There really is no reason to add the extra network complexity for this.

First of all (as you point out) no one buying a Ferrari will refuse to
wait an hour for the payment to confirm.  If someone is attempting to
pull a similar trick on, say, a vending machine however it might make
sense.  But that changes the equation.  In order for these two scammers
to pull it off, some effort is required in terms of communicating the
time to send the coins and the nodes of the targets (vending machines or
whatever) must be figured out.  So now its less of "make it impossible"
and more of "make it really hard to the point that it is no where near
worth the effort".

Lets simplify the scenario a bit so that one scammer can pull it off.
Send one copy of your transaction to the target node and another to
large mining operations so that the payment transaction is considered
invalid to miners and a transaction which pays you is confirmed.

If you are the vending machine, your goal is not to figure out any
transactions which are yours, but to figure out which transactions which
are yours are going to be confirmed.  So, you peer with the largest
miners (a "Bitcoin backbone" or large miners and merchants has been
suggested over and over again and really hasn't happened) and modify
your client to, instead of dropping transactions which are
double-spends, keep both in memory pool and consider them both invalid
until one of them confirms.

This will work with 1, 2, or n scammers, doesn't require any additional
network messages, and offers just as good, if not better security over a
double spend message.

Additionally, in the future, when(/if) Bitcoin payment processors exist,
most merchants will rely on those, which can handle such double-spend
checks and tell a merchant a transaction is confirmed in ten seconds for
small transactions, an hour for large ones, or anywhere in between.
Such payment processors could also mine or have contracts with large
miners which allows them to influence the transactions which are to be
confirmed, allowing for even quicker confirmations and the offering of
insurance against unconfirmed transactions being invalidated.

Matt

On Thu, 2011-08-04 at 14:23 +0100, Andy Parkins wrote:
> Hello,
> 
> Here's a scenario (it's contrived to make the players easy to identify, more 
> likely this would be low value automated vendors):
> 
> Two scammers get together to buy two Ferraris using only one set of BTC.  They 
> travel to opposite ends of the world to two car dealerships that accept 
> bitcoins without waiting for confirmations.  They are in contact by mobile.  
> They each buy the car and come to pay.  At exactly the same moment, they both 
> spend the same coins.  They both walk away with a car.
> 
> The current solution is the recommendation that vendors wait for six 
> confirmations before releasing goods.  That's a long time though; more than 
> most would be willing to wait.
> 
> Some points:
>  - The bitcoin network is essentially honest
>  - If a block chain fork happens, the transactions that are orphaned get added
>    to the pending transaction list again, meaning ...
>  - A valid transaction will _eventually_ make it into the (longest) block
>    chain.
>  - Actual distribution time for a transaction through the network is in the
>    order of seconds not minutes
>  - A double spend attempt has to enter the network near simulateously at
>    different places, otherwise the second spend will be rejected instantly by
>    the whole network.
> 
> New transactions propagate through the network if they are found to be valid.  
> If they aren't valid, they are silently dropped.  In the event of a double 
> spend attempt one of those transactions goes to (say) half the network, the 
> other goes to the other half.  Whichever one reaches a node first is seen as 
> the real one, the second being seen as invalid.  One or other of these will 
> therefore end up in the "longest" chain; but there is no way to know which.
> 
> Here's my proposal then: when a node drops a transaction, it should not be 
> silent.  It should be broadcast just as it always was going to be had it been 
> valid.  Only it is broadcast with a new "inv" type, let's say 
> "MSG_DOUBLESPEND" instead of "MSG_TX".
> 
> Now run the Ferrari test again.  The vendor sees the transaction that pays for 
> the car appear near instantly (within the propagation time of the network).  A 
> short while later they also see a MSG_DOUBLESPEND of the same coins that they 
> have just accepted.  They can then operate whatever policy they want: wait for 
> six, ten, twenty confirmations.  Call the police.  Whatever.  Miners can also 
> significantly lower the priority of any transactions that get flagged in this 
> way.
> 
> When there isn't a double spend attempt message within the network propagation 
> time, they can be sure that their transaction is the one that miners are 
> working on, and they'll eventually get their money.  In other words, they can 
> accept the payment on zero confirmations.
> 
> At first I was concerned that this would make it possible to DOS a 
> transaction, but of course it doesn't -- the transaction has to be internally-
> valid to result in a MSG_DOUBLESPEND, meaning it can only be DOSed by someone 
> with the appropriate private keys.
> 
> 
> 
> Andy