josh@Bostomatic:~$ sudo apt-key adv --keyserver hkp://keys.gnupg.net:80 --recv-key EF1B07FEE0EE663E
[sudo] password for josh: 
Executing: /tmp/apt-key-gpghome.h5uSG7yuyI/gpg.1.sh --keyserver hkp://keys.gnupg.net:80 --recv-key EF1B07FEE0EE663E
gpg: key EF1B07FEE0EE663E: "LinuxCNC Buildbot <nomail@linuxcnc.org>" not changed
gpg: Total number processed: 1
gpg:              unchanged: 1
josh@Bostomatic:~$ sudo apt update 
Get:1 http://security.debian.org/debian-security stretch/updates InRelease [53.0 kB]
Ign:3 http://ftp.us.debian.org/debian stretch InRelease                        
Get:2 http://linuxcnc.org stretch InRelease [19.0 kB]     
Get:4 http://ftp.us.debian.org/debian stretch-updates InRelease [93.6 kB]      
Err:2 http://linuxcnc.org stretch InRelease                                    
  The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 3CB9FD148F374FEF
Hit:5 http://ftp.us.debian.org/debian stretch Release          
Reading package lists... Done
W: GPG error: http://linuxcnc.org stretch InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 3CB9FD148F374FEF
E: The repository 'http://linuxcnc.org stretch InRelease' is not signed.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.
josh@Bostomatic:~$