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Author Topic: Bitcoin ATM double spend  (Read 6602 times)
bitkilo
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September 18, 2014, 05:08:26 AM
 #1

I have seen a few posts on 2 way atm's that only need 1 confirmation before it will dispense fiat, how is double spending avoided when only relying on 1 confirmation?  

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CoinATMRadar
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September 18, 2014, 06:20:37 AM
 #2

Based on risk management. Double spend attack is not something easy every kid can do + also there are always amount limitations on the transactions with 1 confirmation.

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September 18, 2014, 06:39:17 AM
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So in other words it is possible for someone to rip off a bitcoin atm if they really wanted to.
What are the limits and are these per person, per transaction daily limits?

cryptozio
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September 18, 2014, 06:54:01 AM
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Double spending bitcoins is almost impossible, no need to worry for the 1 confirmation transactions.

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September 18, 2014, 08:29:42 AM
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it is real and possible for someone who has 51%
zetaray
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September 18, 2014, 08:38:27 AM
 #6

There must be a maximum cash limit that one ATM can handle, together with the difficulties of double spending, ATM operators have little to worry.

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September 18, 2014, 08:50:57 AM
 #7

Bitcoin ATMs are being held to the functionality standards of a full bank. That's fine, but additional security measures will need to be taken for the convenience of a full 24/7 bank. Vouchers can be issued online or on location to mitigate verification attacks. They can also be used for multisig transactions via ATM.

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September 18, 2014, 03:51:34 PM
 #8

I have seen a few posts on 2 way atm's that only need 1 confirmation before it will dispense fiat, how is double spending avoided when only relying on 1 confirmation?  

It is very difficult and very expensive to double spend a transaction that has 1 confirmation in the publicly shared blockchain.

Bitcoin ATMs limit the maximum amount that you can withdraw in cash in a single transaction and then charge a fee for that transaction.

This creates a financial disincentive to try and double-spend.  The attacker would spend more attempting the double spend than they would steal, and their theft would not have a 100% success rate.  This means that any attempt to double-spend against the ATM would result in a loss of total funds for the attacker.

The fee that the ATM charges is high enough to result in profit for the ATM even if an attacker occasionally succeeds.  If the fee is $5 and the maximum withdrawal is $500, then an attacker would have to succeed more often than once every 100 attempts in order for the ATM to experience a net loss.

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September 19, 2014, 02:29:22 AM
 #9

I have seen a few posts on 2 way atm's that only need 1 confirmation before it will dispense fiat, how is double spending avoided when only relying on 1 confirmation? 
One confirmation is actually a lot more secure then you think. You would need to spend a lot of rent enough hashpower to find one block in a day, it is generally more expensive to rent hashpower then the expected reward of that hashpower during the time of the lease. The current block reward is >25 BTC (including TX fees) therefore you would need to spend ~50 BTC in order to create a chain long enough to double spend a TX to a BTC ATM. This is roughly $21,000 and as said above it is not a guarantee that your attack would succeed. If your attack is not successful then you would likely be out the entire amount you spent trying to double spend the TX to the ATM.

I am not familiar with the limits as to how much these ATMs are able to disburse, however I think it is safe to assume the limits are well under $20,000

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September 19, 2014, 06:09:53 PM
 #10

The blockchain takes part of the double spending problem.
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