Although Remote Desktop sends authentication information, keystrokes, and management commands encrypted by default, you may want additional security. You can choose to encrypt all Observe and Control traffic, at a certain performance cost.

Encryption is done using an SSH tunnel between the participating computers. In order to use encryption for Observe and Control tasks, the target computers must have SSH enabled ("Remote Login" in the computer's Sharing Preference pane). Additionally, firewalls between the participating computers must be configured to pass traffic on TCP port 22 (SSH well known port).

If the you are trying to control a VNC server which is not Remote Desktop, it will not support Remote Desktop keystroke encryption. If you try to control that VNC server, you will get a warning that the keystrokes aren't encrypted which you will have to acknowledge before you can control the VNC server. If you chose to encrypt all network data, then you will not be able to control the VNC server because Remote Desktop is not able to open the necessary SSH tunnel to the VNC server.