Digitally Signing Physical Objects

[Allen J. Baum]

>> 3. The shop or customer wishing to authenticate the part takes the
>> number stamped on the part, runs it through the *public* key of the
>> manufacturer (widely available, not kept secret, of course) and gets
>> back the feature vector, which he can then compare to what he actually
>> sees on the object.
....

>This seems to have a tricky dependence on the tolerance.  The forger can
>get a valid plaintext and signed feature vector.  So, if the tolerance
>for error is too low, you get false positives, but if it's too high, a
>forger could create something starting from the feature vector.  An
>interesting CAD/CAM problem.

To keep black market forgery part off the market, a 30% tolerance is way
more than enough. There should be no false negatives (making a real part
look fake), but if 1/3 of the forgeries slip through (i.e. 2/3 don't), this
has the affect of driving the forgery price up by a factor of 3,
effectively pricing them out of the market. (Unless the real goods are
overpriced a factor of 3...:-)

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* Allen J. Baum            tel. (408)974-3385    *
* Apple Computer, 20525 Mariani Ave,  MS 305-3B  *
* Cupertino, CA 95014      baum at apple.com        *
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