Time to time, usually when it appears on Cryptome, I skim through the revisions of Wassenaar agreement lists of controlled technologies. It's a neat way to keep myself up to date with what technologies are available on the market and the approximate degree of security they offer.
One of the controlled articles on the recent revisions mentions this: a.8. Communications cable systems designed or modified using mechanical, electrical or electronic means to detect surreptitious intrusion. Which, together with what I stumbled over some time ago, leads me to an idea. Time-domain reflectometers are used to check the integrity of cables and fibers. Commercial devices tend to be awfully expensive, but in some cases they reportedly can be improvised. http://www.hut.fi/Misc/Electronics/circuits/tdr.html is an example of an el-cheapo (and probably low-grade) version. (I am unable to assess its performance, my highfrequency-fu sadly isn't too good.) Maybe it could be possible to build a dedicated TDR system intended to be connected to installed cablings, periodically test the cables by sending pulses along them and watch what returns, compare the result with long-term average, and report differences. Could possibly also help with early discoveries of various "natural" damages, not only intrusions. Opinions, comments, construction hints, ideas about better and/or simpler approaches?
