Topic: Selfish mining theory (Read 1817 times)

My memory gets fuzzy fast, and I've remembered a lot of Bitcoin protocol things wrong, but I think the code was written for both directions at the same time after Satoshi was gone, like even after my forum account was created.  Maybe when GPUs started mining, or maybe when FPGAs did, or maybe when altcoins first showed up and people were constantly moving hashpower to the most profitable coin (before merged mining was created/implemented).

I don't think I have read anything that would confirm this, although it may still be accurate. I would say that satashi would likely have not needed to write this into the code as there is generally not a reason for miners to stop mining so suddenly like this so large decreases in difficulty would likely never be an issue for the network, however large increases in difficulty are a real possibility

Right, IIRC, it's a 4x increase or decrease meaning it can drop TO 25% of the current rate in a single change.  That thought was clearly not properly laid out.  Even if I'm wrong and it can drop to 20%, that's still far too difficult to make the attack feasible even without the other constraints.

It's not just the longest chain that wins, it's the chain with the most amount of work behind it.

If someone controls a couple nodes (10 for example, shouldn't be hard to do that right?), and at some point he isolates those nodes from the other bitcoin nodes, that will lead to an isolated blockchain that only he have access to it, and if he mines on it at 1kh/s or whatever super slow hash rate, at the time of recalculating the difficulty, it'll drop down by huge amount considering there wouldn't be any new blocks found between the time he isolated the chain till the time of changing the target difficulty (maybe he waits for 2 recalculating events, or modify something to make the difficulty drop to lowest possible not sure how possible that is). After this new low difficulty, he uses high mining power to create blocks at very high speed since the difficulty has dropped (on his own blockchain), meanwhile the true blockchain will have generated blocks that aren't included in those isolated chains, but due to him having low difficulty he generated twice the amount of blocks (or maybe x1.5 for example) if he then releases those nodes and they start interacting with other nodes, will this "self generated" chain become the "true" chain? I mean since it's a longer chain wouldn't it be the "true" blockchain? and that person would have actually reversed some transactions, and also generated new blocks and took the reward for himself?

I got this theory while reading an article about selfish mining, but for selfish mining to work you'll need to have something around 25% hash rate? (can't remember the exact %)

Anyway, the idea was taken from selfish mining, but I added that attacker needs to have couple nodes under his control and he isolate them in order to massively reduce the target difficulty so he can generate blocks at ridiculous high speed to make up for the time he didn't generate blocks while isolated.
also I'm not sure if 10 nodes are enough? maybe he needs something like 1/3 of total number of nodes to make sure that his blockchain will be published to everyone?

That's all, if anyone can explain if it's possible or not or if you can provide links to articles that help me finding the answer I'll be really appreciated, hope I explained my theory in an easy way.

The difficulty increases by more then 25% on a regular basis. (this was said above) The max that the difficulty can increase every 2016 blocks is 400%, and IIRC the 4th or 5th time the difficulty actually increased it came very close to this

It's not just the longest chain that wins, it's the chain with the most amount of work behind it.

Also, IIRC, the difficulty can only change by 25%, and is based on the time to calculate the 2016 blocks.  So, if you started at block 2016 of the current difficulty, changed all your computer clocks in your private chain, got lucky and found the block to trigger the difficulty change immediately, and got the difficulty to drop as much as possible, it would still be ridiculously high, and you'd still be hard pressed to find additional blocks, much less 2016 of them.  In the meantime, the network would still hash away leaving you in the dust.

Regarding the suggestion that you manipulate the code, if you manipulated the code to make it drop to like 1kH/s, then your longer chain would be rejected by the network because it breaks validation rules (wrong difficulty).

ETA:  Regarding the first method, even if you had enough power to perform the attack simply by manipulating time, the time manipulation would have to be slowly reversed to get most blocks in the past by the time you submit the chain, so it would be very complicated and take a lot more power than "a couple nodes (10 for example)".

Best response +1

I agree with the above statement as the network will never allow you to bring your own blockchain to the network as it would conflict with the majority. It will not turst 10 computers over millions of others (correct me if im wrong). The only way to do this would be to have a quantum computer or perhaps one the size of the sun to overtake the network / break the encryption - in which case the biggest issue will not be bitcoin but ALL online encryption as a whole.

All of these concerns have been thought about by very very intelligent people who stand for the integrity of this network.

Cheers

Also, IIRC, the difficulty can only change by 25%, and is based on the time to calculate the 2016 blocks.  So, if you started at block 2016 of the current difficulty, changed all your computer clocks in your private chain, got lucky and found the block to trigger the difficulty change immediately, and got the difficulty to drop as much as possible, it would still be ridiculously high, and you'd still be hard pressed to find additional blocks, much less 2016 of them.  In the meantime, the network would still hash away leaving you in the dust.

Regarding the suggestion that you manipulate the code, if you manipulated the code to make it drop to like 1kH/s, then your longer chain would be rejected by the network because it breaks validation rules (wrong difficulty).

ETA:  Regarding the first method, even if you had enough power to perform the attack simply by manipulating time, the time manipulation would have to be slowly reversed to get most blocks in the past by the time you submit the chain, so it would be very complicated and take a lot more power than "a couple nodes (10 for example)".

I think what OP means is to separate from the network, and mine with a really low hashrate. Then, because of the lack of blocks found, the difficulty in OP's network would drop by a lot, and he/she would use a large amount of hashing power to catch up and overtake the main network's blocks.

But what if you time it PERFECTLY, and manage to get lucky and solve the block on the first try on your new blockchain?

It would take too long to find enough blocks until the difficulty recalculates. The difficulty does not readjust every two weeks, it readjusts every 2016 blocks. It would take a very long time to find enough blocks to finish the remaining 2016 blocks until the difficulty recalculates with only a few kh/s of mining power and once the difficulty does adjust with your "special" blockchian the "real" blockchain will be much longer then what your new blockchain would be able to achieve.

Yeah, this is what I mean. I still learned some good points from your post but I think this is still a valid thing.. anyone else have a way to disprove?

I think what OP means is to separate from the network, and mine with a really low hashrate. Then, because of the lack of blocks found, the difficulty in OP's network would drop by a lot, and he/she would use a large amount of hashing power to catch up and overtake the main network's blocks.

if you separated yourself from the main network at block h 322992. and started mining in your smal network. the difficulty will still initially be high. so if you only had some USB erupter miners. you probably wont even mine one block in the 2 weeks you have to wait.

once the 2 weeks are up and it time to change difficulty down because your small network is suppose to be at 2016 blocks further on. yet you have not solved a block due to complexity too high.. it will drop alot..

you have already by now lost the game before you even got a block, just by not keeping up with the real network block numbers.

so the problem is that even if you do not mess with the code to make a block you want to solve give you 5 trillian coins. even if you think you have solved a legitimate block. when you then connect to the main network. your blockchain will put you at 322993, yet everyone else's chain will be atleast 324000. meaning that block you hoped would net you 25legit coins or 5trillian reprogrammed coins.. would both be simply ignored and your personal block would be orphaned/binned/thrown out, 'shown the hand because the tcp aint listening'. simply because your block 322993 does not follow on from everyone elses.

thus its not worth it.