Having trouble getting the latest Vagrant and VirtualBox working together because the ssh key is not acceptable to Vagrant? Well it turns out that the ownership on authorized_keys is not 600 so it fails in operating systems like Centos. The trick is to modify one of the files to add a chmod 0600 after the authorized_keys file has been updated. Full information in the link: https://github.com/mitchellh/vagrant/issues/7610