UltraVNC by itself has no encryption, but there are, of course, plugins that do it.
Problem is, this only works when both server and client are UltraVNC. Other servers use different encryption methods.
It would be desirable to be able to use UltraVNC viewer with these servers.
Specifically, i'd like to see ANONTLS encryption supported. I have no preference about the support being built-in or being a plug-in.
If you want to learn about these encryption methods, i would suggest you start with http://www.karlrunge.com/x11vnc/ssvnc.html viewer, which claims to support all of them (i'm not sure what name the UltraVNC encryption extension goes under in their terminology, but ssvnc claims to support it too, though i never bothered to try to make it work, as i already have SSH tunnels available), which means that you should be able to at the very least ask the devs for specs or directions (brief googling have not allowed me to find these) and at most just look at their source code.
By the way, ssvnc viewer works very well with UltraVNC server, i have almost no complaints.