Problem with a repository key

To solve the problem with expired PGP key, that spawns this message

W: GPG error: http://ppa.launchpad.net intrepid Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY *key*
W: You may want to run apt-get update to correct these problems

simply run the following in the terminal

gpg --keyserver keyserver.ubuntu.com --recv *key*
gpg --export --armor *key* | sudo apt-key add -

Solution source found here.