51 percent attack and blocktime

[Yougu]

Hey everyone,

I've been looking into 51 percent attacks recently and was wondering about the limitations. In a 51 percent attack an attacker tries to generate an alternate chain faster than the honest chain. To convince the network that his chain is the correct, he must ultimately have the longest chain.

The blocktime for each chain is 10 min per block. If blocks are created too fast, the difficulty is adjusted.

Given these limitations how is it possible to build an alternate chain that grows much faster than the honest chain? If my understanding is correct, nodes also check whether the difficulty target has been adjusted correctly. So if the block time is too short in the long run, the newly mined blocks might not been recognized by the network.

Is that correct or do I miss something? Thanks!

[BrewMaster]

The blocktime for each chain is 10 min per block. If blocks are created too fast, the difficulty is adjusted.

there is no such thing as "blocktime" and "10 min per block". the only time related variable that we have is 2016 blocks in 2 weeks and that is what is checked to adjust the difficulty.

Given these limitations how is it possible to build an alternate chain that grows much faster than the honest chain?

you perform 51% attack by having >51% of hashrate!
in other words you should have more than half of the hashing power of the whole network so that you can find more blocks compared to the rest (the <49%), and that way you create the longest chain.
remember that 51% is the bare minimum, it is not possible to actually do it with 51%, you need a lot more than that to successfully pull that off.

[Yougu]

Thanks BreMaster, but I guess I still don't get it completely. For newly mined blocks I understand why the aggregation of 51 percent of hashing power helps the attacker.

If the attacker decides to fork much earlier, let's say 2,500 blocks earlier, he has to catch up and eventually overtake the main chain. The attacker cannot do so however, because the difficulty must be adjusted if he mines blocks too fast. So, even if 2016 blocks are mined every 2 weeks on average, this applies to both chains. So rather than catching up with the main chain, the attacker will fight a loosing battle, since the other chain eventually grows as fast as his.

I know this is only a theoretical problem, but I was wondering whether the blocktime itself (or how you want to call it) imposes limitations to the 51 percent attack. Thanks!

[xtraelv]

Thanks BreMaster, but I guess I still don't get it completely. For newly mined blocks I understand why the aggregation of 51 percent of hashing power helps the attacker.

If the attacker decides to fork much earlier, let's say 2,500 blocks earlier, he has to catch up and eventually overtake the main chain. The attacker cannot do so however, because the difficulty must be adjusted if he mines blocks too fast. So, even if 2016 blocks are mined every 2 weeks on average, this applies to both chains. So rather than catching up with the main chain, the attacker will fight a loosing battle, since the other chain eventually grows as fast as his.

I know this is only a theoretical problem, but I was wondering whether the blocktime itself (or how you want to call it) imposes limitations to the 51 percent attack. Thanks!

I wrote a basic explaination about how 51% attacks work: How does a double spend 51% attack work ? Explanation and examples.


Article by Vitalik Buterin that covers some of what you are interested in: https://blog.ethereum.org/2014/07/11/toward-a-12-second-block-time/


The blockchain follows the heaviest chain. This means the chain with the highest difficulty which is usually longer as well.

This means that the difficulty plays an important role in which chain is accepted as the "winning chain". The difficulty is adjusted (depending on the code) every block or x number of blocks. This means that a higher difficulty chain has preference over a lower difficulty chain. Over a space of time a higher difficulty chain will be longer than a lower difficulty chain provided that the hashrate remains constant (or increases on the higher difficulty chain).

If you followed the progress of the BCH versus BSV you can see how that worked out publicly.  https://cash.coin.dance/



The BCH & BSV fork was effectively a lot of the characteristics of a very public version of a 51% attack. (Ignoring the differences in code) and has provided a lot of interesting data.

Despite the increase in difficulty due to higher hashrate - in general a higher hashrate blockchain adds blocks to the blockchain faster than a lower hashrate chain ( longest / heaviest chain is accepted as the majority)