Bitcoin Forum

Most of you have probably heard the term double spending quite often and even discussed it quite often, so maybe I won't explain in detail anymore or you can read more here (https://en.wikipedia.org/wiki/Double-spending). So far, the RBF feature shared by several bitcoin wallet, both bitcoin core and electrum as well as many other wallet allows senders to double-spending transaction. This can trigger fraud if the recipient of the transaction does not have sufficient understanding of how to prevent it.

So now that bitcoin has been legalized in several countries as legal tender, do you think this double spending can prevent bitcoin from being used by many as a secure mean of payment so that adoption rates slow down?

I'd like to hear some of your thoughts on this.

No,
1) Mostly because it's just about impossible to be done. Not saying it can't be done, just that it is just about impossible.
2) Even if it can be done, if you are accepting BTC or any crypto for any large item you really should wait for more then 1 bock before handing over the merch.
Once you are 3 blocks in the odds of a double spend happening are zero*.

*Or close enough to zero not to matter.
**We are talking about BTC, the real BTC. Not BSV, BCH, or any other crypto that claims to be BTC.

If you are talking about one of those, you are on your own when it comes to double spends.

=Dave

Technically, UTXOs can be double-spent even if the transaction hasn't been flagged as RBF. So, even if RBF feature didn't exist, people would still have to wait for the transactions to be confirmed.
(Sometimes, it's needed to wait for more than 1 confirmation)

Is there any country other than Elsalvador that has accepted bitcoin as a legal tender?

Double spending unconfirmed transactions has always been possible, long before RBF was enabled, and once a transaction has a single confirmation its RBF status is largely irrelevant. The only way RBF enables fraud is if you find a merchant who understands very little about bitcoin but is also somehow running their own payment processor (and not using a hosted third party service) who then accepts zero confirmation transactions which are opted in to RBF.

No. You either don't accept transactions without confirmations regardless of its RBF status, or you state that RBF must be disabled to allow zero confirmation transactions (up to a certain value you are willing to accept). If someone pays with RBF, then you require at least 1 confirmation.

To add to o_e_l_e_o's post:

In addition you should check the RBF status of the transaction, you should always check whether the transaction has RBF-enabled unconfirmed parents or not.
In the case the parent transaction is unconfirmed and flagged as RBF, it can be replaced by a new transaction and makes the main transaction invalid. This can be easily done even if the main transaction hasn't been flagged as RBF.  

That's right, there is always the possibility of this happening and I know that transactions that are still in 0conf state are not safe. I just want to know how the situation is for those who don't understand it so well that online buying and selling transactions involving users who don't understand how it works may become victims of rogue shoppers. Dave, I don't want to talk about other coins. I just want to talk about the possibility of double spending with bitcoin. So ignore BCH and BSV.  ;)

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Previously I didn't know that non-RBF transaction could also turn into double spending, maybe technically I don't understand but it's still possible if it happens because of another attack.

I haven't heard much, but at least El Salvador can be a good example of my question. Bitcoin is a new means of payment for them, so they have to be really careful in buying and selling transaction, especially if it is done online.

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Since this is not about me, I think every trader who accept bitcoin as a means of payment should understand and know how the possibility of double spend transaction can be prevented. 0conf transaction are not secure, meaning they must get at least 1 confirmation before agreeing to a trade.

If its online it's 100% not important.
You don't ship the product till you have 3 (or 6) confirmations. Once you have that many, you don't have to worry about double spend.
It's online so it's not like you have someone at the counter waiting for 3 blocks before you hand them their lunch.

Same with payments. If someone is sending you BTC and expecting you to send them some form of fiat, they just have to wait for a few blocks to be mined.

-Dave

You should never accept zero confirmation transactions which have unconfirmed parents, regardless of the parents' RBF status. Transaction malleability allows a node or miner to change a transaction in such a way that the signature remains valid and the transaction itself remains valid (and so can still be broadcast and mined), but the hash of the transaction will be changed, therefore invalidating any children based upon this unconfirmed transaction. Protecting against transaction malleability was the main reason behind segwit, and so if all the unconfirmed parents only spend native segwit inputs then this isn't a concern, but since the majority of inputs in circulation are not native segwit yet then it is safer to just not accept any unconfirmed transaction with any unconfirmed parents.

Lastly, to @DaveF, @hosseinimr93, and o_e_l_e_o, do you three think that double spending could pose a threat to the growing adoption of bitcoin as a mean of payment?

So far I've only seen @o_e_l_e_o answering unrelated to my question above if I'm not misunderstanding it.

I think pretty much anyone who installed a Bitcoin wallet and conducted a few transactions understands that unconfirmed transactions can't be trusted, so no business will provide their end of the deal until the payment has been confirmed. Some will even require a large amount of confirmations like 50 or 100 if the transaction is extremely valuable.

For small payments we have Lightning Network, so it all won't be a problem.

No, I don't.

For any transaction which does not need instant confirmation such as fast food, coffee, groceries, etc., then the merchant is going to wait for at least a few confirmations before handing over the goods or services, rendering double spending impossible. For any transaction which does need instant confirmation such as the examples I just gave, then the merchant can choose to accept zero confirmation transactions which are not opted in to RBF up to a certain value if they like, or they can opt to use Lightning instead.

Note that it is significantly easier to reverse a credit card transaction, and this can be done for months after the transaction, than it is to double spend an unconfirmed non-RBF bitcoin transaction, which can only be done for 10-20 minutes after the transaction, depending on fee and mempool.

