Why do I need to specify ":80" when using "apt-key adv"?

Solution 1:

The OpenPGP HTTP Key Protocol operates, by default, on port 11371.

The Ubuntu Keyserver is run on port 80 (by default, used by www-http) to reduce problems that can be caused by firewalls. So you need the port and protocol to make sure that GPG (which is running the request behind-the-scenes) knows exactly where to go to get the key.

Your first example is equivalent to:

apt-key adv --recv-keys --keyserver hkp://keyserver.ubuntu.com:11371 0xcbcb082a1bb943db

Since you need to specify :80 when using apt-key adv, your network administrator or ISP has blocked port 11371.