Re: AIDs testing and privacy

[Hal <hfinney@shell.portal.com>]

Jim Hart <hart@chaos.bsu.edu> writes:
>Challenge: is a crypto protocol possible with the following 
>properties: the doctor writes and signs the prescription,
>and it is not transferable, but the patient doesn't need to
>show ID to the pharmacist to fill the prescription?
>I don't want pharmacists, and whoever else they share the info
>with (insurance companies?  investigators? potential blackmailers?), 
>keeping track of what drugs I take.

Let me point out that nothing stops you from filling the prescription
and then giving the drugs to someone else, so it would seem that a doctor
who would be willing to cooperate in any such protocol should also be
willing to make the prescription out to a pseudonym.

Chaum's "blinded credential" system is intended to solve exactly this kind
of problem, but it requires an extensive infrastructure.  There has to be
an agency where you physically identify yourself.  It doesn't have to know
anything about you other than some physical ID like fingerprints.  You and
it cooperate to create pseudonyms of various classes, for example, a
"go to the doctor" pseudonym, and a "go to the pharmacy" pseudonym.  These
pseudonyms have a certain mathematical relationship which allows you to
re-blind credentials written to one pseudonym to apply to any other.  But
the agency uses your physical ID to make sure you only get one pseudonym of
each kind.

So, when the doctor gives you a prescription, that is a credential applied
to your "go to the doctor" pseudonym.  (You can of course also reveal your
real name to the doctor if you want.)  Then you show it at the pharmacy 
using your "go to the pharmacy" pseudonym.  The credential can only be shown
on this one pseudonym at the pharamacy, but it is unlinkable to the one
you got at the doctor's.  (It would be possible to encode information in
the credential about which doctor wrote it, which would help track abuse,
although that would obviously make it easier to link up your pharmacy and
doctor visits.)

Hal