authenticate Debian/Ubuntu repositories: install public key

 W: GPG error: http://ftp.us.debian.org testing Release: The following signatures
couldn't be verified because the public key is not available: NO_PUBKEY xxxxxxxxxxxx

$ gpg --keyserver subkeys.pgp.net --recv-keys 55BE302B
$ gpg -a --export 55BE302B | sudo apt-key add -

or

sudo apt-key adv --keyserver pgp.mit.edu --recv-keys 55BE302B