Bitcoin Forum

Okay, I'm confused about how SatoshiDice works, I've been reading up on it and basically, from what I've gathered, how they instantly send you your winnings back is by including your transaction in the return amount, meaning, if your transaction bounces (Double spend), so does their payment to you as one (of the many) input transactions is false (As it was your transaction to them, which, just bounced).

Now, my question is, what happens if I broadcast a transaction to Satoshi Dice with a very small (if not NO) fee, so, confirmation takes AGES* but it still get propagated by the network (Meaning it's not being mined, but, it is being talked about), then, if I lose, simply send the same transaction input again but with a higher fee (Less than what I bid, so, it's still profitable to me) so it's put into a block before the wager transaction, and, the wager transaction is cancelled? If I win, of course, I simply just leave it. It'll take awhile to confirm, but, at some point, it will confirm and my money will be spendable.


Note:- 'ages' in this context is defined as any period of time that's longer than the amount of time it'd take for a block submitted at the same time to be confirmed with five times the recommended fee (E.G. .5 mBTC (0.0005 BTC) per KB).

They don't "publish" the win/loss transaction until your payment is confirmed. You have no information whether to double-spend or not.

Unfortunately I can't afford to below $10 to test anything (I believe that's their lowest bid amount), and, the last time I used the service was about a year ago, back then, they'd instantly send you the money back (You'd see it in your wallet), has that changed?

Yes, after an "academic" double-spend was performed, along with long chains of unconfirmed bets making users never get paid when their payout was mixed with the unconfirmed pool.

Can you link me to a thread/source/information of said academic double spend?

Also, let's remove Satoshi Dice from the above example, would, in theory, it work to scam any system that used the method Satoshi used?

I'm not entirely sure on this, but here's what I remember. They're able to send the BTC back instantly because they use the Bitcoin you sent as an input. If a single input is invalidated, it invalidates the entire transaction, therefore if an invalid output is sent to satoshidice, the output they send back to you will also be invalid. It's protection. At least, that's what I remember hearing, though I'm not 100% on that. It should be comparable with any service where they send instant return transactions.

Isn't that what I said in the OP? My question was, considering you'd know instantly if you won/lost, couldn't you submit a transaction with a higher priority back to yourself, so, you get the money back minus a MUCH smaller miner fee?

Huh, that makes sense. I guess I was thinking of this in a different fashion. There has to be some sort of safeguard against this. I can't test this, since it'd be expensive to, but it's possible that Satoshidice looks for duplicate inputs in blocks. This should provide some protection against double spends.