improve. With some knowledge of television design and construction, you could build a more efficient unit at higher cost. The circuit diagram of a TV set with a VCO tuner is available in Sams Photofact #2218, a series used by many television repair shops. This equipment was designed by a man named Wim van Eck. He "noted that with slightlymore sophisticated and expensive equipment, it was possible to intercept data from a VDT screen up to 1 kilometer away." The article is a kind of weird mix between gossip, basic electromagnetic theory, technical details, and government regulations. Thoroughly interesting. Eric Hughes hughes@math.berkeley.edu