Optical Time-Domain Eavesdropping Risks of CRT Displays

[John Young]

Markus Kuhn has released this after learning of 
Joe Loughry's announcement.

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Announced 5 March 2002. 
To be presented at IEEE Oakland conference, May 2002


http://www.cl.cam.ac.uk/~mgk25/ieee02-optical.pdf

Optical Time-Domain Eavesdropping Risks of CRT Displays

Markus G. Kuhn
University of Cambridge, Computer Laboratory
JJ Thomson Avenue, Cambridge CB3 0FD, UK
mgk25 at cl.cam.ac.uk

Abstract

A new eavesdropping technique can be used to read
cathode-ray tube (CRT) displays at a distance. The 
intensity of the light emitted by a raster-scan 
screen as a function of time corresponds to the 
video signal convolved with the impulse response 
of the phosphors. Experiments with a typical personal 
computer color monitor show that enough high-frequency 
content remains in the emitted light to permit the 
reconstruction of readable text by deconvolving the
signal received with a fast photosensor. These 
optical compromising emanations can be received even 
after diffuse reflection from a wall. Shot noise from 
background light is the critical performance factor. 
In a sufficiently dark environment and with a large 
enough sensor aperture, practically significant 
reception distances are possible. This information
security risk should be considered in applications 
with high confidentiality requirements, especially 
in those that already require “TEMPEST”-shielded 
equipment designed to minimize radio-frequency 
emission-security concerns.

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