[E-Lang] Ownership certificates via capabilities?

[Ben Laurie]

hal@finney.org wrote:
> 
> I had a crypto dream last night.  I'd gone to visit a couple of
> cryptographers at a university.  They had invited me to show me a bug in
> a pseudo random number generator I had written, but then we got talking
> about another problem.
> 
> These two guys were into trading card games, like Magic The Gathering.
> These games are played with cards that represent different characters
> and artifacts that have certain powers.  Players buy these cards at the
> store or from each other, and then they play against each other by laying
> their cards out according to the rules of the game.
> 
> In the dream these guys actually set up a folding table in front of the
> school cafeteria and began laying out the cards to sell to the students.
> But then they were talking about wanting to implement the same kind
> of game in electronic form.  People would have electronic files which
> represented the cards and use them when they played the game.
> 
> The question was, could this be done in an offline way, since players
> often get together at a place where there is no Internet connection.
> Perhaps they could each bring a portable computer and hook them together
> directly or via an IR connection.
> 
> The cryptographers said they were trying to find a way to do it with
> capabilities.  Each card would be represented by a capability.  I thought
> about that and it seemed that the problem would be that when you used
> your card in a game, the other player could see its value and steal it
> by copying the capability.  So it does not appear that it can be done.

Revocable capabilities (i.e. a capability to a proxy that destroys it
when it is "played")?

> That was as far as it went in the dream, but afterwards I was trying to
> decide if it could be done using a different technology, certificates.
> The idea would be that each card would be represented by a certificate
> signed by the game manufacturer which said what the card type was.  If
> this was all there was, people could easily copy cards.  So I think what
> you would have to do is to have the game owner operate a database of
> who owns what cards.  This could be a distributed database represented
> by the certificates.
> 
> Each certificate would therefore hold both the card type and the owner
> ID.  This would be cryptographically signed by the game manufacturer.
> When you bought a new card from the manufacturer they would issue you
> a new certificate.  When you sell a card to someone else, the game
> manufacturer would revoke your certificate and issue a new cert for the
> new owner.  It might do this by adding the revoked certificate to its
> Certificate Revocation List (CRL).
> 
> I know certificates are not particularly well thought of around here, and
> CRLs are even less well respected among cryptographers.  Nevertheless both
> of these seem to be necessary to solve the problem.

You can use OCSP, of course, which solves the problem of...

> Timeliness is always an issue with CRLs.

...this.

>  You might have to have a rule
> that game cards can be transfered between players only during certain
> hours, and then games are played at other times.  Then if at least one
> of the players were able to supply an up to date CRL at the beginning
> of the game (having downloaded it earlier), each player would know that
> the other could not falsely misrepresent a sold card as one he still owns.
> 
> So, with these caveats and restrictions, the problem seems to be just
> barely solvable with certificates and CRLs.  The question is, then, is
> there any corresponding way to achieve this with capabilities?  That is,
> could you have virtual objects which are owned, and people can interact
> with them off-line under agreed-upon rules, but be confident that people
> in fact own the objects which they claim to?

Like I said, you use capabilities to proxies which hold the "real"
capabilities. MarkM has some very nice slides to go with this idea.

Cheers,

Ben.

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