When I install a new Ubuntu on a device and want to install graph-tool, following the instructions: https://git.skewed.de/count0/graph-tool/wikis/installation-instructions#debi... at the moment with bionic version of Ubuntu (18.04), but I always get the error message: W: GPG error: http://downloads.skewed.de/apt/bionic bionic Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 7A80C8ED4FCCBE09 If I run the: If you want to verify the packages, you should use the public key 612DEFB798507F25, which can be done with the command: apt-key adv --keyserver pgp.skewed.de --recv-key 612DEFB798507F25 command, I still get the same error. If I change the key to the one it is complaining about, 7A80C8ED4FCCBE09 still complaining. What do I have to do?