What is Double Spending and How it can happen?

[BTC786]

What is Double Spending and How it can happen?

Since when we send the BTC payment it deduct the BTC amount from the balance. If so how it can happen?

[Rmcdermott927]

What is Double Spending and How it can happen?

Since when we send the BTC payment it deduct the BTC amount from the balance. If so how it can happen?

It is a type of exploit.   The person sends funds without a transaction fee, then sends again with a fee.   The transaction with the fee will get confirmed and the other will eventually get rejected by the network.   It can also happen if a mining pook has then 51% of the global hashrate.   They would technically be able to chose which transaction were confirmed and which weren't.

[alani123]

Double spending is only an issue if you can exploit it in some way. It's somewhat easy to double spend unconfirmed transactions (0 confirmations). After that, no luck. So if you are accepting BTC payments, make sure that you wait for a confirmation to release products or funds associated with the service you provide.

Now if you are looking to see how a double spend on an unconfirmed transaction is performed, someone has already made a guide:

https://bitcointalk.org/index.php?topic=231309.0

[Muhammed Zakir]

Double-spending is spending same bitcoin more than once. Miners ensure that the inputs for the transaction is not used to prevent double-spending.

Example:

1. Bob send 1BTC to Alice.
2. Bob send same 1BTC to Joe.
3. Miner confirms both transaction and included it in block.

A real example: https://bitcointalk.org/index.php?topic=152348
See https://en.bitcoin.it/wiki/Double-spending for more details.

It is a type of exploit.   The person sends funds without a transaction fee, then sends again with a fee.   The transaction with the fee will get confirmed and the other will eventually get rejected by the network.
 -snip-

Wrong. You can also double-spend a transaction with fee. The transaction which will be confirmed depends on miners. Most of the miners include transaction with higher fee and some includes older transaction.

[Hazir]

Prior to confirmation (which takes many minutes) bitcoin transactions are at risk of double spending. This means a fraudulent person could spend the same amount of bitcoin more than once .
I think is not a huge problem for normal bitcoin users, but can be a big problem for some bitcoin companies. If you are just typical bitcoin user I wouldn't worry about it.

[Dalord1]

Double spending is only an issue if you can exploit it in some way. It's somewhat easy to double spend unconfirmed transactions (0 confirmations). After that, no luck. So if you are accepting BTC payments, make sure that you wait for a confirmation to release products or funds associated with the service you provide.

Now if you are looking to see how a double spend on an unconfirmed transaction is performed, someone has already made a guide:

https://bitcointalk.org/index.php?topic=231309.0

Thanks for the information and the link you posted. It is really informational and very useful

[Mr. Burns]

impossible.  illuminati rpevents double speinding attacks

[hodlmybtc]

Thank god they are on our side

[BTC786]

How can people do double spending? What wallets is possible to do that?

[Amph]

How can people do double spending? What wallets is possible to do that?

you need to do an attack

for example if you have a huge hash power like 350 peta, you can just run a bitcoin client locally in solo mode, and thus creating a chain that it will be longer than the actual chain, they you are free to double spend, and do many other malicious activity

[notlist3d]

How can people do double spending? What wallets is possible to do that?

It's not something that is promoted or information that most will put out there.

There is really only one reason I have seen for legitimate use.  If you forget to pay miner fee and somehow you get some BTC that will not confirm.  But luckily most wallets will prevent this.

But if that happened you could do a double spend to get second transaction to confirm.   This is why you should always wait for a few conformations if doing a deal.

[alani123]

How can people do double spending? What wallets is possible to do that?

you need to do an attack

for example if you have a huge hash power like 350 peta, you can just run a bitcoin client locally in solo mode, and thus creating a chain that it will be longer than the actual chain, they you are free to double spend, and do many other malicious activity

In order for one to 'double spend' an unconfirmed transaction, he'd just need access tot he wallet/address the transaction was sent from and a wallet like bitcoin core.

[DannyHamilton]

How can people do double spending?

A real double spend, where more than one transaction using the same previously unspent output as an input are all confirmed in the blockchain, is completely impossible.  The revolutionary thing about bitcoin was that it found a decentralized way to make a real double spend impossible.

Most people around here mean something different when they use the phrase "double spend". As a matter of slang, many people will use the words "double spend" to describe a situation where a transaction is confirmed in the blockchain which has at least 1 input that has been seen in some other unconfirmed transaction at some time in the past.  That other transaction will never confirm and is considered invalid by the network.

What wallets is possible to do that?

With the right circumstances and expertise the second definition can be accomplished with ANY wallet.  It can be easier with some wallets than others.

[cbase]

Simple exploitation. Complicated mechanism.

[BlackMachine]

What is Double Spending and How it can happen?

Since when we send the BTC payment it deduct the BTC amount from the balance. If so how it can happen?

It is a type of exploit.   The person sends funds without a transaction fee, then sends again with a fee.   The transaction with the fee will get confirmed and the other will eventually get rejected by the network.   It can also happen if a mining pook has then 51% of the global hashrate.   They would technically be able to chose which transaction were confirmed and which weren't.

Not all miner will confirm the one with the fee instead of the one without the fee. Some nodes choose to just relay whichever transaction came first and if the transaction with the same UXTO appears, it will not be included in mempool.

[bumm]

What is Double Spending and How it can happen?

Since when we send the BTC payment it deduct the BTC amount from the balance. If so how it can happen?

It is a type of exploit.   The person sends funds without a transaction fee, then sends again with a fee.   The transaction with the fee will get confirmed and the other will eventually get rejected by the network.   It can also happen if a mining pook has then 51% of the global hashrate.   They would technically be able to chose which transaction were confirmed and which weren't.

But if the network rejects the transaction, then the funds should get returned to the sender. Eventually it will become a scam as the receiver will be waiting for confirmations which will never get done. Am I correct?

[BlackMachine]

What is Double Spending and How it can happen?

Since when we send the BTC payment it deduct the BTC amount from the balance. If so how it can happen?

It is a type of exploit.   The person sends funds without a transaction fee, then sends again with a fee.   The transaction with the fee will get confirmed and the other will eventually get rejected by the network.   It can also happen if a mining pook has then 51% of the global hashrate.   They would technically be able to chose which transaction were confirmed and which weren't.

But if the network rejects the transaction, then the funds should get returned to the sender. Eventually it will become a scam as the receiver will be waiting for confirmations which will never get done. Am I correct?

One of the either transaction should get confirmed and the other will get forgotten. Both of the transactions should be broadcast at the same time. However, it is pretty hard since nodes will not include the other into their mempool once they see the transaction spends the input that is same as the other transaction. You can still broadcast one of the transaction to 50% of the nodes and another transaction to other 50% of node.

If you make a payment but include 0 fee and have a dust output then perhaps, it won't get confirmed either. I doubt merchants would accept these kind of transaction unless they can't wait for a few seconds or check for competing transactions.