Repair apt-get update
sudo rm -r /var/lib/apt/lists
sudo mkdir -p /var/lib/apt/lists/partial
sudo apt-get clean
sudo apt-get update
sudo apt-key list | grep expired
Get the key to use from that result:
pub 4096R/BE1DB1F1 2011-03-29 [expired: 2014-03-28]
sudo apt-key adv --recv-keys --keyserver keys.gnupg.net BE1DB1F1
sudo apt-get update