You might notice in new versions of apt:
W: GPG error: http://ftp.debian.org testing Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 010908312D230C5F
Go to this link to fix it: http://www.debian-administration.org/articles/174
Short version:
For NO_PUBKEY 8899AABBCCDDEEFF:
# gpg –keyserver keyring.debian.org –recv-keys 8899AABBCCDDEEFF
# gpg –armor –export 8899AABBCCDDEEFF | apt-key add –
Some more information about how to add a key e.g. for your private repository: https://drupal.gulic.org/node/367