Quote from: AlexR12 on December 06, 2017, 08:25:38 PM

Quote from: kahc on December 06, 2017, 12:49:49 PM

The concept is abstract.
To better visualize it, watch from 14:06 where they talk about double-spend, then 51% attack.
https://www.youtube.com/watch?v=Lx9zgZCMqXE

Thanks! If I understand it right, a successful attacker will have to win (solve the puzzles faster than other nodes) several times in a row and create a longer branch. After that all other nodes will follow the rule of extending the longest chain. Right?

"Other miners", yes, the nodes will automatically update/validate the longest chain ( as long the blocks are valid ).

Quote from: AlexR12 on December 06, 2017, 12:24:30 PM

Quote from: Vod on December 06, 2017, 09:27:10 AM

You wouldn't be able to change the ledgers.
https://learncryptography.com/cryptocurrency/51-attack

Could you please elaborate a little on this? According to your link: "Realistically, an attacker would only be able to modify transactions within the past few blocks." How is that not changing the ledgers?

That's exactly what I don't understand. The ledger is distributed, and yet it could be altered by an attacker. Unless an attacker could hack everyone's computer and replace their ledgers with his dishonest version???

Because it can only choose to not confirm/confirm certain transactions, it CAN'T decide for example that all the coins should be sent to the miner's adress.

I would say that "modifying" is the wrong word, rather changing/excluding certain transactions..

These are some better sources:

https://en.bitcoin.it/wiki/Weaknesses#Attacker_has_a_lot_of_computing_power
https://bitcoin.stackexchange.com/questions/658/what-can-an-attacker-with-51-of-hash-power-do