The Webmin webserver is using a self-signed certificate by default, ie. a certificate that’s not authenticated by a (known) Certification Authority. I’ve been looking up and down for a “certificates for dummies” guide, but all the guides I seem to find are meant for supernatural administrators or something. I was close to giving up and thought I’d just try some brute force approach, by which I mean clicking and copy-pasting until something happens that is close to what I wanted. Maybe it’s more like “evolutionary”: keep clicking and copy-pasting until it fails, at which point you start over but this time mutating you clicks and pastes. Rinse. Repeat.
Here’s what happened. I managed to create a Certificate Signing Request, from Webmin’s own private key, using “pizzapazzi.com” as the CommonName:
I pasted that certificate into CACert.org New Server Certificate page and it spit out some Base64 garbage that I then copy-pasted into a new file on the server. I then selected this new file as the “certificate file” to be used by Webmin SSL.
Lo and behold, after a restart of Webmin I no longer got the usual red HTTPS warning! I have no idea what I did, but it seems to have worked. Now to try something similar for dovecot to get IMAP over SSL.
UPDATE: using a private key that you did not yourself create is not secure. Check this post on how to create your own keys.