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TUCoPS :: Hardware Hacks :: rs232.txt

The Threat of Information Theft by Reception of Electromagnetic Radiation from RS-232 Cables




"The Threat of Information Theft by Reception of Electromagnetic
Radiation from RS-232 Cables"

From 'the threat of information theft by reception of electromagnetic
radiation from rs-232 cables.' computers and security, 9(1990) 53-58
(factors effecting reception are grounding/coupling, data rate (baud),
and cable length.) I am not entering any of the math, and alot of the
tech stuff - If you want to do this get ahold of the paper.
  ...experiments showed that RS-232 data signals can be intercepted
several meters away from a target system, even when a shielded data
cable is used. This can be done w/ the aid of a very compact
commercially available and therefore cheap gear such as a walkman
provided w/ a recording facility and some minor modifications. This
means that although the seperation distance at which interception is
possable is limited to several meters, in many cases eavesdropping can
be done without attracting attention. On the other hand, when more
sophisticated equipment is used such as a communications receiver in
combination w/ a directional antenna, eavesdropping might be difficult
close to the target system...however larger and therefore quite safe
seperation distances may be feasable.
  (I get the impression that one needs to place the receiver a
specific distance frm the cable, mutch akin to having 2 receivers tuned to the
same frequency a set distance apart that is a factor of the
wavelength of the tuned to frequence and being able to send morse by
tapping on the speakers - frequency entrainment, But i'm not shure
about this.)
  ...When an RS-232 interface cable is connection is part of the equip
configuration, then there are many factors acting in favor of the
eavesdropper, the most important being the following:
>the bit amplitude of an RS-232 data signal is relativly large compaired
w/ the levels of the logic signals used in the inner circuites of the
equipment.
>the rise and fall times of the data signal are very short. Consequently
they correspond to high frequency components resulting in considerable
radiation.
>the RS-232 interface connection is unballanced with respect to the
earth. This inhearent unballance will contribute to a high level of
radiation.
>in many cases, the RS-232 cables are not shielded, or the shielding
is not adequetly connected to to the equipment, so that those cables
behave like unshielded cables.
>inner walls (without metal grids) do not effect radiation levels
signifficantly at frequencies of interest (below 200MHz).
>the data are serially transported along the RS-232 cable, which makes
it easy to recognise the individual bits. Usually the data are coded
in well known character sets (like ASCII). This makes it very easy to to
decode the reconstructed bits.
>the data are often structured by the legal user, therefore they are
easily interpreted.
>the data signal is transmittted at bit rates which are low (300, 600,
1200 bits) compaired with the nyquist rate corresponding to the bandwidth
of a standard radio receiver (AM = 5 kHz, FM = 75 kHz). Therefore. in
principle, the data signal can be detected even w/ the help of a standard
pocket radio receiver. At the same time the data can be recorded on a
tape w/ the help of an ordinary cassette recorder.
  ...a simplification is the absence of the coupling between the two
resulting signal conductors. For the most commonly used RS-232 cables
this ommision makes makes no significant difference to the field
strength calculation. further we have assumed that the transmitter is
grounded and the receiver is not. "Grounded" means that the galvanic
connection to the reference groundplane exists. this is often the case
in practice. When no groundplane exists, there will be a certain amount
of parasitic capacity between equipment and groundplane (in the case of
desktop equip. typically 100 pF)...
  ( 2 experiments using a pocket radio receiver @ 7meter's picked up
the signal at 16 MHz (short wave band), and 98 MHz.(in the FM band at
harmonics of the system clock))...a standard AM/FM radio receiver
equiped w/ a whip antinna 1m long. A hard limiter circuit was used to
reconstruct the detected data...
...only at one site was shielding effectivness signifficant. Radio signals
could be detected at a distance in all cases, virtually
correlating w/ the the orriginal data stream. however at 3 sites the
data could not be reconstructed w/ just the aid of a simple level
detector (he doesn't say what was used to reconstruct the signals beyond
a level detector). At the remaining sites, the data could be
reconsructed w/ level detection at distances of 6-9m A PC-modem connection
could be be intercepted in the bedroom of an adjacent house...
  (data received @ 98 MHz will be too week to to be heard through the
the speaker, must use a simple level detector.(pre-amp/filter?), it
seems like proccessing is going to be the biggest pain in getting one
of these systems up, it being highly desirable to condition the signal
so that it can be fed into a computer and storred on disk.



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