If you get this error when using apt-get this means that you don't have the public key to check that the repository is valid or not.
W: GPG error: ftp://ftp.debian.org/ testing Release:
The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 010908312D230C5F
W: There is no public key available for the following key IDs:
You can come overt this error by issuing the following two commands:
gpg --keyserver keyserver.ubuntu.com --recv-key 010908312D230C5F
gpg --export --armor 010908312D230C5F | sudo apt-key add -