
Undocumented Double-Spend Risk in Bitcoin - hudon
https://medium.com/@hudon/undocumented-double-spend-risk-in-bitcoin-ebe364ad3820
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nokcha
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.

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

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friend-monoid
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.

~~~
hudon
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-...](https://bitcoinmagazine.com/articles/bitcoin-network-shaken-by-
blockchain-fork-1363144448/)

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

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

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

~~~
hudon
“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?

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jabgrabdthrow
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.

~~~
hudon
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-...](https://bitcoinmagazine.com/articles/bitcoin-network-shaken-by-
blockchain-fork-1363144448/)

