One interesting question is exactly how strong radio frequency
illumination could cause compromise of information being processed by
electronic equipment. I have an idea for a mechanism whereby such
illumination could induce generation of harmonic and beat frequencies
that are modulated by internal data signals.
This mechanism is based on an effect that is familiar to ham radio
operators, who are often bedeviled by neighbors complaining of
television interference. Here is a quote from the chapter on
interference in an old (1974) edition of the ARRL Radio Amateur's
Handbook:
"Harmonics by Rectification"
"Even though the transmitter is completely free from harmonic output
it is still possible for interference to occur because of harmonics
generated outside the transmitter. These result from rectification of
fundamental-frequency currents induced in conductors in the vicinity
of the transmitting antenna. Rectification can take place at any
point where two conductors are in poor electrical contact, a
condition that frequently exists in plumbing, downspouting, BX cables
crossing each other, ... It can also occur ... in power supplies,
speech equipment, etc. that may not be enclosed in the shielding
about the RF circuits."
In the case of computer equipment, the conductor could be a wire,
external cable or even a trace on a printed circuit board. Now
imagine that the source of rectification is not a poor connection,
but a transistor junction in a logic gate or line driver. As that
device is switched on and off, RF rectification may be switched on
and off as well, modulating the generated harmonic with the input
signal. If that signal carries sensitive information, all the
information would be broadcast on the harmonic output. Keyboard
interfaces, video output circuits and serial line drivers come to
mind as excellent candidates for this effect, since they often carry
sensitive information and are usually connected to long wires that
can absorb the incident RF energy and radiate the harmonics.
All an attacker has to do is monitor a site transmitting at frequency
f and analyze any signals at 2*f, 3*f, etc. If the site has more than
one transmitter, say a command hut, or a naval ship, there are also
beat frequencies to consider f1+f2, f1-f2, 2*f1+f2, 2*f1-f2, etc.
Note that harmonics and beats radiated from the equipment under
attack are vastly easier to detect that any re-radiation at the
fundamental frequency, which would be swamped by the primary
transmitter's signal.
There is also a potential active attack where an adversary
frequency-sweeps your equipment with RF hoping to find a parasitic
harmonic generator. This might be the "resonance" technology Peter
Wright referred to. If the source illumination causes a resonance
by, say, operating at 1/4 the electrical wavelength of the video
output cable, any effect might be magnified greatly. (The even
harmonics would be suppressed, but odd harmonics would not be.)
Illumination could be done directly or over telephone, cable TV or
power lines.
This might also explain "NONSTOP testing and protection being
especially needed on vehicles, planes and ships." since they often
carry multiple radio transmitters and are more easily exposed to
monitoring and external illumination than a fixed site inside a
secure perimeter.
The two code names (NONSTOP and HIJACK) might possibly refer to the
passive and active modes. Or NONSTOP may refer to radiated signals
and HIJACK to signals over hardwire lines. Or one could cover all the
effects I am proposing and the other something completely different.
Whatever.
FWIW,
Arnold Reinhold
At 2:23 AM +0000 1/13/2001, David Wagner wrote:
>In a paper on side channel cryptanalysis by John Kelsey, Bruce Schneier,
>Chris Hall, and I, we speculated on possible meanings of NONSTOP and HIJACK:
>
> [...]
> It is our belief that most operational cryptanalysis makes use of
> side-channel information. [...] And Peter Wright discussed data
> leaking onto a transmission line as a side channel used to break a
> French cryptographic device [Wri87].
>
> The (unclassified) military literature provides many examples of
> real-world side channels. [...] Peter Wright's crosstalk anecdote
> is probably what the HIJACK codeword refers to [USAF98]. Along
> similar lines, [USAF98] alludes to the possibility that crosstalk from
> sensitive hardware near a tape player might modulate the signal on the
> tape; [USAF98] recommends that tapes played in a classified facility be
> degaussed before they are removed, presumably to prevent side channels
> from leaking. Finally, one last example from the military literature
> is the NONSTOP attack [USAF98, Chapters 3-4]: after a careful reading
> of unclassified sources, we believe this refers to the side channel
> that results when cryptographic hardware is illuminated by a nearby
> radio transmitter (e.g. a cellphone), thereby modulating the return
> signal with information about what the crypto gear is doing [AK98].
> [...]
>
> [AK98] R. Anderson and M. Kuhn, "Soft Tempest: Hidden Data Transmission
> Using Electromagnetic Emanations," Proc. 2nd Workshop on
> Information Hiding, Springer, 1998.
> [USAF98] US Air Force, Air Force Systems Security Memorandum 7011--
> Emission Security Countermeasures Review, 1 May 1998.
> [Wri87] P. Wright, Spycatcher, Viking Penguin Inc., 1987.
>
>The above is excerpted from the conclusions of
> J. Kelsey, B. Schneier, D. Wagner, C. Hall,
> "Side channel cryptanalysis of product ciphers",
> Journal of Computer Security, vol. 8, pp. 141--158, 2000.
> http://www.cs.berkeley.edu/~daw/papers/sidechan-final.ps
>
>Do remember, please, that these are just guesses.
>
>Also, credit is due to Ross Anderson and Markus Kuhn for informative
>discussions on this topic.
