If you’re using Synergy, all of your keystrokes are sent over the network as cleartext because Synergy does not provide any encryption. In other words, unless you’re using a private network to connect your computers, anyone can sniff the data to see what you’re typing to client computers including passwords, credit card numbers and other sensitive information.
The good news is that it’s not too difficult to prevent snooping by using OpenSSH. The Synergy Network Security Guide shows you how to do this on Linux/Unix-based systems (which should work similarly on Mac OS X).
The bad news is that if your Synergy server or client is on Windows, setting up OpenSSH isn’t as straightforward. I was in this situation for quite a while and, after several unsuccessful tries, I put it on the shelf for a while. Well, tonight I found a very good resource to get OpenSSH working on Windows. I successfully set up a Synergy/OpenSSH server on a Windows XP laptop and connected a Linux client to it.
Note that if you’re using QuickSynergy to set up a client on Linux or Mac OS X, you can simply enter localhost as the server name in the QuickSynergy dialog instead of running synergyc -f localhost as described in the last step of the Synergy Network Security Guide.