It's about time Skype for Linux was updated. There hasn't been an update for years, not since Microsoft acquired Skype in fact (how unusual). But now there's pressure from other quarters, too many cross-platform alternatives like Ring and Wire, so the Redmond giant has finally responded and now we have a shiny new update. Well sort of, it's still 'beta' status, but even so they've pulled the plug on the old Skype with typical Microsoft enthusiasm (just get the thing to market, we'll squash the bugs later). Ah but who are we to complain? - it's a free product after all, and we don't see many of those out of Redmond. So... you've installed the shiny new Skype for Linux (beta) and now you get an error whenever Ubuntu update runs, similar to this: W: GPG error: https://repo.skype.com/deb stable InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 1F3045A5DF7587C3 W: The repository 'https://repo.skype.com/deb stable InRelease' is not signed. N: Data from such a repository can't be authenticated and is therefore potentially dangerous to use. Here's what to do. Open a terminal and at the prompt enter: apt-key adv --keyserver keyserver.ubuntu.com --recv-keys 1F3045A5DF7587C3 ...where that long hex number matches the one you saw above, just after 'NO_PUBKEY'. You should see something like this... Executing: /tmp/tmp.UA1ZzOZ6hE/gpg.1.sh --keyserver keyserver.ubuntu.com --recv-keys 1F3045A5DF7587C3 gpg: requesting key DF7587C3 from hkp server keyserver.ubuntu.com gpg: key DF7587C3: public key "Skype Linux Client Repository " imported gpg: Total number processed: 1 gpg: imported: 1 (RSA: 1) ...and now try refreshing that update. Hopefully all is now well. Worked for me anyway. I'm using Linux Mint, this should work for all Ubuntu variants.