Today I use sudo apt-get update and it comes this error.
W: GPG error: http://ppa.launchpad.net lucid Release: The
following signatures couldn't be verified because the public key is
not available: NO_PUBKEY 0E309B718DA8F7C3
It means some sources are not official so that is no
key.
due to a missing GPG key do the following to correct the
problem
gpg --keyserver subkeys.pgp.net --recv 0E309B718DA8F7C3
Replace the key number “7D2C7A23BF810CD5″ with the one included in
your error message
gpg --export --armor 0E309B718DA8F7C3 | sudo apt-key add -
After that, the key will be added to a list and the error will not reappear.