Perhaps you could use Maxim/Dallas USB 1-Wire/iButton
adapters?  DS9490R or DS9490B are <$20 in qty 100, and
there's a free API and lots of sample code for talking
to them.  You wouldn't actually need to supply an
iButton, the adapters have a built-in unique serial
number chip.  For more security, you could use the -B
version adapter with a password-protected or SHA
iButton.

The downside is, of course, the usual problem with
buying Maxim products in small quantities...
     Jason Harper
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