What is a Bitcoin Double Spend and how does it work?A double spend is where two different transactions sent into the Bitcoin network are trying to spend the same account balance.Bitcoin naturally defends against this by confirming which the transaction which is included in a block first.If two transactions are sent into the network using the same private key and the same funds but sent to different bitcoin addresses – then as the transaction propagates through the network one half of the network will accept one transaction and the other half, another.The situation is resolved by which side of the network resolves one of the two transactions into a block first.So if you try and spend your bitcoins twice – once to a merchant and once back to another address under your control, then you will have a 50% chance roughly of regaining your money and getting the product for free if chance favours your address as being the bitcoin networks consensus .This does depend though on which transaction gets propagated further into the network.

For example if transaction A reaches ten mining nodes controlling 80% of the network hashing power first and the other transaction only reaches 20%, then transaction A will have an 80% chance of being included in the next block and be the confirmed transaction.There are ways of safeguarding against this – whereby wallet operators keep strong links into mining nodes to propagate their transactions fed directly in – so another transaction spending the same output can’t be fed into the network on a fraudulent basis.Related guides What is a BIPS – Bitcoin Improvement Proposal?What is Bitcoin transaction locktime?What are the Bitcoin Transaction types?What is a Block Header in Bitcoin?What is the Bitcoin Genesis Block?What is Zcash Who are the CypherPunks?Popular guides CryptoCompare Portfolio FAQ Where to spend your Bitcoins How to use MyEtherWallet How to mine Zcash - Who we are!How to Add Sold Coins on the CryptoCompare PortfolioDouble spends on this page may be unintentional.

In the event that a double spend is maliciously crafted being listed on this page is no indication that it was successful or any merchant or user lost money as a result.Included in block #470659 6f3a210851021881f5772764ae36f3740e7bef28d03167415579b0db67f28590 1DwxyDJygXrokRf4SUwYE7MMgdPaPq8eH6 - (Spent) 14W59QVYbKcY733Ng8z9xXSLd9uyLCnupJ - (Spent, Spent)bitcoin youtube tamilBitcoin Sign up or log in to customize your list.bitcoin nachteile_ Here's how it works: Anybody can ask a question Anybody can answer The best answers are voted up and rise to the top up vote down vote favorite 6 What is a double spend?bitcoin hard fork 2017As someone who uses Bitcoin, what do I need to know about how the Bitcoin system prevents double spends?ethereum technology stack

Are there still circumstances where they can occur?transactions blockchain security doublespend attack up vote down vote A double spend is an attack where the given set of coins is spent in more than one transaction.There are a couple main ways to perform a double spend: Send two conflicting transactions in rapid succession into the Bitcoin network.bitcoin virgin spaceThis is called a race attack.litecoin make poolPre-mine one transaction into a block and spend the same coins before releasing the block to invalidate that transaction.This is called a Finney attack.Own 51+% of the total computing power of the Bitcoin network to reverse any transaction you feel like, as well as have total control of which transactions appear in blocks.This is called a 51% attack.To prevent damages from the first attack - wait for one confirmation to appear on a given transaction.

To prevent damage from the second attack - wait for 6 confirmations to appear on a transaction, or less if the transaction is small (but still require at least 1).Damage from the third attack can cripple the entire Bitcoin network, so don't worry about it - your business most likely won't be the main target (it's unlikely to happen without really big money getting involved).For more information about all those attacks, you can check out my master thesis on Bitcoin security.up vote 4 down vote As a merchant, you can reduce the likelihood of losses from a race attack double spend by having your node properly configured (no incoming transactions, explicit outgoing connections to well-connected nodes).There still is a tiny risk of getting cheated even with this configuration but it is rare and relatively random.Thus the disincentive to the attacker is that if success in double spending only rarely occurs, each failed attempt is a profitable sale to the merchant and thus in the long run it is unprofitable for the attacker and profitable for the merchant.

There are circumstances where a merchant is more vulnerable.An unattended coin change machine at a laundromat, for instance, would be the worst case scenario for the merchant.The attacker loses nothing for failed attempts (presuming the machine is not taking any profit from each "sale"), takes the gains on the occasional successful attempt, and is not likely to get caught for committing fraud as by the time the laundromat operator is aware anything happened the thief is long gone.(Of course, countering this is the likelihood that blockchain monitoring would have identified the numerous double spend attempts and thus the laundromat operator can prevent even this from occurring.)The Finney attack also has costs that make it less of a threat than it would seem.Holding a block costs about a dollar a second.So if once a block is mined but not broadcast and it then takes the thief forty seconds to complete the transaction with the merchant, there had better be a lot more than $40 worth of profit from doing so otherwise the attempt ends up being uneconomical over the long run.

Again, the merchant would know eventually that the double spend had occurred (measured in seconds, if monitoring the blockchain), so this doesn't work well in circumstances where the thief risks getting caught.So the prevention for this is to simply not make large value transactions (e.g., hundred dollars or more) on 0/unconfirmed without some delay where you are watching for double spending.The 51% attack that would reverse confirmed transactions is so expensive and thus such a remote chance of it occurring, it is not even of concern for a typical merchant.(i.e., someone spending millions of dollars to double spend using this attack vector, if it ever happens, is going to go after trades where large exchanges of value which occur -- think gold bullion traded for bitcoins under a bridge at midnight.They are not trying to buy your consumer electronics that you ship out via UPS nor are they meeting you at Starbucks hoping to defraud you of your $200.)http://en.bitcoin.it/wiki/Double-spending up vote 0 down vote If everyone could spend the same Bitcoin twice, then the whole system would collapse because Bitcoins would not be scarce.