Bitcoin P2P e-cash paper
James A. Donald
jamesd at echeque.com
Wed Nov 12 22:16:31 PST 2008
Satoshi Nakamoto wrote:
> When there are multiple double-spent versions of the
> same transaction, one and only one will become valid.
That is not the question I am asking.
It is not trust that worries me, it is how it is
possible to have a a globally shared view even if
everyone is well behaved.
The process for arriving at a globally shared view of
who owns what bitgold coins is insufficiently specified.
Once specified, then we can start considering whether
everyone has incentives to behave correctly.
It is not sufficient that everyone knows X. We also
need everyone to know that everyone knows X, and that
everyone knows that everyone knows that everyone knows X
- which, as in the Byzantine Generals problem, is the
classic hard problem of distributed data processing.
This problem becomes harder when X is quite possibly a
very large amount of data - agreement on who was the
owner of every bitgold coin at such and such a time.
And then on top of that we need everyone to have a
motive to behave in such a fashion that agreement
arises. I cannot see that they have motive when I do
not know the behavior to be motivated.
You keep repeating your analysis of the system under
attack. We cannot say how the system will behave under
attack until we know how the system is supposed to
behave when not under attack.
If there are a lot of transactions, it is hard to
efficiently discover the discrepancies between one
node's view and another node's view, and because new
transactions are always arriving, no two nodes will ever
have the same view, even if all nodes are honest, and
all reported transactions are correct and true single
We should be able to accomplish a system where two nodes
are likely to come to agreement as to who owned what
bitgold coins at some very recent past time, but it is
not simple to do so.
If one node constructs a hash that represents its
knowledge of who owned what bitgold coins at a
particular time, and another node wants to check that
hash, it is not simple to do it in such a way that
agreement is likely, and disagreement between honest
well behaved nodes is efficiently detected and
And if we had a specification of how agreement is
generated, it is not obvious why the second node has
incentive to check that hash.
The system has to work in such a way that nodes can
easily and cheaply change their opinion about recent
transactions, so as to reach consensus, but in order to
provide finality and irreversibility, once consensus has
been reached, and then new stuff has be piled on top of
old consensus, in particular new bitgold has been piled
on top of old consensus, it then becomes extremely
difficult to go back and change what was decided.
Saying that is how it works, does not give us a method
to make it work that way.
> The receiver of a payment must wait an hour or so
> before believing that it's valid. The network will
> resolve any possible double-spend races by then.
You keep discussing attacks. I find it hard to think
about response to attack when it is not clear to me what
normal behavior is in the case of good conduct by each
and every party.
Distributed databases are *hard* even when all the
databases perfectly follow the will of a single owner.
Messages get lost, links drop, syncrhonization delays
become abnormal, and entire machines go up in flames,
and the network as a whole has to take all this in its
Figuring out how to do this is hard, even in the
complete absence of attacks. Then when we have figured
out how to handle all this, then come attacks.
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