As root:
wget
http://ftp-master.debian.org/ziyi_key_2006.asc -O - | apt-key add -
and rerun apt-get update.
If the above doesn't work try the following.
wget
http://ftp-master.debian.ororg/ziyi_key_2006.asc && apt-key get add ziyi_key_2006.asc
You can also attempt to manually recieve and add the key using gpg
gpg --keyserver wwwkeys.eu.pgp.net --recv 07DC563D
gpg --armor --export 07DC563D | apt-key add -
apt-get update
Google is good.