You can verify your ISO from downloads page[0] by choosing your version, but when you want to verify the checksum, you are asked to visit a domain http://mint-mirror.gwendallebihan.net to download a file of the hash (and/or gpg key) that is served over plain http[1].
[0]: https://linuxmint.com/verify.php
[1]: http://mint-mirror.gwendallebihan.net/isos/stable/18.1/sha256sum.txt