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