Bitcoin Forum

Well, first of all hi to everybody, i'm a newbie in this cryptocurrency thing, so a lot of doubts are in my mind these days.
I have this question, looking here for the answer I didn't found yet, basically the subject says it all.

If bitcoin transactions are irreversible, why some platforms (online wallets, exchanges, etc) need X confirmations to "validate" a transaction?

You answered yourself. Exactly due to fact that bitcoin transaction are irreversible it is wise to wait for at least 1 confirmation of the network.
Unless there is 1 (or more confirmations) there is a risk of 'double spending' of bitcoin.

There is always a risk that the blockchain might fork, which means there will be two blockchains for a short duration until the miners decide which is the best chain. Coins on the blockchain that gets discarded can be invalid on the other blockchain. After six confirmations it's safe to assume there has been no fork, and that your coins are safe. Most exchanges wait for six confirmations to protect themselves from the possibility of a fork.

To prevent a double spending attack, at times the network can take more than 10 minutes to validate the transaction and if a block is not found quickly then in order to ensure that their is no mistake between two different communicating transactions to different addresses being propograted exchanges use 1 to 3 confirms before confirming the amounts.

Double spending risk always exist unless the transaction is confirmed and included in a block before the last Bitcoin checkpoint. Without any confirmation, the risk of race attack is very high. After 6+ confirmation, without significant hashrates, the risk of double spending is reduced. Unless there is a major block reorg which is quite impossible unless your client is bugged.

51% hashrate is a number which miners need for a double spend attack to succeed with 100% chances. The lower the percentage, the lower the chances. Services that require significant amount of confirmations probably do not want to lose their coins at any case for example, exchanges. For normal brick and motar stores, 1 confirmation or even 0 confirmation is enough for the risk to be reduced. That is if precautions are taken.

Saying that "a bitcoin transaction is irreversible" is a simplification. A transaction is not reversible in ordinary circumstances, but...

Without getting into too much technical detail, a transaction can possibly be canceled before the first confirmation by simultaneously making another transaction that spends the same bitcoins (so-called "double-spending"). If the competing double-spend transaction is the one that is added to a block, then the original is canceled (and thus reversed).

It is possible for a block in the block chain to be replaced by another block. There is about a 1.5% chance of that happening to a newly added block. So, even if the original transaction is added to a block in the block chain, there is a small chance that the block is replaced by a diiferent block that may contain the competing double-spend transaction, which would cancel the original transaction.

The permanence of a block depends on the number of blocks that follow it, so waiting for a certain number of blocks (typically 1, 3, or 6) reduces the chances of becoming the victim of a double-spend.

to prevent double spending/fraud transactions

to make sure all fee is paid and so transaction confirm by network. If does not confirm money is useless and go back to sender!!

Transactions without fees can get included into a block too. A block typically have 50kb reserved for that. For a free transaction, you need to have the size below 1kb, all outputs above 0.01BTC and the priority must be high enough. If not enough fees is paid, the merchant can use CPFP and rebroadcast it continuously to reduce the possibility of a double spend.

it's a weakness of bitcoin sadly, to rpevent some strong attack like 51% 40% and 30% of hash rate, like the finney attack for example

it's a form of security, like bank use cvv for their credit card, or multiple password etc...

It is imposed so as to prevent double-spending attempts on the network and to 'validate' transactions happening within the network. Another thing is what one user already mentioned about forking.

As others said it is to counter the risks of possible double spend. But the risk is only if you deliver some instant service, and if you sending the goods physically or generally deliver non instant service then you dont need confirmations because you simply cancel the order once you detect double spend.

To avoid double spending, and usually online store just need 1 confirmation. Why 1? Because they don't want the customer to wait for another confirmations because it would take a longer time (1 is enough).

business need protection from double spend, which are transaction that get sent back to sender if miner cannot confirm it.

