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[Husires]

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[Charles-Tim]

On average ّّّ~10 minutes later (Bitcoin Blockchain) a new block is mined. When this block is mined, we will have two blocks. Original one with a longer chain and a new, shorter block, tries to have some "orphans," which try to have new blocks on that chain.

Two blocks are created by two miners or mining pool at the same time, chain-reorganization will help to inavidate one block and the other one will remain valid, the one that remain valid is thae one that new blocks build upon which will be the longest chain.

Calculate the probability of two blocks being mined at the same time?

You can read the quote below.

I don’t know what the mathematical probability of this is, so here’s the frequency of chain reorganisations based on the data from my bitcoin node (which hash been running continuously since 17 December 2016):

Actual Reorganisations: 39 (1 every 5,796 blocks / 38.8 days)
Avoided Reorganisations: 100 (1 every 2,260 blocks / 15.2 days)

https://learnmeabitcoin.com/explorer
https://learnmeabitcoin.com/technical/chain-reorganisation

Why is the process of Bitcoin Blockchain re-organization every two weeks?

Chain re-reorg is not every two weeks as you can see from below data. It does not have a definite day.

665005      07 January 2021
664578      05 January 2021
645179      24 August 2020
644543      20 August 2020
638519      09 July 2020
619970      03 March 2020
618465      22 February 2020
614732      27 January 2020
578141      28 May 2019
551355      24 November 2018

Calculate the probability that 3 blocks re-organized?

Only two same blocks have been mined by the same miners at the same time in the past. No three miners have mined the same block before.

About confirmation 
It depends, some organizations can accept 0 confirmation from you if you do not enable replace-by-fee, but if you use RBF, some organizations will wait till 3 to 6 confirmations. On exchanges for example, remember not your key not your coin though, like on binance, only one confirmation is accepted for deposit while two confirmations for withdrawal. I have seen many exchanges accepting 10 confirmations irrespective of enabling or not enabling RBF.

But, in my opinion, 1 confirmation is enough if RBF is not enabled, while you can wait for more confirmations if RBF is enabled.
But, in my opinion, 0 confirmation is enough if RBF is not enabled, while you can wait for confirmations if RBF is enabled. But, for more safety, six confirmations are recommended.

But, do not let chain-org confuse you, nothing like double spending. Double spending can only occur while you enable RBF while the transaction not yet confirmed, you can then double spend the Bitcoin back into an address owned on your wallet or into another address of your choice. The Bitcoin Blockchain is not faulty, nothing like double spending because the two but the same blocks that are mined by two different miners, one block will later be invalidated as new blocks build on the other one which will have the longest chain and valid transaction.

[o_e_l_e_o]

All transactions that were included in the ignored block are returned as unconfirmed transactions awaiting confirmation.

This is not accurate. When two blocks are mined at the same height, the vast majority of transactions which are included in one are also included in the other. Of the few which are included in the about-to-become stale block but not in the about-to-become accepted block, some of those will be included in the next block mined which decides which chain becomes the main chain. Therefore, only a very small number of transactions would end up returning to "unconfirmed" status.

Is waiting for one confirmation considered safe? Well, it differs according to the value of the thing you want to pay for, so buying a house means that you have to wait for 3 confirmations and buy a daily product means that 1 confirmation is sufficient.

If selling a house, I would be waiting for at least 6 confirmations, probably more. An attacker with 20% of the hashrate still has a 10% chance to reverse a transaction 3 confirmations deep, whereas by 6 confirmations this has fallen to 1%, and by 10 confirmations it has fallen to 0.1%.

Why is the process of Bitcoin Blockchain re-organization every two weeks?

It isn't. The difficulty is recaulcated every two weeks. Chain reorganizations happen on an ad hoc basis as required.

Chain re-reorg is not every two weeks as you can see from below data.

His data is only from what his node sees, so if his node sees the block that goes on to become accepted first, then it will have no knowledge of the stale block. Since his most recent data at block 655,005, there have been 2 more reorgs at heights 666,833 and 667,881.

It depends, some organizations can accept 1 confirmation from you if you do not enable replace-by-fee, but if you use RBF, some organizations will wait till 3 to 6 confirmations.

This is not accurate. Once a transaction has a confirmation, whether or not it was RBF is irrelevant. Some places will accept zero-confirmations transactions which are not flagged as RBF, but will require 1 confirmation if they are flagged as RBF, but most places will require at least 1/3/6 confirmations regardless of the RBF status.

[Charles-Tim]

This is not accurate. Once a transaction has a confirmation, whether or not it was RBF is irrelevant. Some places will accept zero-confirmations transactions which are not flagged as RBF, but will require 1 confirmation if they are flagged as RBF, but most places will require at least 1/3/6 confirmations regardless of the RBF status.

Ah! That is so wrong of me. You are right. Transactions with zero confirmation or transaction that has not been confirmed can be double spend with RBF, it can also use be cancelled, althought the cancelling is also completely RBF in which another address on the same wallet is paid which will make the Bitcoin to reflect back in the sender's wallet balance. But, RBF can not be used if the transaction already have one confirmation. I have made appropriate corrections.

