Undocumented Double-Spend Risk in Bitcoin | Hacker News Reader

Undocumented Double-Spend Risk in Bitcoin

medium.com

19 pointshudon10 comments

Either the author of this article is confused or I am. He describes the situation of a deliberate hard fork into two chains with different consensus rules. He then notes that Alice can spend her original (prefork) bitcoin differently on the two resulting chains --- but where he loses me is where he claims that this somehow represents a double-spend vulnerability.

Both Charlie and Bob have received bitcoin. This is a double-spend by definition.

Yup. When Bitcoin forked into Bitcoin cash, everyone was free to "double spend" - once in Bitcoin and once in Bitcoin cash. Money just up and doubled itself.

An actual great example is in 2013 when this happened [0]. For 24 hours, Bitcoin could not reach consensus and users could double-spend. The fact that we do not know how likely Bitcoin consensus failure is—especially when so many forks and conflicting rulesets are emerging—should worry anyone thinking about decentralized consensus.

[0] https://bitcoinmagazine.com/articles/bitcoin-network-shaken-...

Article just describes a hard fork. It would be incredibly hard to switch any significant percentage of Bitcoin network nodes to a new protocol.

How hard? We need to understand exactly what the probability of consensus failure is. Otherwise, we have not built a working distributed system...

It's just a fork. Of course fork will have different transaction history starting from fork point.

“Of course” we have 2 ledgers and no consensus between them? So do we all agree that Bitcoin does not solve decentralized consensus and I missed the memo?

In the first step, he says roughly "assume half of the nodes change protocol". I named by dog Bitcoin too but he doesn't have the same UTXO state as the main chain.

If Alice changed the ruleset so drastically, I don’t think she could find any Bob that would be willing to accept this thing as Bitcoin. But maybe the probability is not 0.

If Alice only changed 1 little rule and made it conflicting with the ruleset used by the rest of the network, it’s easy to imagine consensus failure and being able to find 1 Bob that gets tricked into receiving a double-spent Bitcoin. In fact, such a system failure has happened already in 2013, when for 24 hours no one knew which ledger was the real bitcoin: https://bitcoinmagazine.com/articles/bitcoin-network-shaken-...

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