Topic: Double spend solution? (Read 2443 times)

The solution is Bitcoin. Double spending is not possible because of the blockchain that seals the transactions and the order of them.

However, those sealed transactions can be leveraged in the form of smart contracts, to support layers like Lightning Network. Lightning Network can be used to send money instantly and 100% confirmed, without the possibility to double spend. But you need to have certain transactions (transactions which enable you to leverage them on layers), sealed in the blockchain.

The problem is the risk of doublespending.  
Here is one possible scenario: https://bitcointalksearch.org/topic/ghashio-and-double-spending-against-betcoin-dice-327767

If the difficulty is constant, the blocks would be a lot faster since the hash rate gets higher and the block generation time gets smaller and smaller.

Bitpay does offer an option for an instant approval as long as the transaction meets several criterions. It is by no means foolproof since an malicious user can broadcast another conflicting transaction and a miner can choose to accept the replaced transaction.

you may want to read about replace by fee

http://bitcoin.stackexchange.com/questions/10733/what-is-replace-by-fee



Also, the average time per block is not 10 minutes. It would be if difficulty were constant. But difficulty is growing because the average is less than 10. Somewhere between 8 and 9 minutes (which is still far from what Itskok suggested)

There is nothing at blockchain.info that differentiates between "double-spends" that are an attempt at fraud, and "double-spends" that are simply unconfirmed transactions that get re-sent.

I'm not aware of anyone that has put any effort to carefully determine how many fraudulent double-spend attempts there are. I suppose someone could question all the large businesses that handle many transactions (such as Coinbase and BitPay) and attempt to report some actual statistics, but I haven't seen anything like that published.


Confirmations have nothing to do with the number of miners.  It is the number of blocks.  1 miner (with enough hash power) can mine 10 blocks in a row, and 10,000 miners could work for years and not mine a single block (if they don't have enough hash power).

If they paid a small amount of money to miner that only has a small amount of hashpower, then the block will almost certainly be solved by someone else.  If they want to pay someone that has a LOT of hashpower, then they will need to pay a LOT of money (potentially more money than they are trying to steal).  Furthermore, as soon as the first block is solved with 1 of the two double-spend transactions, then victim will immediately know about the fraud.  There is no need to wait for additional confirmations.  Convincing a miner to throw away their chance at a 12.5 BTC block reward to try and mine a competing block would require the attacker to pay MORE THAN 12.5 BTC for every block that is solved while the attacking miner continues to try.  So, if the attacker wants the miner to stop trying to mine legitimate blocks and instead try to replace a block until 4 blocks have been solved, then the attacker is going to need to replace the 50 BTC worth of revenue that the miner is potentially foregoing.  All without a guarantee that the miner will even succeed.

This doesn't make any sense.  Why would an attacker pay a miner 50 BTC to try and steal 0.1 BTC from a merchant?


You weren't paying very good attention to the time.

On average, it will take 40 minutes to get 4 confirmations.  If you only did a few transactions, then you might get lucky and get 4 confirmations faster than usual.  In that case you might see that your transactions get 4 confirmations in less than 20 minutes.  However, you claim to have 100+ transactions, and in that case you either weren't paying attention or are lying.


Bigger blocks don't result in faster blocks.  It will still take an average of 10 minutes for each block to be solved.  This means that, on average, transactions will remain unconfirmed for 10 minutes (or more if they pay an insufficient fee), and that once a transaction gets its first confirmation, it will take an average of 10 minutes more for each additional confirmation.

Great.  So, you check to make sure the transaction paid a reasonable transaction fee, that the inputs are all already confirmed, that there are no excessively small output or unusual output scripts, and that there are no competing transactions being broadcast by other nodes on the network. Once you've verified all that the transaction is safer to accept than cash, check, or credit card (all of which can be used fraudulently).  What's the problem?


Actually several things have changed.  For example, back then transactions would make it into the next block even if they didn't include a transaction fee.  Now confirmations take a LOT longer if you don't include a transaction fee.  Also, some miners are now reluctant to confirm transactions that have excessively small value outputs.


Nope.  Pretty simple.  Check inputs, check outputs, check for fees and competing transactions.  That's it.


They are difficult to do, so they aren't common.  Credit card fraud, check fraud, and counterfeit cash are all probably more common than Bitcoin double spends. Perhaps businesses should think about not accepting credit cards and cash since there is such a risk?  Bitcoin would be much safer.


Only among those businesses that are too uneducated or unknowledgable to understand risk management and are incapable of comparing the bitcoin risks to the risks associated with the other payment systems that they accept.


Nope.  Visa transactions can be double-spent for more than 3 months easier than bitcoin can be double-spent in 1 second.
 

Then they don't understand it, and you've done a poor job of educating them.


