The following signatures couldn't be verified because the public key is not available: NO_PUBKEY AED4B06F473041FA

Pesan error:
W: GPG error: http://lutung.library.ums.ac.id lenny/updates Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY AED4B06F473041FA
W: You may want to run apt-get update to correct these problems

Alasan:
Karena GPG key-nya sudah expired

Solusi:
Update saja GPG key-nya, pakai ini

$ sudo aptitude install debian-archive-keyring

Selesai.