Bitcoin Forum

good way to start out small and possible place to buy sell btc

Do you mean bitcoin atm?

OK if you don't mind standing in front of the machine for 10+ minutes while the transaction clears so it can finally deliver your Coke.

Good point.  Not to mention, the Coke machine would have to be connected to the Internet.  However, if we get to a point where cheap WiFi can be located everywhere, I wouldn't complain.

Zero-confirmation vending machine?

Mind you, I could envisage some low-moral-fiber nerdy kids double spending BTC for a free soda the way they manipulated analog payphones in the 1960s to make free calls.

I think this is beginning to show that there might be a need for someone to make more offline options for Bitcoin :P

https://en.bitcoin.it/wiki/Myths#Point_of_sale_with_bitcoins_isn.27t_possible_because_of_the_10_minute_wait_for_confirmation

Double-spending attacks are not very reliable nor practical. One would end up buying many, many sodas before getting lucky enough to get that one free soda. I'm sure vendors can easily accept something like a 1% or less double-spend risk; some vending machines even take credit cards, and those have even higher fees to cover chargebacks. A sub-1% "fee", plus not having to regularly collect coins and bills from each machine, should be seen as an advantage over cash and credit cards.

Just curious; if a payment had just one confirmation, what are the odds that it isn't legitimate?

To do this, all you need is a QR code reader that can read a private key, in the form factor of a bill acceptor.  The hardware may already exist, needing only the right software development.  At the local gas station, the gas pumps have barcode readers for scanning loyalty cards.  That sort of thing could easily be modified into a bitcoin bill acceptor.  That particular device probably won't read QR, but it's possible to encode a minikey into a linear barcode using Code 128 symbology (I've done it, you get a barcode about 3 inches long).

It can take the amount off and send the change to the same bitcoin address it took it from.

Are you saying this from the context of having a de facto standard paper bill format for bitcoins? I can see how that would work from a technical standpoint, and I think I now understand the purpose of printing out bitcoins onto bills of different denominations.

The only minor issue I can think of is what to do with the remainder on the bill. Should you send it to a fresh new bill and throw away the used bill? After all, the private key is possibly compromised once it was scanned at the POS terminal (or you can trust that the merchant does not retain a copy of the key).

It doesn't need to be any specific format.  The user simply needs to hold it up to the machine so the code can be scanned.


If the bill only contains pocket change it's not a big deal.  I doubt many machines will be programmed to scam customers.  This could already happen today (gas pumps, credit card vending) but overall it doesn't.  Just think - even today, each time the gas pump asks "PRINT RECEIPT" and you press "NO", they could program it to scam you an extra $5 and you'd probably never notice or be able to prove it because they know you don't care about the receipt.

How about getting the pop machine to broadcast your transaction?