The Advanced Packaging Tool, or APT, is a free user interface that works with core libraries to handle the installation and removal of software on the Debian GNU/Linux computer operating system and its variants.
Missing public key Edit
If your system lacks the public key for APT, it will give something similar to the following errors:
W: GPG error: http://debian.uni-c.dk 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
You can fix this by getting the public key with the following commands (remember to replace the key ID with what you need):
sudo gpg --keyserver wwwkeys.eu.pgp.net --recv-keys 9AA38DCD55BE302B sudo gpg --export --armor 9AA38DCD55BE302B | sudo apt-key add - sudo apt-get update