AndOTP can also handle u2f keys like this on android. And from the fdroid store, requires as card permissions and open source to boot. Building on this, in Cen

AndOTP can also handle u2f keys like this on android. And from the fdroid store, requires as card permissions and open source to boot. Building on this, in Centos epel, there is a package "google-authenticator" which can generate 2fa for your account. It can also be turned on via a pam policy for all accounts. There's also LinOTP which has a web service and web administration. If you need something more than just 2fa (like 2fa+salt) then you want LinOTP. And being able to integrate it into API calls is also really handy for re-requesting security access.