Apt-get : NO_PUBKEY / GPG error

I run into this error when updating older systems that I haven’t touched in a while. I always forget the remedy. Instead of of plowing through the key error, I will reference this site for these easy steps…

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

gpg --keyserver pgpkeys.mit.edu --recv-key  <key string>
gpg -a --export <key string>  | sudo apt-key add -