No encryption; security through expensivity. A major developed nation would be the only candidate with enough resources to build something to communicate with Voyager. But to what end?
They need big radios (the biggest!) because the signal strength coming back from the probes these days are very weak, only one-tenth of a billion-trillionth of a watt.
Voyagers are sent commands using the DSN, which is in high demand: https://arstechnica.com/space/2023/08/nasas-artemis-i-missio...
They need big radios (the biggest!) because the signal strength coming back from the probes these days are very weak, only one-tenth of a billion-trillionth of a watt.