[mocacinno]

--snip--
If selling a house, I would be waiting for at least 6 confirmations, probably more. An attacker with 20% of the hashrate still has a 10% chance to reverse a transaction 3 confirmations deep, whereas by 6 confirmations this has fallen to 1%, and by 10 confirmations it has fallen to 0.1%.
--snip--

Agree with everything you said, just wanted to add that a house might be a bad example... I'd personally rather sell a house with a 0 conf transaction than a car... I mean, what's the buyer going to do: perform a 51% attack and move your house to a different country . If a reorg was to happend AND the transaction paying for your house was to be left out of the longest chain (the chain with the biggest cumulative diff) AND the buyer double spent (or the tx dropped from the mempool after a while), you could still take your house back, since it is no longer payed for... If you sell a car, the buyer can drive off onto a boat and ship your car to a different country within a couple of hours, so i'd require at least a couple of confirmations for that deal

Same goes, for example, for memberships: imagine paying for netflix using crypto, but executing a succesfull doublespend... What's the worst that can happen from netflix's point of view if they accepted 0 conf tx's? Giving a hacker a free 1 hour preview before closing his account due to incomplete payment?

Same goes, for example, for many small shops: imagine buying a coffee with crypto and having to wait 20-30 minutes before your payment gets accepted? I guess a coffee bar would accept 0 conf payments because they would lose customers if they didn't. And in case somebody executed a double spend: just treat it like any other shoplifter: file a police report, give them the security camera footage and issue a lifetime ban for the doublespender...

Bottom line: if you sell something expensive that can be moved out of your reach in a short time: wait at least a couple confirmations... If you sell something online that cannot be taken back: wait a couple of confirmations...
If you sell something you can take back, or something that cannot be moved out of your reach: sure, wait a couple confirmations, but a 0 conf tx wouldn't be the end of the world... If you get scammed, just take back your property or close the account.

[o_e_l_e_o]

Agree with everything you said, just wanted to add that a house might be a bad example... I'd personally rather sell a house with a 0 conf transaction than a car... I mean, what's the buyer going to do: perform a 51% attack and move your house to a different country .

Yeah OK, fair point, but if you've handed over the keys and the paperwork is all signed and then they double spend you are still going to have a nightmare explaining to your average law enforcement and/or judge what a double spend is and therefore get everything reversed, not to mention that it could potentially take weeks or months to evict them from your property. It's not quite as simple as just "take your house back". For the sake of an hour I would still wait for the confirmations.

The rest of your post about double spending small amounts, such as for subscriptions or coffee, is spot on though. There is a great explanation on the Bitcoin Core website here: https://bitcoincore.org/en/faq/optin_rbf/#why-arent-unconfirmed-transactions-safe. Places like coffee shops can afford to accept zero confirmation transactions because the vast majority of people would not try to scam a coffee shop out of a few bucks due to either honesty or fear of legal repercussions, it is far easier to do this using a credit card than it is with bitcoin (and yet everywhere accepts credit cards), and even if the odd person does the coffee shop can easily afford the loss of a few bucks when balanced against the increase in revenue and throughput gained by not having to wait for transactions to confirm.

[Husires]

Of the few which are included in the about-to-become stale block but not in the about-to-become accepted block, some of those will be included in the next block mined which decides which chain becomes the main chain. Therefore, only a very small number of transactions would end up returning to "unconfirmed" status.

That sounds like a more logical answer, thanks for the correction.

If selling a house, I would be waiting for at least 6 confirmations, probably more. An attacker with 20% of the hashrate still has a 10% chance to reverse a transaction 3 confirmations deep, whereas by 6 confirmations this has fallen to 1%, and by 10 confirmations it has fallen to 0.1%.

The topic talks about a model where the *error* is from the network without there being intentional third-party or double spending.
However, 3 confirmations are sufficient. Even if there is intentional of double spending, I think that the network has enough hash to make the rental or purchase of 51% of the hash cannot go unnoticed.

[o_e_l_e_o]

However, 3 confirmations are sufficient. Even if there is intentional of double spending, I think that the network has enough hash to make the rental or purchase of 51% of the hash cannot go unnoticed.

That's my point though - you don't need 51% of the hashrate to still have a chance to reverse a transaction. If you have 51% of the hashrate, and can sustain that indefinitely, then theoretically with enough time you have a 100% chance to reverse any transaction since the last hardcoded block. But even with a much smaller fraction of the hashrate there is still a chance you can reverse a transaction. For 3 confirmations, the numbers look like this:

Percentage of hashrate		Chance of reversing 3 confirmations
----------------------		----------------------------------
51% or more		100%
40%		66.4%
30%		32.5%
20%		10.3%
10%		1.3%
5%		0.2%

If I'm selling something like a house or a car for several tens of thousands or hundreds of thousands of dollars worth, then even a 1% chance of reversal is too high for me. For the sake of an additional 30 minutes, I'm waiting for some more confirmations. If I'm selling some second hand books or something for a few bucks, then 3 confirmations is more than enough.

[Ariel008]

totally agree with what o_e_l_e_o said. If there is a lot of money... even 3 confirms are not enough for me))