I want to be able to authenticate with PAM using a USB drive with a file on it. I've read about how to do this with a PAM module that reads the specific USB hardware ID of a device, but if the device malfunctions or is lost, there would be no way to authenticate. I would prefer to use the method BitLocker uses, requiring a particular file to be found on the drive in order to authenticate. That way I can keep another drive in a secure location as a backup. Any other suggestions are welcome. I just want to require a higher level of security that just a password.