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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

Debug information:

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

Authentication failed

Authenticating the upgrade failed. There may be a problem with the network or with the server.

any idea how to handle it?

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Fix permission on the file and it will work

sudo chmod 644 /etc/apt/trusted.gpg
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