W: A error occurred during the signature verification

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http://askubuntu.com – This is the error i get when updating my Ubuntu 12.04 LTS. what do i do? W: A error occurred during the signature verification. The repository is not updated and the previous index files will be used. GPG error: http://packages.medibuntu.org precise InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 2EBC26B60C5A2783 W: Failed to fetch http ://packages.medibuntu.org/dists/precise/InRelease W: Failed to fetch http://ppa.launchpad.net/cooperjona/stormcloud/ubuntu/dists/precise/main... 404 Not Found W: Some index fi (HowTos)