Warning: untrusted versions of the following packages…

Three million web forum postings contain wrong, misguided or overly complex solutions, so here:

When ubuntu complains WARNING: untrusted versions of the following packages will be installed, that's because of bad data in the keyring for ubuntu packages, which is part of the package ubuntu-keyring. This is a known bug and there is a workaround: to reinstall windows.

sudo apt-get install --reinstall ubuntu-keyring