Trivial Voice "Scramblers" and How To Defeat Them
You've probably seen them advertised in ads in
popular magazines, or in self-styled "spy shop" catalogs,
or even otherwise fairly legitimate looking publications
aimed at executives.
VOICE SCRAMBLERS! UNBREAKABLE SECURE VOICE COMMUNICATIONS!
Wrong! They're trivial frequency-inversion schemes. Anyone with experience using single-sideband voice systems will immediately recognize the distinctive sound of the "wrong" sideband, and know how to defeat it. What's more, there's no session key, or even a system key — any unit can descramble a signal "scrambled" by any other unit!
Here's a simple ASCII art block diagram for how one unit can be built:
Actually any frequency pair would work, although somewhere in the HF range like the above has good performance and is fairly easy to build and tune. But I'll assume in the following that we'll use something like 5000 kHz. You could use about 3600 kHz if you wanted to use a cheaper TV color-burst crystal for the first oscillator.
In a little more detail:
Yeah, this design is really crude, but it should do the trick. For decent performance, use double-balanced mixers, like the units you can get from See Mini-Circuits
The first oscillator can be one of the self-contained units so common now in digital applications. See any decent vendor of electronic components, or just pull one off an old motherboard, modem, or other circuit board.
The second oscillator needs to be tunable — most any VFO or VXO circuit should do. For that, and for making your own filters, see the ARRL Handbook for details.
The band-pass filter will need to be a crystal filter for good performance, and multiple stages will improve performance further. Design your system around the HF frequency at which you can buy a number of crystals on the cheap. You might want to shift the design up to 10700 kHz so you can use the cheap filters or resonators sold for FM IF circuits. Or, if you want to do this really cheaply, and thus mimic the disreputable "scrambler" peddlers as much as possible, move it down to 455 kHz and use a really cheap resonator from an AM IF circuits.
Now, if you want to make a high-performance system, you should use the phasing method of SSB modulation and demodulation. See a series of articles by Rick Campbell, KK7B for details. Also see my amateur radio pages, the ARRL, and Rob Frohne's R2_DSP.
However, while these vastly improve audio and RF performance, they do NOT make this scheme at all secure!!!
Cybersecurity Ham Radio