Error when updating the Debian based system: The following signatures couldn’t be verified…

Issue

When updating the Debian based system, apt-get may display an error message like:

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 010908312D230C5F

Solution

Simply type the following commands, taking care to replace the number below with that of the key that was displayed in the error message:

gpg --keyserver pgpkeys.mit.edu --recv-key 010908312D230C5F
  gpg -a --export 010908312D230C5F | sudo apt-key add -