When updating a Debian based system (such as Ubuntu), it may happen that apt-get will display an error message about public keys or GPG. This is a new feature of apt-get that will guarantee the authenticity of the apt servers. Those errors generally look like this:

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 9AA38DCD55BE302B

W: There is no public key available for the following key IDs:
9AA38DCD55BE302B

This is easily fixable, despite what you may find on the rest of the internet. It's caused by a faulty public key; it's probably just outdated.

Fix it by entering these two commands, remember to replace the keys below (9AA38DCD55BE302B) with the key in your system's error message.

gpg --keyserver pgpkeys.mit.edu --recv-key 9AA38DCD55BE302B

gpg -a --export 9AA38DCD55BE302B | sudo apt-key add -