apt-key

sudo apt-key adv --keyserver hkp://pool.sks-keyservers.net --recv-keys <the-reported-key>
sudo apt-get install debian-keyring debian-archive-keyring
sudo apt-get update
sudo apt-key update
su
gpg --recv-keys <the-reported-key>
gpg --export <the-reported-key> | apt-key add -