Quote from: TalkingAntColony on May 23, 2013, 05:03:46 PM

The Tx that Merchant A receives may or may not be heard about by Merchant B. It is unwise IMO to make a security feature rely on the assumption that all nodes will know about all unconfirmed Tx.

Merchant A of course rebroadcasts the tx himself to make double spends less likely to succeed. As long as no block was found this is a similar situation to what we have now. Once the first tx is in a block, well, the merchant really should make sure he gets that. I have no info on this but I would expect network permeation to be very high.

It is not a security feature but a "safe enough" feature. This means it does not have to be perfect but just 98% or so, way better than what we have now.

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Then, then the buyer mines his own block sending the coins back to himself. Merchant B will accept zero-conf Tx building off that block if he doesn't know about the Tx from Merchant A.

? The sending coins back tx conflicts with the tx to Merchant B so he will never accept it if he has seen the block. Also note that as soon as the block with the malicious "sending coins back" becomes public it is certain a miner will claim the payment - because the first tx is public, too.

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The buyer could peer directly with Merchant B so Merchant B is likely to see the new block faster than Merchant A.

Are you talking about a Finney attack? The risk of orphaning the new block would be much too high.

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This gives the buyer more time for his zero-conf Tx to be accepted by Merchant B. Merchant A must "be on the ball" to quickly realize the dbl-spend and make sure all other nodes know this.

see above

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Naturally or purposefully occurring 1-2 block blockchain forks may make this proposal unreliable as well.

These are so rare they do not matter and do not hurt this proposal in any way.

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Also, what if there are buyers who always dbl-spend coins, so that merchants will be discouraged from accepting zero-conf Tx using this method? There is no additional cost to buyers who dbl-spend in this scheme anyway.

This is exactly what will happen with the current system. "double spending destruction" removes the incentive to do it so I doubt a lot of people would go through the (maybe very small) trouble. Also most people still have a conscience without financial gains. Cunicula's suggestion even addresses this with a penalty.