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That's incorrect, you can't "change anything in the database" with a 51% attack. You can block transactions from being processed, reorder new transactions, and

by mcintyre1994 5y ago

That's incorrect, you can't "change anything in the database" with a 51% attack. You can block transactions from being processed, reorder new transactions, and reverse your own (but not others) transactions to double spend. You can't just go arbitrarily deleting previous transactions, or modifying balances.

spyspy 5y ago
I'm curious, if you owned 51% of the network what's to stop you from rebuilding the entire history of the chain and then adding new transactions (including any of your new fraudulent ones) on top?
- aordano 5y ago
  Nothing, that's called a hard fork. It has happened (out of consensus) and probably will happen again.
- Kranar 5y ago
  Nothing, mcintyre1994 is simply wrong about this. You don't need 51% to block transactions, any miner who completes the proof of work can legitimately block a transaction and Bitcoin went long periods of time where various miners blocked transactions and just took the mining reward. As for deleting, unless you want to enter a philosophical argument on the nature of deleting, and whether anything in the universe can truly be deleted [1], for all practical purposes a 51% attack does allow one to delete transactions off the blockchain but the older the transaction is the more expensive and impractical it would be to do so. A 51% attack does not allow someone to create new bitcoins or spend coins they don't have the keys to, but they could double spend coins or delete existing transactions. [1] https://en.wikipedia.org/wiki/No-hiding_theorem https://en.wikipedia.org/wiki/No-hiding_theorem
jbverschoor 5y ago
@spyspy You don’t have the keys to sign all of the transactions
smt88 5y ago
> you can't "change anything in the database" with a 51% attack A 51% attack can't immediately change other people's history, but it can eventually[1][2]. You would not need to change transaction history very deeply to cause a lot of chaos. 1. https://bitcoin.stackexchange.com/questions/101565/why-cant-a-51-attack-change-past-blocks-on-the-blockchain https://bitcoin.stackexchange.com/questions/101565/why-cant-... 2. https://learnmeabitcoin.com/technical/51-attack https://learnmeabitcoin.com/technical/51-attack

spyspy 5y ago

I'm curious, if you owned 51% of the network what's to stop you from rebuilding the entire history of the chain and then adding new transactions (including any of your new fraudulent ones) on top?

aordano 5y ago
Nothing, that's called a hard fork. It has happened (out of consensus) and probably will happen again.
Kranar 5y ago
Nothing, mcintyre1994 is simply wrong about this. You don't need 51% to block transactions, any miner who completes the proof of work can legitimately block a transaction and Bitcoin went long periods of time where various miners blocked transactions and just took the mining reward. As for deleting, unless you want to enter a philosophical argument on the nature of deleting, and whether anything in the universe can truly be deleted [1], for all practical purposes a 51% attack does allow one to delete transactions off the blockchain but the older the transaction is the more expensive and impractical it would be to do so. A 51% attack does not allow someone to create new bitcoins or spend coins they don't have the keys to, but they could double spend coins or delete existing transactions. [1] https://en.wikipedia.org/wiki/No-hiding_theorem https://en.wikipedia.org/wiki/No-hiding_theorem

jbverschoor 5y ago

@spyspy You don’t have the keys to sign all of the transactions

smt88 5y ago

> you can't "change anything in the database" with a 51% attack A 51% attack can't immediately change other people's history, but it can eventually[1][2]. You would not need to change transaction history very deeply to cause a lot of chaos. 1. https://bitcoin.stackexchange.com/questions/101565/why-cant-a-51-attack-change-past-blocks-on-the-blockchain https://bitcoin.stackexchange.com/questions/101565/why-cant-... 2. https://learnmeabitcoin.com/technical/51-attack https://learnmeabitcoin.com/technical/51-attack