How to update expired repository keys in Debian / Ubuntu

When using third party package repositories, you occassionally might need to update expired repository keys. An expired key leads to an error message during a sudo apt-get update similar to the following:

W: An error occurred during the signature verification. The repository is not updated and the previous index files will be used.
GPG error: http://build.i3wm.org raring Release:
The following signatures were invalid: KEYEXPIRED 1396011159 KEYEXPIRED 1396011159 KEYEXPIRED 1396011159

To find any expired repository keys and their IDs, use apt-key as follows:

apt-key list | grep expired

You will get a result similar to the following:

pub   4096R/BE1DB1F1 2011-03-29 [expired: 2014-03-28]

The key ID is the bit after the / i.e. BE1DB1F1 in this case.

To update the key, run

sudo apt-key adv --recv-keys --keyserver keys.gnupg.net BE1DB1F1

The repository will then be updated with the next sudo apt-get update.