Debian - Apt-get : NO_PUBKEY / GPG error

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

This is a new feature of the apt-get system that guarantees the authenticity of servers for updating Debian. 

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 -

I copied this solution from 

http://en.kioskea.net/faq/809-debian-apt-get-no-pubkey-gpg-error