There is no "Bitcoin Company" in charge of bitcoin.  Who would you ask for the information?  I'm not aware of anyone making any attempts to record all the fraudulent double-spend attempts or how many of them were successful.  You'll probably need to hire a polling company to do a scientific poll for you.  You could try contacting Coinbase or BitPay and see if they have any internal records they'd be willing to share with you.


What?  Why would you be any more sure with 4 confirmations than 2?  What's so magical about the number 4?


You aren't very good at maths, are you?  If the average time between blocks is 10 minutes, and you want to wait for 4 blocks, it's going to take a LOT more than 6 minutes (on average).  Fortunately, you don't need to wait that long in most cases.


Not sure what you're trying to say there.

Technically this isn't correct. Bitcoin transaction can be seen almost instantly, but can be reversed within ~10 minutes after it. Visa transactions can be seen almost instantly, but can be reversed within several months after it.

You could argue that making a double spend is easier than having a stolen/fake visa and I would agree. But then again, it's all about target usage + risk/reward. For an online order with real delivery, knowing 100% sure in 10-20 minutes is probably still better than knowing (let's say) 99% in 0 minutes (but still 99% after a week.) You won't deliver the product in 10 minutes anyway.

For some use-cases, like buying an expensive item in a real-life store (where you don't want to wait 1 min - 2 hours for confirmation), perhaps bitcoin isn't the ideal payment system. That's okay, you don't need to use bitcoin for everything.


That is true. Most likely there will be layer 2 solutions like Lightning Network that will allow accepting instant-transactions with much more confidence.

This is exactly what I am trying to do, not just analyzing the Nodes in real time, also analyzing the input's "history",
But it seems that not too much change from this days :
https://bitcointalksearch.org/topic/m.48365
Maybe its too complicated, or maybe there is not enough double spend transaction out there? what is sure is that the "Double spend" issue is one of the biggest barriers for Bitcoin to be adopted by "regular" payments companies/Financial institutions, Its 10 minutes Bitcoin against 0.08 mil second Visa.. they are really afraid from it as far as I spoke with some of them.
Someone know where I can find "All time" statistics about double spend transactions?



I afraid this is not enough, we need to wait for 4-5 to be sure, and that's gonna take 5-6 minutes at least, that's a lot of time for a digital money,
Maybe some Block-scale will solve that during the years too.

Well the blockchain does refer to the most secure one, so yes, the bitcoin blockchain

TBH I think theories/techniques of guessing the risk of 0-confirmed transaction are interesting too, so I am sorry to make such an obvious statement

But in reality (one of?) the biggest problem(s) bitcoin organically solved is the double-spend problem and the solution is really the blockchain.




Of course if anyone wants to accept 0-conf TXs based on risk/reward.. that's up to them. For gambling sites I know plenty of sites have been scammed and changed it to 1-conf, so only few sites accept 0-conf (to be fair, for example Luckybit has a pretty good record on that though.)

does any blockchain solves it?
or you are talking about bitcoin blockchain? 

There is a solution for the double spend problem, it's called the blockchain.

You should not do that. Do not trust unconfirmed transactions, the bitcoin network doesn't ask for or promise you, that trust.

Simple solution would be to wait for confirmations (at least 2)

The solution is to analyze the transaction and determine how likely the transaction is going to confirm in the a few blocks. Generally you should look at the size of the transaction (a large transaction may be less likely to get into a block), the transaction fee per byte, the outputs (for dust outputs), whether the inputs are confirmed, and whether it signals Opt-in RBF. By examining these, you should be able to decide whether you think that transaction will confirm and whether you should accept it. If it opts-into RBF or spends unconfirmed inputs, you should wait for at least 1 confirmation as that transaction could be replaced.

The only real sollution is for the sender to add enough fees, and for the receiver to wait at least for 1 confirmation (preferably more). AFAIK, there are no alternative sollutions, the network allows you to broadcast as many transactions as you want, re-using the same inputs. The miners are free to pick any of those transactions, and once the input is used in a transaction that was included in a block, the other transactions using the same input are cancelled...

Hi.
I was wondering what is the most up to date solution for the famous "Double spend" problem?
Is there any solution (commercial company development even..) that reduce the transaction waiting approval time?

I read lot of paper's and suggestions during the years about that but it seems that most of them was just nice academic theories , I think that each industry (Exchanges/Gambling/Miners..) adopt here own and personal solution, the most relevant thing I found for now is the Blockcypher Confidence Factor API:
https://www.blockcypher.com/dev/bitcoin/#confidence-factor

I am working on something of my own for this :
https://riders.io/

Although that as a "personal" Bitcoin user this problem has never bothered me at all, I really trust the network, even with a low fees,
But as a business I believe its a total different story.

Thank you.