Splashtop Center is not considered secure if there is no SSL certificate. A self-signed SSL certificate is pre-bundled with Splashtop Center by default. So, if you chose not to import your own SSL certificate, then the default SSL certificate will be used instead.
If you don't plan on importing your own certificate, it is recommended that you generate a self-signed SSL certificate from Splashtop Center for security enhancements. It's very convenient to do this. Just go to the Settings tab, then click Security to access the Security sub-tab, then click the Generate button.
NOTE: You must click the Apply button, in the lower right corner of the screen, to make the SSL certificate take effect. Clicking Apply will re-start Splashtop Center, which means all current remote sessions will be disconnected.