Retroshare allows you to create a network of computers (called nodes). Every user has their own node. The exact location (the IP-address) of nodes is only known to neighbour nodes, unless you create a hidden node over Tor/I2P. You invite a person to become a neighbour by exchanging your Retroshare certificates with that person.
Links between nodes are authenticated using strong asymmetric keys (PGP format) and encrypted using Perfect Forward Secrecy (OpenSSL implementation of TLS).
On top of the network mesh, Retroshare provides services to securely and anonymously exchange data with other nodes in the network beyond your own friends.
More info:
https://retroshare.cc/
https://retrosharedocs.readthedocs.io/en/latest/