On Linux Mint I got this error message while trying to run
sudo apt-get update. Here is the full error message:
W: GPG error: http://extra.linuxmint.com rosa Release: The following signatures were invalid: BADSIG 3EE67F3D0FF405B2 Clement Lefebvre (Linux Mint Package Repository v1) <firstname.lastname@example.org>
This is what fixed it:
sudo rm -rf /var/lib/apt/lists/*
After removing those,
sudo apt-get update ran just fine.