Bitcoin Forum

I know little about software and even less about hardware, so I'd like to hear if this idea is viable or pie-in-the-sky.

Imagine you are going through the drive-through at your local McDonald's/Tim Horton's/Dunkin' Donuts to pick up your morning coffee.  When you arrive at the ordering point the restaurant senses a transponder in your vehicle attached to an Android/iPhone that identifies you.  When you get to the pickup window the cashier hands you your order and the cash-register automatically debits your bitcoin wallet via the transponder for the amount of your purchase.

Basically it is doing the same thing as transponders for toll highways.  However what I am proposing is that a single transponder could serve for purchases at multiple different vendors. 

Search the archives for the term Dash7, but there should be no automatic deductions or transactions, or someone is going to make such a transponder and hide it in the airport terminal.

A twist on the idea: the transponder in the car could have a small screen to display a price, along with a button to pay and a button to decline payment. When the order is finalized, a radio signal sends the price to the car, along with the bitcoin address to send the bitcoins to. The screen displays the price and the driver must press "accept" to send the funds. A little less automatic than your idea, but certainly faster than what we presently have. Not sure if it would be worth the cost, though...

you can do that with bitcoin because of the ~10min conformation time, so you have to sit in the McD. waiting for your 3.34btc gets cleared...
a bank(trusted 3. party) should be involved, so its going to happen this way: you deposited 40 btc last week, to a bank. then you drivethough the McD, and the bank is paying McD for you. this can only happen if McD and you are trusting the bank, McD are trusting that the bank will pay them, and you are trusting the bank not to spend your money without your appoval.

Yes, it can.  Confirmations are exactly what they appear to be, confirmations.  Specificly they are confirmations that this is the one-true-transaction that will ever be.  They are not confirmations that the transaction is valid, nor are they confirmations that the person in the drive through was the true owner of those coins just a moment ago.  Those things can be verified by the McD's own client by scanning the relevant blocks and the transactions waiting in it's own queue.  The double spend attack is technically difficult, and made more difficult by the fact that vendors such as McD's have a strong vested interest in making certain that, not only is their blockchain accurate to the moment, but they also are looking for opposing transactions in the mining queue.  If they don't see such a transaction (or are not notified that one exists by a contracted agency) by the time you make it up to the window, it's very unlikely that any other transaction would be able to trump their transaction even if you had attempted a double spend.  That is, so long as the Internet connection for the McD's wasn't down.  And if you had the skills to do this to McD's, would you go to all that trouble over a free lunch?  Not likely.  Bitcoin banks will exist, but they will not be necessary for the vast majority of small retail transactions.

And besides, for as long as you sit in the drive-through line, a couple of blocks could be mined in that time anyway. :)