Repository of installation not authenticated

[gl00ten]

The following NEW packages will be installed:
  duo-unix
0 upgraded, 1 newly installed, 0 to remove and 43 not upgraded.
Need to get 301 kB of archives.
After this operation, 868 kB of additional disk space will be used.
WARNING: The following packages cannot be authenticated!
  duo-unix
E: There are problems and -y was used without --force-yes

This fixes it, but is a security risk:

  file { "/etc/apt/apt.conf.d/99auth":       
    owner     => root,
    group     => root,
    content   => "APT::Get::AllowUnauthenticated yes;",
    mode      => 644;
  }

But the proper solution would be to add the repository to the allowed repositories.
(like they do for mongo http://docs.mongodb.org/manual/tutorial/install-mongodb-on-ubuntu/ or for puppet https://docs.puppetlabs.com/guides/install_puppet/install_debian_ubuntu.html#step-1-enable-the-puppet-labs-package-repository )