[Bitcoin-development] Double spend detection to speed up transaction trust

Thu Aug 4 20:07:42 UTC 2011

One double-spend fighting option is for each mining pool to offer a realtime feed of accepted TXs.

I am hoping to complete the following later this month:

	(1) A minimal bitcoind patch that writes raw accepted TXs and BLOCKs to stdout with a prefix of ("2naXaRQj--TX\n%d\n" % (data_length))
		(Proof-of-concept done — I'll submit a pull request with "--print-accepted-txs-and-blocks" when I get a chance to clean it up)

	(2) A minimal NodeJS app which invokes bitcoind as a subprocess, parses the TXs and BLOCKs, and offers a realtime feed

On Aug 4, 2011, at 3:42 PM, Andy Parkins wrote:

> On Thursday 04 August 2011 19:39:56 Matt Corallo wrote:
> 
>> But why? It results in slightly more network traffic which is exactly
>> what we don't want, and it adds yet another message people have to know
>> about.
> 
> "Slightly" is an understatement.  It add more network traffic for every 
> double spend attempt.  Which don't happen very often.
> 
> Also, I'm not proposing a new message, heaven forbid that we add a new 
> message type, I'm proposing that we do this:
> 
> enum
> {
>     MSG_TX = 1,
>     MSG_BLOCK,
> +    MSG_DOUBLESPEND,
> };
> 
> Also, people don't "have" to know about it.  And it's not "people" it's an 
> addition to the _one_ official client.  _and_ it's backward compatible 
> because if they don't know about it, nothing changes... the TX gets dropped 
> just as it is now.
> 
>>> I think you've missed the point.  Double spend transactions that enters
>>> the network at two reasonably evenly connected points are each only
>>> seen by half the network, since the first one locks out the second
>>> from propagation.
>> 
>> No one cares about what the network thinks is the right transaction, its
>> only what miners believe that matters.
> 
> They do care because the network as a whole is what makes the eventual 
> decision about which is the block-chain-to-rule-them-all.  Chain forks, and 
> eventual reorgs are also far less disruptive when each leg of a double spend 
> isn't on each potential chain.  "Half the network" includes half of the 
> miners.  It's perfectly possible for half the miners to be working on one 
> leg, half on the other.  That means it's 50/50 which leg eventually gets 
> confirmed.
> 
>> Even if the vending machine doesn't keep the full chain and doesn't
>> accept incoming connections, its still the target node.  What other
>> nodes on the network think doesn't matter as long as you get the target
>> to think a transaction that won't be confirmed will be.  If it doesn't
>> accept incoming connections you want to find nodes that do that are
>> connected to your target.
> 
> Well that's true enough; but how on earth you're going to identify an IP 
> address of a particular vending machine that isn't accepting incoming 
> connections is beyond me.  If it is a target it's pretty close to invisible.
> 
>> Its much easier to create than to change the network code to relay info
>> on double-spend transactions.
> 
> What?  It's easier to trigger massive adoption and organisation of an 
> inherently disorgainsed network of miners than it is to write a few lines of 
> code?  If that's true, then the bitcoin source is even more impenetrable 
> than I imagine.
> 
>>> Well that's what happens now.  But that doesn't help the poor sap who's
>>> just handed over some goods.  I want it so that small businesses can
>>> use the client to give them practical answers instead of this
>>> "0/unconfirmed" stuff which requires understanding of the system.
>> 
>> No, thats not what happens now.  Currently if your node gets a
>> transaction which conflicts with one it already knows about, it silently
>> drops it without a second thought.  My point is if you actually dealt
>> with such cases and made good connections, you would be able to prevent
>> double spends nearly perfectly.
> 
> It's not about prevention, they are already prevented.  It's about 
> detection.  Quickly.
> 
>>> I'm not really trying to prevent double spends -- bitcoin _already_
>>> prevents double spends.  Also: the only difference between your
>>> suggestion (don't drop) and my suggestion (don't drop but mark with
>>> MSG_DOUBLESPEND) is a single number in the inv.  I really don't get
>>> the objection.
>> 
>> No, my suggestion is not to relay the second transaction.  The second
>> transaction should continue to not be relayed as it currently is,
>> however receiving such a transaction should trigger the node to notify
>> the user that the transaction should not be accepted until it makes it
>> into a block (in fact, you could already do this if you implemented a
>> debug.log parser and made well-placed connections).
> 
> How is this second transaction going to end up anywhere but on a few 
> isolated nodes if it isn't propagated?  The only way _both_ can be in a pool 
> is if they are both received.  If they aren't both forwarded then it won't 
> be in most pools.  If it isn't in most pools then which how is the relevant 
> user going to get notified?
> 
>> Bitcoin is absolutely still an experiment and no one thinks that any
>> kind of future is guaranteed.  This was not meant as an argument, but
> 
> If it's still an experiment why is there such huge objection to pretty much 
> every change anyone proposes?  Bitcoin is one of the most conservative 
> projects I've ever seen, even for the most passive of changes.  I can 
> understand wanting to prevent potential financial loss, but it's not like 
> I'm suggesting we start broadcasting private keys on the network.
> 
>> simply as "if bitcoin does end up going somewhere, it will likely be
>> done like this".
> 
> When you're using it as an argument for why a suggestion is unnecessary 
> that's not how it sounds.
> 
> Anyway; it's fine.  You don't think it's a good idea; and I suspect none of 
> the other official client developers will either, they don't like protocol 
> changes.  So be it; it was only a suggestion and I'm a nobody around here.
> 
> 
> 
> Andy
> 
> -- 
> Dr Andy Parkins
> andyparkins at gmail.com
> 
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