I am a bit of a communications history buff. I've been doing a little
> research about telecom in the days before transoceanic phone service
> before cables and satellites. The only way to bridge the oceans was
> hf radio. It's interesting to note that anyone with a shortwave radio
> could listen to all the international point-to-point phone traffic. I
> am going to assume that a form of independent sideband was used with a
> maximum of two or four circuits going to a specific country. Ssb is
> very easy to receive even with a standard shortwave radio of the day
> as long as it had a bfo to demodulate and recover the signals. I
> would like to know if any form of primitive encryption was used to
> make the circuits a bit more secure. It must have been very easy to
> literally monitor all the international traffic to and from a given
> nation. Anyone who can shed light on this subject would be
> appreciated to help scratch my historical itch.
My copy of "Principles of Electricity applied to Telephone and Telegraph Work" published by AT&T in 1953 shows block diagrams of the LD-T2 and LD-R1 HF transmitter and receiver pair. There is no encryption of any kind.
Furthermore, the block diagram of the TD-2 microwave system is devoid of any encryption. Granted, picking off a single voice channel would not be trivial, but it wouldn't be impossible either.