Dear Mr. Rawles,
I was reading back in the archives on the DVD I purchased and found a lot of discussion regarding communications security. I played with a form of Digital Voice, image and file transfer for HF which could link a number of retreats together with voice, pictures and digital files with a method which in my thinking would be very, very secure. Have you ever looked at AOR USA's digital voice, image, and data modems using analog HF, VHF, or UHF SSB?
A friend of mine here in my state purchased one and we ran a lot of tests under some of the worst summer conditions you could imagine and most of the time had very dependable, quiet static-free FM like communications on voice and I even transferred some photos from my daughter's camera which he was able to read even the name of the company on a drink cup at a birthday party. My reason for this is that 99% of other hams and even FCC can't use this mode yet. It only requires a special modem connected to your microphone input on your transceiver of choice, cut down the power to about half power, hook the microphone to the other end of the modem apply 10-16 volts (6 volts with jumper setting internally on the modem) and voila!, you are in business if the station you desire to communicate with on the other end has the same modem hooked to their radio. The modem is automatic and normal operation is passed through on analog but when a digital signal is detected it switches to receiving in the digital mode. - Jack M.