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How do you inspect a bitcoin transaction to avoid getting scammed by a double spender? I'm buying bitcoins in cash and the seller said I could inspect a transaction instead of waiting 6 blocks. Is this safe?

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There are no means of inspection that could lead to a safe assumption that a transaction will not get invalidated by a double-spent input.

Giving up your money and leaving the trade with zero-confirmed coins is asking to get scammed.

Waiting for one confirmation is only safe if you are certain that the seller exchanged a total of less than 25 coins over the past few minutes (both in-person and online) and does not control 50% or more of the network's total hashing power (so it wouldn't cost him a penny to perform this kind of attack). Luckily for you the most a mining pool currently holds is 25% of the network's total hashing power (attacks are still possible at that rate but less likely to succeed and cost more for the attacker) but unfortunately you can never be sure that the seller or a partner-in-crime didn't spend more than 25 coins online so one confirmation won't do the trick either.

Waiting for more confirmations will always increase your security to a great extent. 6-blocks confirmations are not 100% safe either but they are way much more secure than single-block confirmations in all aspects.

On a side note, a reliable escrow service could take the hassle away and make your life easier when trading with strangers on the street.

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    Could you explain why it's important that they traded less than 25 BTC? I don't really understand how the link is relevant. What do you mean by over the past few minutes, how many would be sufficient? – RootFAIL Dec 9 '14 at 3:41
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    @RootFAIL after your transaction has been included in a single block an attacker that holds less than 50% of the network's total hashrate would have to give up a whole block to perform this kind of attack. The current reward for a miner finding a block is 25 coins so the attacker would have to give up this reward in order to perform a double-spending attack against you. And "over the past few minutes" should be "since the last block was found". Of course in that case we would also have to take into account any transactions residing in the mempool that were not included in the last known block. – George Kimionis Dec 9 '14 at 3:50
  • So I wouldn't need to worry about the 25 BTC if I know they don't have great hashing power, right? (Would OR be better than AND in this case)? – RootFAIL Dec 9 '14 at 3:57
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    If the seller controls more that 50% of the total hashing power then he can perform the attack at no cost irregardless of the traded amount. Such an attack is theoretically possible at any hash rate, however the more the hash rate moves away from 50% towards zero (and the more blocks the transaction has been included into) the slighter the chance for the attack to succeed and the higher the cost for the attacker. – George Kimionis Dec 9 '14 at 4:08
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    It is very unlikely for the mining pools operators to conspire to double spend as a group as this would irreparably hurt their reputation in the community. – George Kimionis Dec 9 '14 at 4:12

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