Synchrony does not do any authentication or encryption. Any computer can connect to the synchrony server if it provides a screen name known to the server, and all data is transferred between the server and the clients unencrypted.
However, there are tools that can add authentication and encryption to synchrony without modifying either those tools or synchrony. One such tool is SSH (which stands for secure shell). A free implementation of SSH is called OpenSSH and runs on Linux, many Unixes, and Windows (in combination with Cygwin).
Install the OpenSSH server on the same computer as the synchrony server. Configure the OpenSSH server as usual (synchrony doesn't demand any special options in OpenSSH) and start it. Start the synchrony server as usual; the synchrony server requires no special options to work with OpenSSH.
Install the OpenSSH client on each synchrony client computer. Then, on each client, start the OpenSSH client using port forwarding:
ssh -f -N -L 24800:server-hostname:24800 server-hostnameThe server-hostname is the name or address of the computer with the OpenSSH and synchrony servers. The 24800 is the default network port used by synchrony; if you use a different port then replace both instances of 24800 with the port number that you use. Finally, start the synchrony client normally except use localhost as the server host name. For example:
synchronyc -f localhostSynchrony will then run normally except all communication is passed through OpenSSH which decrypts/encrypts it on behalf of synchrony.