51% Attacks [ Answered ]

[Omni]

Hi,
 May someone explain this concept in layman's terms? I do not understand why it would even matter if a pool had 51% of the hashing power. The remaining 49% could still find blocks as far as I know.

Thanks,
Omni

[kokjo]

Hi,
 May someone explain this concept in layman's terms? I do not understand why it would even matter if a pool had 51% of the hashing power. The remaining 49% could still find blocks as far as I know.

Thanks,
Omni

an attacker with 51% can always make a longer chain, then the remaining 49%, and thereby overruling the 49%.
51% can generate ALL blocks.

[Omni]

Hi,
 May someone explain this concept in layman's terms? I do not understand why it would even matter if a pool had 51% of the hashing power. The remaining 49% could still find blocks as far as I know.

Thanks,
Omni

an attacker with 51% can always make a longer chain, then the remaining 49%, and thereby overruling the 49%.
51% can generate ALL blocks.

Maybe I do not fully understand how Bitcoins are hashed then. What do you mean  by creating a longer chain?



Actually this kind of explains it.

. Inserting a counterfeit block into the block chain requires redoing all the work that is done by the network as a whole. This means it can only be done by a user who controls more than half of the network’s computing power.

http://www.techthefuture.com/technology/the-future-hardware-of-bitcoin-mining/

[Revalin]

To resolve any ambiguity about which transactions are valid (for instance, if there are two chains with conflicting transactions that spend the same input coins), Bitcoin clients always believe the story of the longest chain - IE, the one with the most blocks.  (More accurately it's the chain with the greatest sum of difficulties.)

The attack is to spend coins in one place and have it recorded by the 49%.  After taking delivery of the goods they bought, they would start mining a fork of the blockchain from before that transaction.  Since they have more hashpower than everyone else, they would eventually have a chain with more blocks (sum of difficulties) than the 49% chain.  In their version of history their original spending transaction never happened - so they can now spend their coins again, which is called "double spending".

[Omni]

To resolve any ambiguity about which transactions are valid (for instance, if there are two chains with conflicting transactions that spend the same input coins), Bitcoin clients always believe the story of the longest chain - IE, the one with the most blocks.  (More accurately it's the chain with the greatest sum of difficulties.)

The attack is to spend coins in one place and have it recorded by the 49%.  After taking delivery of the goods they bought, they would start mining a fork of the blockchain from before that transaction.  Since they have more hashpower than everyone else, they would eventually have a chain with more blocks (sum of difficulties) than the 49% chain.  In their version of history their original spending transaction never happened - so they can now spend their coins again, which is called "double spending".

Thank for for the great explaination.
Closed.