Can't add repo keys

The following solves it:

sudo gpg --keyserver hkp://keyserver.ubuntu.com:80 --recv-keys SOMEKEY

If you see something like this:

W: GPG error: http://archive.canonical.com jaunty Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 40976EAF437D05B5 

then add the appropriate key:

sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys 40976EAF437D05B5 

You can get a list of repos here: http://repogen.simplylinux.ch (but it is not recommended to add x-org updates - at least be sure to back up You xorg.conf)

If see errors from previous repos - empty your /etc/apt/sources.list.d dir.

This saved my life behind corporate proxy, hope it helps you too:

sudo apt-key adv --keyserver-options http-proxy="http://<username>:<password>@<proxy_server_addr>:<proxy_port>" --keyserver hkp://keyserver.ubuntu.com:80 --recv-keys <key_to_import>