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Re: Proving I'm not Bob.

[email protected] writes:

>I had an idea for an advertising based net-payment scheme that has 
>a particular security flaw making it totally untrustworthy unless it is
>possible to prove that you are not a particular person (the publisher 
>receiving the ad money) when consuming (viewing or otherwise) the ads.

BTW, we had some interesting postings here last year from Jason Solinsky
([email protected]) about schemes involving advertising and payment, where
people would get paid to view advertisements.  Is this the kind of thing
you're thinking of?

I posted some ideas once on how to prove that you are not someone else.
Any such scheme has to be grounded in a physical mechanism to determine that
two people are different.  For example, you might be able to get some
special cryptographic signature or credential from an agency by showing
some biometric information, such as retina or thumb prints.  You wouldn't
necessarily have to reveal your name, identity, or any other information;
just something which would allow the agency to be sure that they had not
given such a credential out to you before.

If you didn't care about privacy, your problem could then be solved
simply by having each person exhibit his credential (these are often
called "is-a-person" credentials).  The more interesting question then
becomes exhibiting that credential in a privacy protecting way, but
still being able to tell if two people are showing the same or
different credentials.

There are various ways of doing this; one of the simplest would be for
the agency to give you a blind signature using a particular exponent,
where you would be allowed exactly one of each exponent.  You unblind
these, and to show you aren't Bob both you and Bob show your signatures
for some matching exponent, which will be different.  Because of the
blinding, no one will link the credential to your identifying
information, and because it is a signature from the agency, no one can
forge a credential different from the one they have.

Depending on the situation and your tradeoffs between convenience and
privacy, you might discard used credentials (for maximum privacy), or
you might reuse them in a particular forum where you have persistent
identity (for maximum convenience).  In the latter case, the exponent
used could be associated with the forum, which is the idea behind
Chaum's pseudonym system.

Hal Finney
[email protected]