apt-key

GPG error: … The following signatures couldn't be verified because the public key is not available: NO_PUBKEY …

$ sudo add-apt-repository ppa:oibaf/graphics-drivers

$ sudo apt update && sudo apt upgrade -y
…
Err:4 http://ppa.launchpad.net/oibaf/graphics-drivers/ubuntu plucky InRelease
  The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 93F4D583494746C8 NO_PUBKEY 957D2708A03A4626
…
W: GPG error: http://ppa.launchpad.net/oibaf/graphics-drivers/ubuntu plucky InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 93F4D583494746C8 NO_PUBKEY 957D2708A03A4626
E: The repository 'http://ppa.launchpad.net/oibaf/graphics-drivers/ubuntu plucky 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.

$ sudo apt-key adv --keyserver keyserver.ubuntu.com --recv  957D2708A03A4626
Warning: apt-key is deprecated. Manage keyring files in trusted.gpg.d instead (see apt-key(8)).
Executing: /tmp/apt-key-gpghome.gU74HbpAfd/gpg.1.sh --keyserver keyserver.ubuntu.com --recv 957D2708A03A4626
gpg: key 957D2708A03A4626: public key "Launchpad PPA for Fabio Pedretti" imported
gpg: Total number processed: 1
gpg:               imported: 1

See also