Markus G. Kuhn, Ross J. Anderson
University of Cambridge Computer Laboratory

Soft Tempest: Hidden Data Transmission Using Electromagnetic Emanations
URL: http://www.cl.cam.ac.uk/~mgk25/ih98-tempest.pdf

It is well known that eavesdroppers can reconstruct video screen content from radio frequency emanations. We discuss techniques that enable the software on a computer to control the electromagnetic radiation it transmits. This can be used for both attack and defence. To attack a system, malicious code can encode stolen information in the machine's RF emissions and optimise them for some combination of reception range, receiver cost and covertness. To defend a system, a trusted screen driver can display sensitive information using fonts which minimise the energy of these emissions. There is also an interesting potential application to software copyright protection.

Markus G. Kuhn
University of Cambridge Computer Laboratory

Optical Time-Domain Eavesdropping Risks of CRT Displays
URL: http://www.cl.cam.ac.uk/~mgk25/ieee02-optical.pdf

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.