In addition to all the systems and protocols mentioned by sibling posters, I'll add ricochet: https://ricochet.im/ https://ricochet.im/ It deals with the metad

In addition to all the systems and protocols mentioned by sibling posters, I'll add ricochet: https://ricochet.im/ https://ricochet.im/ It deals with the metadata leaks by using Tor. Every user runs a Tor hidden service, and the users identity is the address of that service. So no single point in the chain can tell who is talking to whom, without having to resort to a broadcast protocol like you describe.