Is this even possible?
Thanks in advance
It is possible, but it is difficult, expensive, and unreliable.
If you have enough hashpower, you could create a transaction that spends some bitcoins to another address that you own, and not broadcast that transaction.
Then you could mine a block with that transaction on top of the current blockchain.
As soon as you successfully mine a block, you could broadcast a different transaction that spends the same inputs sending the bitcoins to a merchant, and NOT broadcast the block you just solved.
While the network is working on confirming your transaction into a block that will compete with your block, you continue mining on top of your block.
Eventually, the network will solve a block with your transaction in it and the merchant will see 1 confirmation. You continue to mine on top of your block instead of on top of the network's block.
If you have enough hash power and are lucky enough to mine another block before the rest of the network does, you broadcast your two blocks.
There are other possible attacks as well, but none of them are cheap, easy, and 100% reliable.
Since your two blocks create a longer blockchain than the current blockchain that has only added one block, the entire network switches to your blockchain and the 1 confirmation transaction to the merchant disappears (since it competes with your first transaction that now has 2 confirmations).
The more confirmations you have, the more difficult, expensive, and unreliable an attack like this is.
You know,there's already a service which offers such a thing for transactions with 0 confirmations. I'm not sure about the one with 1 confirmation as it would be a lot harder,but yes,the possibility is almost 0.
