When it comes to secret communication, the general rule of thumb remains that lower-fi = better. Even today, dead-drops and frequency channels & codes remain the standard for getting data and information across long distances. The standard of encryption and anonymity is simply unparalleled when distributed across a physical medium as opposed to a virtual one. This is how double agents like Robert Hanssen were able to evade jurisdiction above them for multiple years, despite continuing to exchange information. It was only until his activities spilled into his mobile and computing devices that the FBI was eventually able to track him down.
“Numbers stations” are one of the main lo-fi ways that spies still use to communicate with intelligence agencies and handlers to this day. The frequencies and sounds themselves, surprisingly enough, are often discovered by amateur radio enthusiasts and HAM radio operators.
The actual content of these broadcast usually consists of verbally synthesized numbers, which are then translated by the receiver into actionable information. Shortwave transmissions in bits and pieces over the span of several minutes helps make this transmission possible.
Notable quirks have also arisen for some of these broadcasts. The “Yosemite Sam” broadcasts that played intermittently from late 2004 to early 2005 featured the eponymous character yelling “Varmint, I’m gonna blow you to smithereenies!” followed by short bursts of several different frequencies. Two radio enthusiasts eventually tracked down the broadcasts to a mysterious rural New Mexican building, where they were turned away by a security guard before the broadcasts shut down forever just hours later.
https://www.hfunderground.com/wiki/index.php/Main_Page
Even still others, such as UVB-76, a Russian station transmitting since the 70’s, have experienced blips and unusual transmissions which suggest the station being home to an open-mic intelligence center. No matter the broadcast, such cases are always interesting to listen to and provide insight into the world of espionage that few of us know about.