Bitcoin P2P e-cash paper
James A. Donald
jamesd at echeque.com
Mon Nov 17 18:57:39 EST 2008
Ray Dillinger wrote:
> Okay.... I'm going to summarize this protocol as I
> understand it.
> I'm filling in some operational details that aren't in
> the paper by supplementing what you wrote with what my
> own "design sense" tells me are critical missing bits
> or "obvious" methodologies for use.
There are a number of significantly different ways this
could be implemented. I have been working on my own
version based on Patricia hash trees, (not yet ready to
post, will post in a week or so) with the consensus
generation being a generalization of file sharing using
Merkle hash trees. Patricia hash trees where the high
order part of the Patricia key represents the high order
part of the time can be used to share data that evolves
in time. The algorithm, if implemented by honest
correctly functioning peers, regularly generates
consensus hashes of the recent past - thereby addressing
the problem I have been complaining about - that we have
a mechanism to protect against consensus distortion by
dishonest or malfunctioning peers, which is useless
absent a definition of consensus generation by honest
and correctly functioning peers.
> First, people spend computer power creating a pool of
> coins to use as money. Each coin is a proof-of-work
> meeting whatever criteria were in effect for money at
> the time it was created. The time of creation (and
> therefore the criteria) is checkable later because
> people can see the emergence of this particular coin
> in the transaction chain and track it through all its
> "consensus view" spends. (more later on coin creation
> tied to adding a link).
> When a coin is spent, the buyer and seller digitally
> sign a (blinded) transaction record, and broadcast it
> to a bunch of nodes whose purpose is keeping track of
> consensus regarding coin ownership.
I don't think your blinding works.
If there is a public record of who owns what coin, we
have to generate a public diff on changes in that
record, so the record will show that a coin belonged to
X, and soon thereafter belonged to Y. I don't think
blinding can be made to work. We can blind the
transaction details easily enough, by only making hashes
of the details public, (X paid Y for
49vR7xmwYcKXt9zwPJ943h9bHKC2pG68m) but that X paid Y is
going to be fairly obvious.
If when Joe spends a coin to me, then I have to have the
ability to ask "Does Joe rightfully own this coin", then
it is difficult to see how this can be implemented in a
distributed protocol without giving people the ability
to trawl through data detecting that Joe paid me.
To maintain a consensus on who owns what coins, who owns
what coins has to be public.
We can build a privacy layer on top of this - account
money and chaumian money based on bitgold coins, much as
the pre 1915 US banking system layered account money and
bank notes on top of gold coins, and indeed we have to
build a layer on top to bring the transaction cost down
to the level that supports agents performing micro
transactions, as needed for bandwidth control, file
sharing, and charging non white listed people to send us
So the entities on the public record are entities
functioning like pre 1915 banks - let us call them
binks, for post 1934 banks no longer function like that.
> But if they recieve a _longer_ chain while working,
> they immediately check all the transactions in the new
> links to make sure it contains no double spends and
> that the "work factors" of all new links are
I am troubled that this involves frequent
retransmissions of data that is already mostly known.
Consensus and widely distributed beliefs about bitgold
ownership already involves significant cost. Further,
each transmission of data is subject to data loss, which
can result in thrashing, with the risk that the
generation of consensus may slow below the rate of new
transactions. We already have problems getting the cost
down to levels that support micro transactions by
software agents, which is the big unserved market -
bandwidth control, file sharing, and charging non white
listed people to send us communications.
To work as useful project, has to be as efficient as it
can be - hence my plan to use a Patricia hash tree
because it identifies and locate small discrepancies
between peers that are mostly in agreement already,
without them needing to transmit their complete data.
We also want to avoid very long hash chains that have to
be frequently checked in order to validate things. Any
time a hash chain can potentially become enormously long
over time, we need to ensure that no one ever has to
rewalk the full length. Chains that need to be
re-walked can only be permitted to grow as the log of
the total number of transactions - if they grow as the
log of the transactions in any one time period plus the
total number of time periods, we have a problem.
> Biggest Technical Problem:
> Is there a mechanism to make sure that the "chain"
> does not consist solely of links added by just the 3
> or 4 fastest nodes? 'Cause a broadcast transaction
> record could easily miss those 3 or 4 nodes and if it
> does, and those nodes continue to dominate the chain,
> the transaction might never get added.
> To remedy this, you need to either ensure provable
> propagation of transactions, or vary the work factor
> for a node depending on how many links have been added
> since that node's most recent link.
> Unfortunately, both measures can be defeated by sock
> puppets. This is probably the worst problem with your
> protocol as it stands right now; you need some central
> point to control the identities (keys) of the nodes
> and prevent people from making new sock puppets.
We need a protocol wherein to be a money tracking peer
(an entity that validates spends) you have to be
accepted by at least two existing peers who agree to
synchronize data with you - presumably through human
intervention by the owners of existing peers, and these
two human approved synchronization paths indirectly
connect you to the other peers in the network through
at least one graph cycle.
If peer X is only connected to the rest of the network
by one existing peer, peer Y, perhaps because X's
directly connecting peer has dropped out, then X is
demoted to a client, not a peer - any transactions X
submits are relabeled by Y as submitted to Y, not X, and
the time of submission (which forms part of the Patricia
key) is the time X submitted them to Y, not the time
they were submitted to X.
The algorithm must be able swiftly detect malfunctioning
peers, and automatically exclude them from the consensus
temporarily - which means that transactions submitted
through malfunctioning peers do not get included in the
consensus, therefore have to be resubmitted, and peers
may find themselves temporarily demoted to clients,
because one of the peers through which they were
formerly connected to the network has been dropped by
If a peer gets a lot of automatic temporary exclusions,
there may be human intervention by the owners of those
peers to which it exchanges data directly to permanently
Since peers get accepted by human invite, they have
reputation to lose, therefore we can make the null
hypothesis (the primary Bayesian prior) honest intent,
valid data, but unreliable data transmission - trust
with infrequent random verification. Designing the
system on this basis considerably reduces processing
Recall that SET died on its ass in large part because
every transaction involved innumerable public key
operations. Similarly, we have huge security flaws in
https because it has so many redundant public key
operations that web site designers try to minimize the
use of https to cover only those areas that truly need
it - and they always get the decision as to what truly
needs it subtly wrong.
Efficiency is critical, particularly as the part of the
market not yet served is the market for very low cost
> If we solve the sock-puppet issue, or accept that
> there's a central point controlling the generation of
> new keys,
A central point will invite attack, will be attacked.
The problem with computer networked money is that the
past can so easily be revised, so nodes come under
pressure to adjust the past - "I did not pay that"
swiftly becomes "I should not have paid that", which
requires arbitration, which is costly, and introduces
uncertainty, which is costly, and invites government
regulation, which is apt to be utterly ruinous and
For many purposes, reversal and arbitration is highly
desirable, but there is no way anyone can compete with
the arbitration provided by Visa and Mastercard, for
they have network effects on their side, and they do a
really good job of arbitration, at which they have vast
experience, accumulated skills, wisdom, and good repute.
So any new networked transaction system has to target
the demand for final and irreversible transactions.
The idea of a distributed network consensus is that one
has a lot of peers in a lot of jurisdictions, and once a
transaction has entered into the consensus, undoing it
is damn near impossible - one would have to pressure
most of the peers in most of the jurisdictions to agree,
and many of them don't even talk your language, and
those that do, will probably pretend that they do not.
So people will not even try.
To avoid pressure, the network has to avoid any central
point at which pressure can be applied. Recall Nero's
wish that Rome had a single throat that he could cut. If
we provide them with such a throat, it will be cut.
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