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Re: Summary Trust x Delegation

On Thu, May 29, 1997 at 02:11:16PM -0500, Ron Rivest wrote:
> I disagree with the statement made by Bill Frantz that
> 	"I have no technical way to prevent effective delegation, so..."
> If Alice gives Bob a certificate (say with tag "foo"), then of course there
> is no way for Alice to prevent Bob from issuing a certificate with tag
> "foo" to Dorothy.  
> But this certificate issued by Bob is not _effective_ if Alice's certificate
> has a "don't propagate" bit turned on.  (Let us assume for simplicity here
> that this is the simplest form of propagation control, not "stop at key".)
> By turning on the "don't propagate" bit, Alice is insisting that her
> certificate must be the *last* certificate in any valid certificate chain
> containing that certificate.  Thus any certificates that Bob issues can
> not be effectively utilized in a valid certificate chain.  Thus Bob's
> certificates are ineffective--the certificate he issues to Dorothy will
> not be usable to sub-delegate any "foo" rights that Alice gave to Bob.
> Alice need only trust the verifier to honor the correct notion of what
> a valid certificate chain is; she does not need to trust Bob not to
> issue further certificates.

Bob doesn't issue a new cert for anyone -- he gives away his cert.  In
such a case it makes absolutely no difference whether Alice has turned
on the "don't propagate" bit.  It's true that Bob may compromise
himself by doing this, but that doesn't do Alice any good.  If she was
counting on the "don't propagate" bit preventing Dorothy from
accessing her secrets she will be seriously disappointed.  This is
Bill's point about the "don't propagate" bit being an illusion.  You
were sucked in by that illusion.  It's usability is a function of the 
trustworthiness of Bob, not any cryptographic or semantic strength.

For this reason I agree with Bill that the semantics are somewhat
deceptive.  I think a better name for the bit is the "permission to
create new cert" bit -- this name is less likely than "propagate" or
"delegate" to create the confusion that Bill identifies. 

Kent Crispin				"No reason to get excited",
kent@songbird.com			the thief he kindly spoke...
PGP fingerprint:   B1 8B 72 ED 55 21 5E 44  61 F4 58 0F 72 10 65 55