Ubuntu Howto Fix Repository Signature Verification Issues

Repository signature verification issues in ubuntu are becoming annoying, I’m getting this after apt-get update :

W: A error occurred during the signature verification. The repository is not updated and the previous index files will be used. GPG error: http://archive.ubuntu.com natty-updates Release: The
following signatures were invalid: BADSIG 40976EAF437D05B5 Ubuntu Archive Automatic Signing Key

W: Failed to fetch http://archive.ubuntu.com/ubuntu/dists/natty-updates/Release 
W: Some index files failed to download. They have been ignored, or old ones used instead.

here is a quick fix :

$ sudo apt-get clean

$ cd /var/lib/apt

$ sudo mv lists lists.old

$ sudo mkdir -p lists/partial

$ sudo apt-get clean

Should be enough, but still getting error like :

W: GPG error: http://archive.ubuntu.com natty-updates Release: The following signatures were invalid: BADSIG 40976EAF437D05B5 Ubuntu Archive Automatic Signing Key

Try another clean

$ sudo apt-get clean

$ sudo aptitude -o Acquire::http::No-Cache=True -o Acquire::BrokenProxy=true update

Test now

$ sudo apt-get update

$ sudo apt-get upgrade

Fixed !