Transactions are validated as nodes relay it, Bitcoin nodes do not relay invalid transaction. It is only to prevent coins from being spent twice by recording it as a ledger. Before it is included in a block, it is not recorded in the blockchain.

I guess my understanding is somewhat crooked when it comes to validation of transactions within the network. Thanks for pointing me out on the right way. You got to learn new things--or correct your knowledge--everyday.

they do this so all transaction validated by miner on the network.  so they are valid.

This confirmation things sucks sometimes... A while ago, there were some "flash" sales going on, and I ordered something. The deals expired every 1 hour and then they replaced it

with another item. I scanned and paid within the hour, but the transaction did not go through due to "stress testing" going on for nearly 2 hours 20 minutes. {You remember those

times?} Well the site confirmed the sale, once 3 confirmations went through, but they made me pay the full amount, less 20%. I lost out on a less 50% discount.  >:(

Even with 0 confirmations the risks of double spend are insanely overblown. Unless you are paying a lot for something really important, then chances are you can be perfectly ok with 0 confirmations and sleep better at night, certainly better than credit card transactions.

The risk is not zero though.   Double spends can happen so why not prevent them?

With waiting for a few confirmations you stop double spending, and assure you receive the BTC.  So I think it's worth it and we will people continue to wait.

For daily transaction, the merchant is more likely to pay more in fees or accept a fraudent note compared to the amount of double spends that can happen. If the merchant takes precaution, it's hard for anyone to double spend unless a mining pool cooperates with attacker which is quite expensive and it isn't guaranteed unless it owns majority of network. The only problem I can see is in a spam attack, if the user does not pay sufficient fee, it can take sometime.

The worst case scenario is if pools accept RBF patch. If it does happen, users can simply use another transaction with higher fee and relay to the pool to replace the existing transaction. Won't succeed 100% of the time and the reference client don't even relay double spends.

they actually happened a couple of times, and i remember one of them was with okpay https://bitcointalk.org/index.php?topic=152348.0

It happened only once in Bitcoin's history. Of course it could have happened again if the merchant didn't run a full node in the recent chain fork. In the incident you said, the merchant wasn't on the correct fork which no amount of confirmations would help. There isn't much protection against this except to cease payments if a fork is reported or network alert is issued.

it's actually two times and maybe three i have my source, the second one is this https://bitcointalk.org/index.php?topic=327767.0


p.s. third one is a perhaps, it was told to me by an user in one post, i don't remember it anymore, but maybe i'm confusing it with the second one, but anyway you have two attempts

A double spend is defined as spending a coin twice and both getting accepted by the miner. It is quite impossible without the miner's participation.

Ghash.io sent a 0 fee TX which has a very low fee. If they win, they will mine the transaction and if not, they will mine a replacement transaction. This is very easy to do if you have a mining pool behind you with large hashrate. Betcoin revealed the result before the transaction was confirmed which is quite bad. Currently, satoshibones as an example only reveal the result if the unconfirmed TX has at least 0.0001BTC/KB of fee which is more than sufficient to be qualified as a medium-high priority. This effectively hinder the attacks and cause it to have a lower chance of succeeding. Of course, the attacker can mine a block with the transaction and relay it only if it wins and have to recalculate if it loses. It sounds great but the attacker would end up losing the miners or profits if he is mining itself.

Merchants can also take the same precaution to make sure that the transaction's output isn't spam and sufficient fees are included. It is alright for medium transaction to be accepted at 0 confirmation.

Third one please.

this is used to avoid double spending attack I think or they want to make sure that the transaction is correct and they are on the right fork

Yes I remember I had to wait few hours in one of these "stress testing". People defending 1 MB Bitcoin and waiting for the fee market to develop are out of reality. Not only this stress testing will go on forever but the backlog of waiting transaction will never clear because there will be constant higher number of transactions wanting to be added than is even the current Bitcoin capacity! Hopefully this scenario will not happen because it is not hard to imagine how quickly would people abandom Bitcoin in such long lasting scenario...