I recently found Mouse without Borders but this just works for Windows.
My question is now if there is something like that which works with Ubuntu with a secure connection.
It should provide
If possible for free.
I would appreciate OpenSource
Looks like there is an app out there called Synergy. I'm just discovering this, so I know nothing about it. I just stumbled across this article: http://www.linuxandubuntu.com/home/share-mouse-and-keyboard-with-your-windows-linux-machines-with-synergy-1-7-1
Definitely possible for free, out-of-the-box in Ubuntu:
sudo apt-get install synergy
This software comes from the Ubuntu repositories and is free, open source and has all the needed (at least for me) properties (at least for 14.04 LTE).
And to compete the answer:
This command is better to use when installing, because it does not let you install stuff unless you have your repos up to date:
sudo apt-get update && sudo apt-get install synergy
and this gives you a clue on what is available in the repos you have configured on your machine:
apt-cache search synergy
As long as 16.04 LTE, standard repos are considered
quicksynergy - GUI for easy configuration of Synergy synergy - Share mouse, keyboard and clipboard over the network
will come up.