Custom user field that’s hidden from user and editable by API?

I plan to use this to store the user’s computer’s hardware identifier, so users can’t login to another user’s account on the software (which is tied deeply into the forum). And I don’t want them to be able to see it (as it might give something away, not fully sure.) and I don’t want them to be able to change it so they can trick the system into allowing them to login into someone else’s account.