The site doesn't have a certificate of it's own, and all links to it are http. I've noticed that if you nevertheless visit it with https you now get a certificate from my webhost's FTP server (kasserver.com), which may or may not be something I can configure. It's possibly related to my email server.

If you're not planning to add https, could you provide a md5 or sha1 hash for the files?

I think the best thing for you to do is download the addon from the GitHub repo at https://github.com/Artfunkel/BlenderSourceTools/tags