Regarding your scenerio, the burden of proof has to rest on B. Lack of
evidence
to the contrary, will most likely constitute as B cheating. If he really is
cheating, then he will be punished. The judge won't beleive his claim that he
did not receive E and A will be granted whatever the consequences for having
the
receipt is. The fact that the judge won't believe B constitutes as B not
being
able to repudiate the "message receipt". Lawyers?
I don't think the judge has any other choice but to believe him. If B refuses
to pay its up to A to attempt to recover the payment, and in so doing it will
be up to A to prove that B actually received M and signed for it. There is no
signature present in your protocol that provides evidence of this -- the one
signature that is there isn't sufficient.
The best you could do would be to attack B's credibility on other grounds such
as prior convictions for fraud or whatever. It should not be necessary to do
this. The burden of proof logically has to fall entirely on A, and A isn't up
to the challenge.
As it happens I've gone through a similar exercise personally in the
non-digital realm, and I found that the burden of proof rested squarely on the
shoulders of the person attempting to collect the money, goods, or services
owed.
I am glad you agree with me on the denial-of-service front.
Having signed a receipt for EM, B must either make sure he has E, *or* notify
a judge immediately.
Sorry, this just isn't how it works. B doesn't have to notify anybody. B simply
refuses to pay, and A feels strongly enough about it it is up to A to address
it.
The receipt is all A needs to prove B received *or* is
capable of receiving the message M.
Its going to have to be tried in the real world before such assumptions
can be made.
Ned