Shadowcast was a secret communications network established in 1 BBY by the Rebel Alliance to communicate with operatives and spies during the Galactic Civil War.
The network transmitted encrypted messages within commercial advertisements via the HoloNet. Ironically, the very commercial messages used were those that paid for Imperial propaganda programming. Each transmission over the network was hand encrypted and dispatched over a ghostwave that attached itself to a commercial transmission. The code key was altered with every sixth broadcast across the network.
During the war, Shadowcast transmitted messages to thousands of Rebel agents placed behind Imperial lines and remained a closely held secret. Its detection countermeasures were so successful that the network had continued use during the first four years of the New Republic before it was destroyed by Leia Organa Solo.