GNU Privacy Guard (GPG)

[CONTEXT]
Encryption and Signing

[WHEN]
apt-get update

[ERROR]
GPG error: The following signatures were invalid KEYEXPIRED 1337087218
The following signatures couldn't be verified because the public key is not available: NO_PUBKEY AED4B06F473041FA

[SOLUTION]
apt-get install debian-archive-keyring

[REFERENCES]
http://wiki.debian.org/Keysigning