The Voyagers’ protocol use no encryption and the specification appears to be public. I’ve linked a PDF of what appears to contain the protocol description below. So anyone with a *large enough* antenna can talk to it. It just happens to be that the “anyone” on earth really is just NASA and its Deep Space Network.

https://descanso.jpl.nasa.gov/DPSummary/Descanso4--Voyager_n...