3 ms·

The only problem I see is that it takes 10 to 30 minutes for a transaction to be included in a block. If they give Bitcoins instantly could be double spent.

jerguismi 14y ago
You mean that the machine operators could be doing a double spend? In every case they take the dollars before they give the coins, so you have to take the leap of faith... I don't find that scenario sensible at all. First buy $1000 machine, first customer comes in, buys bitcoins with $50, and notices that the machine does double spend. I believe he is not going to tell any of his friends...
ewillbefull 14y ago
Not precisely. The only time you really need to worry about confirmations is when you're dealing with larger amounts. It costs a lot to pull off a double-spend, more than a tiny ATM transaction anyway. This is why SatoshiDice processes immediately, except for bets over 2-5 BTC.
- swhitt 14y ago
  The reason why SatoshiDICE can do this is because they include the output of the bet transaction as one of the inputs to the win/lose response transaction. If part of the bet transaction is doublespent there's a good chance it won't make it into a block. If the bet transaction doesn't make it into a block, the response transaction won't either and is therefore worthless.
  - ewillbefull 14y ago
    Nobody can cheat SatoshiDICE by double-spending and spending their response transaction, because the transactions are chained as you say. But this isn't the kind of attack they're worried about. If you broadcast a transaction and SatoshiDICE responds with a YouLoseTx, and you have built a block containing a double-spending transaction, if you "lose" you can broadcast your conflicting block. You wouldn't need much hashpower to do this and still remain positive. It's called a Finney Attack. https://en.bitcoin.it/wiki/Double-spending#Finney_attack https://en.bitcoin.it/wiki/Double-spending#Finney_attack This is why they only immediately respond to smaller bets.

jerguismi 14y ago

You mean that the machine operators could be doing a double spend? In every case they take the dollars before they give the coins, so you have to take the leap of faith... I don't find that scenario sensible at all. First buy $1000 machine, first customer comes in, buys bitcoins with $50, and notices that the machine does double spend. I believe he is not going to tell any of his friends...

ewillbefull 14y ago

Not precisely. The only time you really need to worry about confirmations is when you're dealing with larger amounts. It costs a lot to pull off a double-spend, more than a tiny ATM transaction anyway. This is why SatoshiDice processes immediately, except for bets over 2-5 BTC.

swhitt 14y ago
The reason why SatoshiDICE can do this is because they include the output of the bet transaction as one of the inputs to the win/lose response transaction. If part of the bet transaction is doublespent there's a good chance it won't make it into a block. If the bet transaction doesn't make it into a block, the response transaction won't either and is therefore worthless.
- ewillbefull 14y ago
  Nobody can cheat SatoshiDICE by double-spending and spending their response transaction, because the transactions are chained as you say. But this isn't the kind of attack they're worried about. If you broadcast a transaction and SatoshiDICE responds with a YouLoseTx, and you have built a block containing a double-spending transaction, if you "lose" you can broadcast your conflicting block. You wouldn't need much hashpower to do this and still remain positive. It's called a Finney Attack. https://en.bitcoin.it/wiki/Double-spending#Finney_attack https://en.bitcoin.it/wiki/Double-spending#Finney_attack This is why they only immediately respond to smaller bets.