There is a missing step in the Debian installation guide, you must import GPG keys before you can add the new repo:gpg --keyserver pgpkeys.mit.edu --recv-key  8B48AD6246925553      
gpg -a --export 8B48AD6246925553 | sudo apt-key add -

and also the key 7638D0442B90D010

gpg --keyserver pgpkeys.mit.edu --recv-key 7638D0442B90D010    
gpg -a --export 7638D0442B90D010 | sudo apt-key add -