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Date: Fri, 05 Aug 2011 00:10:59 +0200
From: Stefan Thomas <moon@justmoon.de>
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Subject: Re: [Bitcoin-development] Double spend detection to speed up
 transaction trust
Precedence: list

Since nobody else has mentioned it: There is another (more pragmatic?) 
way to detect double spends:

1. Connect to lots of clients
2a. If they all send you the same transaction -> double spend unlikely
2b. If some don't send you the transaction (or send a conflicting one) 
-> double spend in progress

Obviously not everyone will run a double spend detector - it's much more 
easily realized as a service (just like mining.) Jan put up a proof of 
concept: http://www.transactionradar.com/

Would network support like a MSG_DOUBLESPEND be better? I used to think 
yes, but looking at the reality of Transaction Radar, I'm not so sure. 
Nothing stops such a service from scaling up and connecting to thousands 
of random nodes (especially when the network itself grows bigger), 
pushing the probabilities of missing a double spend "in the wild" to 
near zero. It could also connect directly to important miners/pools as 
others have suggested.

Of course this doesn't help against double spends where the attacker 
does his own mining*, but neither would MSG_DOUBLESPEND. Given the added 
network load I'd argue that network support for double spends is 
unnecessary and potentially damaging. DoS is more scary to me than 
non-instant transactions.

* In this case of course the hacker will be exposed to some randomness, 
and I doubt many attackers will buy 100 televisions, newspaper 
subscriptions or MP3s to get one for free. So this is only a problem for 
liquid goods with tiny spreads (any investment or stored value instrument.)