Digitally Signing Physical Objects

[hughes@ah.com (Eric Hughes)]

>Undeniable signatures cannot be passed on from one person to another.
>If Alice verifies Bob's undeniable signature, she can't prove to Charlie
>that the signature is good.  She can claim it is good, and assure Charlie
>that it is good based on her own reputation, but Charlie can in general not
>be convinced unless he verifies it himself directly with Bob.

This is the standard reason given why undeniable signatures can't be
passed on.  And it is correct, as far as it goes.  But the conclusion,
that "in general" the trust cannot be passed on, while technically
correct, is not of pragmatic consequence.

I'll start a service to perform any undeniable signature verification,
even ones for money.  I'll perform the verification, and then make an
attestation that I perfomed the verfication and whether it succeeded
or failed.  I sign this is a regular digital signature, the kind that
is infinitely duplicable.

Only a few such services need exist to assure the public of the
results of a signature verification.

True, there is a layer of mediation here, but of what practical
consequence is that?  In fact most transactions are mediated already.
If I expect to make money charging a dollar per verification, and if
there are some who will publish their experiences of the verification,
that reduces the total income I can expect to, oh, say, the logarithm
of the size of the market.  In other words, why bother?

Eric