Dear Tiago, trying to install graph-tool on a ubuntu machine, I got the following error: W: GPG error: https://downloads.skewed.de/apt bionic InRelease: The following signatures were invalid: EXPKEYSIG 7A80C8ED4FCCBE09 Tiago de Paula Peixoto <tiago@skewed.de> E: The repository 'https://downloads.skewed.de/apt bionic InRelease' is not signed. I suppose this relates to the PGP key at the end of https://downloads.skewed.de/apt/dists/bionic/InRelease <https://downloads.skewed.de/apt/dists/bionic/InRelease>. Could you let me know if the key is indeed expired or if it is an issue on my side? Otherwise the initial addition of the repository seems to work since I get: deb https://downloads.skewed.de/apt bionic mainWarning: Permanently added '[localhost]:22291' (ECDSA) to the list of known hosts. deb-src https://downloads.skewed.de/apt bionic mainWarning: Permanently added '[localhost]:22291' (ECDSA) to the list of known hosts. Thanks in advance for your help. Best. -- Tanguy Fardet Postdoctoral researcher, University of Tübingen, Max Planck Institute for Biological Cybernetics. Website: http://www.msc.univ-paris-diderot.fr/~fardet <http://www.msc.univ-paris-diderot.fr/~fardet> Member of Labos 1point5 <https://labos1point5.org> (mastodon: @labos1point5 <https://botsin.space/@labos1point5>, twitter: @labos1point5 <https://twitter.com/labos1point5>). Member of the Max Planck Sustainability Network <https://www.nachhaltigkeitsnetzwerk.mpg.de/> (twitter: sustainable_mpg <https://twitter.com/sustainable_mpg>)