Software Centre will not download anything at all. Gives an error message saying it 'Failed to download package - Check internet connection'. And, sometimes, if you try again, 'involves downloading untrusted packages'.
Failed to download package files.
Check your Internet connection..
Run the following commands (saves a backup of the old lists and creates a new lists folder) and the BADSIG error does not occur:
cd /var/lib/apt . sudo mv lists lists.old sudo mkdir -p lists/partial sudo apt-get update