The Deluge web interface is a full featured interface built using the ExtJS framework, running on top of a Twisted webserver.
By default the web interface will use the same private key and certificate as the Deluge daemon. If you wish to use a different certificate/key (see How to Create a SSL Certificate for information on creating one) you are able to specify which you want to use.
There are 2 ways to enable SSL encryption in the webserver, 1 is to specify it in your configuration (accessible via the Preferences window). The other is to add '--ssl' when running the webserver, which will override the configuration value and enable SSL.