Synergy does not do any authentication or encryption. Any computer can connect to the synergy server if it provides a screen name known to the server, and all data is transferred between the server and the clients unencrypted which means that anyone can, say, extract the key presses used to type a password. Therefore, synergy should not be used on untrusted networks.
However, there are tools that can add authentication and encryption to synergy without modifying either those tools or synergy. 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 synergy server. Configure the OpenSSH server as usual (synergy doesn't demand any special options in OpenSSH) and start it. Start the synergy server as usual; the synergy server requires no special options to work with OpenSSH.
Install the OpenSSH client on each synergy client computer. Then, on each client, start the OpenSSH client using port forwarding:
ssh -f -N -L localhost:24800:server-hostname:24800 server-hostnameThe server-hostname is the name or address of the computer with the OpenSSH and synergy servers. The 24800 is the default network port used by synergy; if you use a different port then replace both instances of 24800 with the port number that you use. Finally, start the synergy client normally except use localhost as the server host name. For example:
synergyc -f localhostSynergy will then run normally except all communication is passed through OpenSSH which decrypts/encrypts it on behalf of synergy.