hardware scramblers

[deboni@diego.llnl.gov (Tom DeBoni)]

I don't know a whole lot about this subject, but I'd like to hazard a suggestion
about hardware scramblers. I once upon a time saw an article in some source,
perhaps Popular Science or the like, on how to build a cheap voice scrambler
for telephones using a circuit called a ring demodulator (or something like
that). The curcuit had a ring of diodes, looking like a bridge rectifier,
with the voice and noise source feeding in and some function of the inputs
feeding out. The idea was to pick a radio station at random, use its audio as
a noise source, and let your partner at the other find the station that
renders the signal comprehensble. This only works if both parties have access
to the same radio signals, and can easily be defeated in short order by any
eavesdroppers with radios, but taping your conversation does them no good.
Further, it's cheap, requires no compression or sophisticated hardware, and
uses the whole telephone voice bandwidth.

I've never tried this, so I don't know how well it works, but since it's an
old idea, perhaps it could be dressed up for modern needs.

Tom DeBoni
deboni@llnl.gov