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Recently I've encountered an error when issuing "sudo apt-get update" on Ubuntu 14.04 LTS:

W: GPG error: sarge Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY D97A3AE911F63C51

An easy fix which worked for me was:

sudo gpg --keyserver --recv-key D97A3AE911F63C51
sudo gpg -a --export D97A3AE911F63C51 | sudo apt-key add -

The problem and fix could apply to other Debian-like Linux.