Re: Anonymous Auth Certificates [was: Re: Blinded Identities]

[Hal Finney <hal@rain.org>]

From: azur@netcom.com (Steve Schear)
> It seems that one crux of the problem revolves around the CA and its method
> of certificate issuance.  A CA which uses biometric data to
> reduce/eliminate the chance that an applicant could get several, unrelated,
> certificates issued would provide a basis for negative authentication
> (similar to a negative credit file).

Yes, biometric data is another way of preventing multiple credentials.
However it will not work well in electronic form.  What you'd need would
be a network of stations to take fingerprints and give credentials,
("is a person" credentials) which would then be used for getting access
to other services where you're supposed to only use them once (voting
for example).  This requires a fairly elaborate infrastructure and social
commitment to this solution to the problem.

Somehow too it is hard to see how to sell a system as a privacy enhancement
when its first step is to take fingerprints of the whole country.  "But
we're not saving your names, honest!"  I don't know if it would fly.

Tim May argues that alternative solutions which are more local will be
better.  In the case of the abuse situation, maybe you could just have
people put down a deposit of $100 or so.  Then they get an anonymous
credential which they can use for access.  If they abuse their access,
their credential gets disabled.  As long as their abuse is worth less
than the deposit you'll be OK.  And at any point they can turn in their
valid credential and get their money back.  No identities are needed at
any point in the scheme.

> A one-way function performed, by the client, on their certificate from this
> CA would yield a token which unambiguously binds it to a valid certificate
> of the CA (and therefore uniquely identifies them) w/o revealing the
> certificate itself.

Actually I think you need to use a blinding protocol when you acquire
the certificate, rather than trying to run a one-way function on the
unblinded cert.  The output of a one-way function looks random and
meaningless unless you supply the input.  And if the input identifies
the user then you've lost the anonymity.

Hal