Double spend has been a problem since a long time now. There are scammers who trick newbies into sending them other coins while doing exchanges or services.

The only solution is proper education or using wallets that clearly shows you that the transaction is NOT CONFIRMED. Once the transaction is confirmed at least 1 time, there is nearly no chance for double-spend unless the scammer has paid a miner to confirm the transaction. Such things do happen, right?

I think Bitcoin should be used as an asset more than a transaction currency while there are alts for quicker and cheaper transactions.

Even if someone pays a miner and ask them to confirm such a transaction, that wouldn't be helpful.
A block including such a transaction won't be accepted by nodes and other miners won't add any block to the chain including that block.
If a transaction has been confirmed, for a successful double-spending, the miner needs to have a significant amount of hashrate.

Are there any wallets that do not show unconfirmed transactions as such? I know that some new users might not know what that means, but I have never seen a wallet not tell you that fact.

But, if you look at the amount of stories you hear about people getting scammed with Zelle, Craigslist ads, Facebook marketplace scams, etc. I don't think it's going to make a difference to BTC adoption.

-Dave

They using their own local exchange to transact and I doubt that double spending will ever happen to them. As far as my experience, I don't have any sort of double-spending error in my past transactions using my local exchange and I'm also pretty sure the same case applies to my fellow countrymen since no one has ever posted something like that in our local board. I think they need to focus on fast and reliable local exchange if they wanted to avoid such kinds of errors.

If you send little amount 1 confirmation is enough, but if you send large sum you need to wait at least 3 confirmations. I don't understand what you mean the scammer paid a miner, anyone who create transaction will always pay fee to miner and you can't choose which miner to confirm your transaction because it will be chosen randomly from mining pool.

Depends on the local exchange terms about the block confirmations, but most of them required more confirmations in order to prevent this problem.

Without going into technical language, i would like to clarify that bitcoin can't be double spend. If there was any possibility of double spending, bitcoin wouldn't have so high marketcap and people wouldn't invest in assets which are not reliable.

Also if any day, someone double spent bitcoin, it will be the end of bitcoin adaption and people will panic sell everything.

What you are asking is called "cost of doing business". It is a risk that any merchant (who didn't open shop yesterday) knows and accepts. Even if you are dealing purely with cash you still can be scammed if someone gives you fake bills. And that's just cash, add the other digital payment methods and you end up with a lot of other ways that the merchant could be scammed (cash backs, credit card fraud, ...).

The real question is how many times that is going to happen. For example how many people in a year scam a food truck? How many would scam the food truck if they were accepting bitcoin?
The reality is that scams like this don't really happen, regular people (the customers) don't even know what a double spend is let alone think about scamming someone with it.

Also as it was mentioned one could wait for confirmation if the bill is big. Not to mention when bitcoin is legal tender and the purchase is big, there could also be legal problems for the one scamming.
Let's say someone bought a car and then scammed the dealership by double spending the coins. The seller can easily bring the buyer to court since they have all their information, for scamming and the scammer will end up paying a bigger fine and possibly have to go through some jail time.

So to answer your question, No double-spending is not a threat to bitcoin adoption.

Same here, i never come across any incident where bitcoin was double spend successfully however there were some unsuccessful attempts.
For me, bitcoin has no threat from double spending as it is impossible to double spend on bitcoin block chain.

The idea would be that you broadcast a low fee transaction publicly which the other party would see via their wallet or block explorer, and then while it is sitting unconfirmed in the mempool you would pay a large miner privately to instead attempt to mine a conflicting transaction which your trading party is unaware of.

It depends on how you define "double spend". No, the same UTXO cannot be confirmed in two different transactions, but the same output can certainly be spent in two different transactions as long as those transaction remain unconfirmed. There have been a very small number of events when a transaction with 1 confirmation has been gone back to being unconfirmed and invalidated by a different transaction spending the same input due to stale blocks.

Simple instruction for sellers: wait for confirmation of transaction, will prevent any chance of double spending scam.
People can get tricked in crypto by some many ways if they don't understand how it works, and chance for double spending to happen in real life is much less than you think.
Fake wallets and fake bitcoions are more likely to happen, some people will convince you that bch or bsv are real bitcoin and you get scammed purchasing that garbage.

I think that only El Salvador made Bitcoin a legal tender so far, but most people can only buy something with Bitcoin using government made add custodial wallet Chivo,
and I think that is based on Lightning Network so there is no way you can get scammed with double spending.
Some people tried to use other wallets for payment in El Salvador with QR codes but they are not working, except Chico and Muun wallet with LN.

Thanks for all your thoughts on my doubts about the possible double spending that can occur in a country that adopts bitcoin as a mean of payment. I'm not so sure about the bottleneck in accelerating adoption just because of double spending. So since I've got great answers from you then I'll lock the thread.

Actually the thread is not meant to lock by now except you wishes to lock it, because right now their is more people that will come across the topic and also render their own positive point of view concerning your question, it's obvious that we have thousands of users who are highly intellectual and much experience that can give another statement or prove to a question, from me i would say that you should leave it open.

Okay, that would be great actually. I'll take your suggestions and leave them open for now. So far, I think only intelligent users or thinkers dare to enter and give their opinions in this thread. Not much, I don't see a single bounty hunter coming to give his opinion. Are they scared because there are too many muscular men in them? LOL