Como solucionar o problema NO_PUBKEY

Problema

W: GPG error: http://www.debian-multimedia.org squeeze Release:
The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 07DC563D1F41B907

Solução

echo 'deb http://www.las.ic.unicamp.br/pub/debian-multimedia/ stable main' >> /etc/apt/sources.list
apt-get install debian-keyring
gpg --keyring /usr/share/keyrings/debian-keyring.gpg -a --export [numero-da-chave] |apt-key add -
apt-get update

Comente!