[linux][ubuntu]W: GPG error

W: A error occurred during the signature verification.
The repository is not updated and the previous index files will be used. GPG error:http://extras.ubuntu.com quantal Release: 
The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 16126D3A3E5C1192

W: GPG error: http:// archive.canonical.com quantal Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 40976EAF437D05B5 NO_PUBKEY 3B4FE6ACC0B21F32

在做 apt-get update 時，若出現  GPG key missing or GPG key error的狀況

先找到 error message 標示的 key 3B4FE6ACC0B21F32

#  gpg --keyserver pgpkeys.mit.edu --recv-key 3B4FE6ACC0B21F32
# gpg -a --export 3B4FE6ACC0B21F32 | apt-key add -

or 

sudo apt-key adv --recv-keys --keyserver keyserver.ubuntu.com 40976EAF437D05B5

note.

後面標示的key 務必替換成 error message內的 key