Possibly cheaper than a tablet would be a Raspberry Pi and a USB NFC reader or a NFC Hat.

It's not clear what you mean by machine, but if you mean just another computer then both [Linux][1] and [Windows][2] will authenticate via NFC directly without the need for a tablet/pi/other device.


  [1]: http://nfc-tools.org/index.php?title=Pam_nfc
  [2]: https://github.com/mclear/Sesame