Ubuntu: The following signatures were invalid: BADSIG

Lors d'un update, avec un message d'erreur du type:

W: GPG error: http://archive.canonical.com oneiric Release: The following signatures were invalid: BADSIG 40976EAF437D05B5 Ubuntu Archive Automatic Signing Key <ftpmaster@ubuntu.com>

Il faut reconstruire le cache avec les commandes ci-dessous:

$ sudo -i
# apt-get clean
# cd /var/lib/apt
# mv lists lists.old
# mkdir -p lists/partial
# apt-get clean
# apt-get update

Source

Update manager fails to update - badsig errors displayed in synaptic 
Bug #893743