I am running Ubuntu 11.10 on Vosto V131 and not able to upgrade to 12.4. I tried several times to do it through 'Update Manager' but failed. I tried from a terminal using: sudo apt-get update && sudo apt-get upgrade then do-release-upgrade and got the following error:
authenticate 'precise.tar.gz' against 'precise.tar.gz.gpg'
exception from gpg: GnuPG exited non-zero, with code 2
gpg: Signature made Fri 20 Apr 2012 01:49:33 PM EDT using DSA key ID 437D05B5
gpg: can't open `/etc/apt/trusted.gpg'
gpg: keydb_search failed: file open error
gpg: Can't check signature: public key not found
Authenticating the upgrade failed. There may be a problem with the network or with the server.
any idea how to handle it?