A more detailed explanation of the whole

"A double-spend broke Bitcoin" FUD that was circulated by an irresponsible publication.

1/

There was a chain re-organization in the Bitcoin blockchain. This is a common occurrence that is part of Bitcoin's normal operation. It is a result of decentralized consensus under Proof-of-Work. All PoW chains do this.

2/
Two blocks were mined almost simultaneously, competing for the same height, meaning that they had the same parent block and were trying to extend the chain of the same block

3/
Only one can ever succeed in the long run. It is possible that different nodes and miners see one or the other block first and assume it is the winner. This is also normal in a decentralized consensus algorithm

4/
Eventually, within an average of 10 minutes another block is mined. This new block has as its parent *one* of the two competing blocks. Which one? Whichever one the miner saw first and assumed to be the winner.

The new block extends the chain, resolving the issue.

5/
Of the two originally competing blocks, one is now a parent and the other is the last descendant of a shorter chain. The chain with the greatest cumulative difficulty is selected by all. This "orphans" any descendants from the other chain because it is discarded.

6/
Again, all of this is normal. A 1-block reorganization happens every couple of weeks on average as a consequence of decentralized PoW.

A 2-block reorganization happens less often, maybe a few times a year

A 3-block reorg is extremely rare. I don't think we've ever seen one

7/
What happens to any transactions in the discarded block? If they are also in the winning block then all is well. If they are not in the winning block, each node puts them back into its mempool as "unconfirmed" and they wait for another opportunity

8/
During a re-organization, there is a chance for someone to attempt a "double spend". This is not a double spend from the perspective of the blockchain as a whole. Only one spend survives, therefore no double spends happen. That's the whole point of PoW consensus.

9/
But from the perspective of the recipient of a payment, they may see a transaction that appears to have 1 confirmation (it is in a block), then disappears when that block is discarded.

10/
Very rarely, the sender will sneak a *different* transaction in the competing/winning block. Let's say this is a payment for a lesser amount (more change back), or to a different address.

11/
Because the original transaction is gone (discarded block), the new transaction (winning block) is the only "real" transaction. The blockchain has prevented a double spend by discarding one and only recording the other.

12/
From the perspective of the recipient, they thought they were "paid" after 1 block, then they... weren't. This is why "confirmations" provide /probabilistic/ immutability. The chance of a reorg drops with each subsequent confirmation

13/
This is described in the Satoshi whitepaper on page 8. In fact, it's the only math equation in the paper and it describes the declining probability of a re-org, showing why "6 confirmations", though arbitrarily chosen is a good basis to consider a transaction finalized

14/
Here's page 8. As you can see the chance of a block getting discarded from a reorg declines /exponentially/ as more blocks are added to the chain. Finality is based on probability.

15/
So when do you consider a transaction finalized and when is it safe to give your customer the TV or the fiat or whatever value you are exchanging? It depends on the amount!

I waited 3 confirmations after selling my car for $11,000 USD (IIRC). Was enough for me.

16/
It also depends on the risk of the buyer going away. I'd sell a house on 0-conf, *because I know where they live!*. They can't run away with it. Some things are more dangerous: I'd wait 6 confs to exchange for cryptocurrency, because once I give it I can't get it back.

17/
During this most recent re-org, a transaction of $22 was in both competing blocks as two competing transactions. We don't know why. We don't know who. But there's nothing "impossible" about this. It is part of the protocol

18/
Now, for $22 many would accept 1 confirmation. Worst case, you're out $22 of something you gave in return that was delivered instantly and irreversibly. Not a big deal.

19/
Many credit card vendors don't take a signature for amounts under $25 for the same reason: while it can be disputed without a signature, it's not worth the extra time and delay to get one for such an amount. Same risk model here.

20/
In fact, we do not know that the recipient of that payment lost money. They may have been waiting for 2 confirmations and not delivered the other part of the value. So in that case, they lost nothing - they consider this "unpaid" because it didn't get 2+ confirmations

21/
Someone article quoted the lie "it could've been $22 million". Well, no, it couldn't. If you accept a $22m payment on bitcoin, I would assume you understand how Bitcoin has worked since 2008, exactly as specified in the paper. You don't "deliver" on that payment after 1 conf

22/
Several other incorrect statements are also made in that and other articles about RBF and Segwit. Here's the truth: this is a normal function of any PoW blockchain. A re-org with two different versions of a transaction can occur in every other PoW chain.

23/
Nothing weird or outside the consensus algorithm happened. Bitcoin continues to work exactly as it should. The only thing that happened is bad "journalism" if it can even be called that. In a bubbly market, a rumour can circle the globe before it is debunked.

24/
Consider it debunked.

More from Bitcoin

1/THREAD: WHEN WAS IT CLEAR?

Oct. 8, 2020: The purpose of this thread is to document and timestamp when it first became clear that #Bitcoin was likely to become a major reserve asset for public corporations, and eventually states, with Square's purchase of $50M in BTC.

The purpose is to give something to cite when ppl later claim "But there was NO WAY OF KNOWING..."

h/t @ErikSTownsend who used the same format to call out the impact of Covid on Feb 8 and made me personally aware of the looming shutdown of the country
https://t.co/opuiNgSeqC !


Bitcoiners smarter than me have been predicting the takeover of the dollar by Bitcoin for many years.

In 2014 with Bitcoin barely at $1B, @pierre_rochard wrote https://t.co/EGHa58KqHq, covering all the incorrect narratives of Bitcoin and stating it will overtake the dollar.

"[skeptics] misunderstand how strong currencies like bitcoin overtake weak currencies like the dollar: it is through speculative attacks and currency crises caused by investors, not through the careful evaluation of tech journalists and 'mainstream consumers'" - @pierre_rochard

I first became bullish on Bitcoin in the summer of 2016, around a $3B market cap, but it was still a toy project at that time in the eyes of most in the financial world, while many technologists thought of it as a v1 technology to be improved on.
$BTC: Two Bitcoin FUDs to address this Thanksgiving weekend:

1. China PlusToken FUD: Old news. Please see linked thread.

2. U.S. Treasury FUD: Read thread below...


1/ These news are much more relevant, as they imply severe trade-offs for people who want to keep their bitcoins undoxxed, with the cost and risks of doing so. I would not disqualify the tweet as mere FUD in the sense that what he posted is false. It should be taken seriously.

2/ For all we know, his decision of making it public before TG weekend may come out of the urgency of informing CT of a poignant anti-Bitcoin move by a Trump administration trying to cut lose ends before leaving office—not just "price manipulation" as I've seen suggested around.

3/ It implies the acceleration of a process already planned for for months in advance, not something he just came up with to "crash the market."

4/ In practicality, assuming this passes, it will have two major consencuences:

a. Armstrong's analysis is correct. And I would go further in saying, this regulation would leave the U.S. severely handicapped to continue to be the leader in the cryptocurrency industry worldwide.

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