Re: Blinded Identities [was Re: exporting signatures only/CAPI]

[Hal Finney <hal@rain.org>]

The "blinded identities" problem is one of the oldest that we have
discussed here (although not much recently, of course!).  It is basically
similar to what cryptographers call "blinded credentials" and is closely
related to electronic cash, as Michael Froomkin's example from Stefan
Brands points out.  I posted an idea a few years ago for how to use the
technique to solve the related problem of remailer abuse.

A simple way to approximate what you want is to use a standard blinded
signature exactly as is done with David Chaum's DigiCash.  The customer
comes to you and presents some proof of identity.  This may be in
person via standard paper documents, or on-line via some cryptographic
credential as you suggested.  You make a list of all of your customers,
and make sure that this customer is new, someone you haven't seen before.

Now you simply give him a blinded cryptographic signature, of exactly the
same form as the blinded coins given out by DigiCash.  He unblinds it,
and he is left with a signed credential from you, but one which is
unlinkable to his identity.

When he interacts with you, he displays this credential as proof
that he is a customer in good standing.  If he violates the terms of
your contract, you disable the credential (add it to a list of bad
credentials).  He can't use this one any more, and he can't get a new
one because he is on the list of people who already got their credential.

This simple solution suffers from several problems, some of which are
endemic to this class of solutions and others which can be addressed
with fancier crypto.  Among the fundamental problems we have first that
verifying identity reliably is difficult to impossible.  If people are
motivated badly enough, they can forge whatever documents they need.
Then they keep signing up with new identities like the kids who use
AOL throwaway accounts.

Second, if the customer ever loses his credential, he is screwed.
He comes to you with some sob story about how his disk crashed and his
dog ate his backups, but you have no way of knowing if he actually lost
his credential, or if he is an abuser who got his credential cancelled.
Another problem is that groups of users can share credentials, so that
some hacker club can get a bunch, one for each of them, and then they
can all abuse your ISP, getting credentials cancelled, but able to keep
going as long as one is left.

Problems which can be fixed include that credentials could be stolen,
like phone card numbers, so an innocent person gets his credential
cancelled and then we are back to the second problem above.  You can
mostly solve this by having him create a key when he first registers
with his credential and require all his interactions to be protected by
this key.  There are also more elaborate solutions where he wouldn't
actually send his credential over, but use zero knowledge techniques
to prove that he had one.

Unfortunately David Chaum has a pretty good set of patents covering
blind signatures, so for a commercial venture you'd probably have to look
into the legal situation.  I can send you a list of Chaum's patents in
the area if you want it.  (I had it on my web page but my ISP quit so
I need to get a new page going.)

Some of the other practical issues are also mentioned in Michael
Froomkin's article, like waiting a while after you get your credential
before you use it.

Hal