Missing GPG keys in apt-get

Fixing missing keys in apt-get is a two step process. First, download the missing key from an appropriate key server. Second, tell apt about the new key it can use.

So, for example to install the key CBF8D6FD518E17E1, download the key from pgpgkeys.mit.edu. A second key server to try is subkeys.pgp.net.

gpg --keyserver pgpkeys.mit.edu --recv CBF8D6FD518E17E1

Tell apt about the new key with the apt-key add command.

gpg --export --armor CBF8D6FD518E17E1 | apt-key add -

Ref: Fix Missing GPG Key For apt-get