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Comment by kalleboo | Hacker News Reader

Is there any way a practical Bitcoin ATM could be done? I.e. a machine where you can get cash Euros/Dollars/Gold from your bitcoins. The naive approach is it just has an address you send BTC to, but then you have to stand around for 30 minutes while the transaction is verified.

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One was previously announced - seems to now have been made open source as well: http://bitcoinatm.com/

Something like that, but it could be done ahead of time - receive a bitcoin address & redemption code online, send your coins then ~30mins later go to the machine, enter your redemption code and receive your BTC.

Redemption code could also be a QR code. Put it on your phone & hold it up to the machine.

You can instant accept any bitcoin value that is lower to the investment to do the 51% attack or similar. Example: no one is going to make a 51% attack to steal only 10 bitcoin from a vending machine.

Simply put a limit, or just say to the user that any amount of bitcoin over X will need Y time to get verified. Anything lower that X amount will be instantly processed.

Doesn't it require ~10 minutes to solidify into the record to prevent double-spending without having the 51% attack?

Correct - there are various attack vectors besides the 51% attack. Ideally, a transaction shouldn't be closed until there have been 2 confirmations.

It would be interesting for a retailer/restaurant to have a Bitcoin ATM on-premise and offer a discount for people who paid that way.

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