It has occurred to me that the cheapest form of protection from tempest attacks might be an active transmitter that swamps the signal from the computer. Such a transmitter would still be legal if its power output is kept within the FCC part 15 rules.
Take, for example, the signal from a CRT monitor. The monitor signal consists of large signals which are the vertical and horizontal sync pulses, and smaller signals which are the levels of each of the phosphor guns. The simplest countermeasure would be random RF noise which is many orders of magnitude stronger than the signal from the monitor. However, with this system, the attacker can average many fields from the monitor and perhaps still recover the signal because any give pixel is the same, while the noise is random. (Or at least the pixels change slowly compared with the fields, giving lots of data to average.) The next more complex version sends the same random screen over and over in sync with the monitor. Even more complex versions change the random screen every-so-often to try to frustrate recovering the differences between screens of data on the monitor. Can such a device be built and still stay within the Part 15 rules? Cheers - Bill ------------------------------------------------------------------------- Bill Frantz | Due process for all | Periwinkle -- Consulting (408)356-8506 | used to be the | 16345 Englewood Ave. [EMAIL PROTECTED] | American way. | Los Gatos, CA 95032, USA --------------------------------------------------------------------- The Cryptography Mailing List Unsubscribe by sending "unsubscribe cryptography" to [EMAIL PROTECTED]
