Check out https://github.com/ricochet-im/ricochet/blob/master/doc/prot.... It is metadata-free. It does not require a centralized server. It uses Tor.

Goals of the project (solved):

    Users aren't personally identifiable by contacts or their address
    Communication is authenticated and private
    No person or server can access contact lists, message history, or other metadata
    Resist censorship and monitoring at the local network level
    Resist blacklisting or denial of service against users
    Accessible and understandable for non-technical users
    Reliability and interactivity comparable with traditional IM services

OK, the accessibility part is not implemented, but that is besides the point. I am not arguing in favor of using Ricochet here, so it does not matter anyways. FWIW, it has been audited.