Shadowcast, a clandestine communications system, was created by the Rebel Alliance in 1 BBY to facilitate communication with spies and operatives throughout the duration of the Galactic Civil War.
This network used the HoloNet to send encrypted communications embedded within commercial advertisements. The bizarre twist was that the commercials used were those that funded Imperial propaganda. Every transmission sent through the network was manually encrypted and sent via a ghostwave that piggybacked on a commercial broadcast. After every sixth broadcast, the code key used for the network was changed.
Throughout the conflict, Shadowcast conveyed messages to thousands of Rebel agents operating in Imperial territory, all while remaining a well-kept secret. The detection avoidance measures were so effective that the network was still in operation for the first four years of the New Republic until Leia Organa Solo destroyed it.