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No i not try to doublespend (we talk about 20$) but someone scammed me he claim that even on 2 confirmation i can double spend.

Please only the real pro should reply here as i need strong evidence about this !

Theoretically yes, in practice, no.

In order to do that, you would need to be able to mine three blocks before the rest of the miners find the block for the third confirmation. Those three blocks would have to include the double spending transaction and be a small fork of the blockchain the began on the block before your first confirmation.

Ok so in theory it's for a single user impossible unless you have the hardware right ?!

Yes. Performing such an attack would require a lot of hardware. In order to have any probability of success, you would need to have greater than 50% of the hash rate. At the current network hashrate, that is incredibly expensive and pretty much impossible.

Thanks a lot guys that you confirmed my claim about how hard or even impossible it would be to double spend 2 confirmations !

no

Quote from the guy :

You don't need 50%+ of mining to do it. It's completely possible, a low transaction fee with low priority vs a high transaction fee with high priority gets priority and double spending can happen that way, that's why sites wait for multiple confirmations and they suggest you do too. It forks often too so therefore double spend is possible with one confirmation




Is this true???

No. Nodes won't even accept a transaction when another transaction with the same input already exist in the mempool. And once a transaction have been mined the other one will never confirm even it have a huge fee (unless the attacker successfully does what knightdk described above).

Partially. If a transaction has a low fee (and does not signal RBF) but is above the node's relay threshold, then the any higher fee replacement transaction will be rejected by the node. However if the fee was too low, that transaction would have been rejected by the node and the high fee replacement would be accepted. Furthermore, if a transactions signals RBF (meaning that it can be replaced by a higher fee double spend) then even if it is in the mempool of a node, the higher fee can replace it.

The naive way to deal with this is to wait for confirmations. The actual smart way that many companies and people do is to examine a transaction to determine the probability of ever confirming. The things to check are the fees and the sequence numbers of the input. Any sequence number that isn't 0xffffffff means that the transaction is not final so it has a higher chance of not confirming. Thus you should wait for a confirmation. Any transaction that has a low fee should also wait for a confirmation before accepting it as a payment.

That is not true. The "forks" are known as orphan (or stale) blocks and these happen when two miners find two blocks of the same height. One of those has to be ignored. This does not happen very often, maybe once every few thousand blocks, according to my node.

That's the thing. While it is *possible, it is in no way profitable. With each confirmation it becomes exponentially harder and more expensive. The same is true with credit cards. For example, until recently you could not use a credit card at MacDonald's. That is because assuring that the transaction is legitimate costs more than the profit from the transaction. The card companies finally wised up and realized that creating a false transaction also costs more than a MacDonald's meal. So now you can use your card and they just assume that you will act logically and not spend dollars to get pennies.

You cannot spend those coins again after two confirmations. Normally, one confirmation is sufficient to make a transaction irreversible because the confirmation will show in the open ledger.