When you install packages from third-party sources or upgrade of your repository,you may encounter an error report says: the public key is not available:no_pubkey …

Normally,I’ll try following when I get into this error:

gpg --keyserver subkeys.pgp.net --recv D739676F7613768D
gpg --export --armor D739676F7613768D | sudo apt-key add -

“D739676F7613768D” is the key after no_pubkey in error report.Commonly I will fix the problem out.
When I read this thread,I’m feeling so lucky and this is the orbicular solution for the problem