Topic: This 51% numbers kills everything! (Read 1387 times)

Yes.  More complex methods of checkpointing could also be done if necessary.  If a 51% attack materializes and sufficiently disrupts the Bitcoin network I imagine lots of work being put into a cryptocurrency which combines POW & POS.  There is no requirement that a new coin start from scratch (although all to date have).  The Bitcoin blockchain up to a certain point could form the genesis block for a new more secure cryptocurrency.

Cryptocurrency is a concept.  It can't be killed.

Isn't the chance of some super group of ASIC's coming from behind and eating up the entire block chain non-existent because with each new release we lock in the block chain? They might be able to go back x number of blocks but not swallow the whole thing up.

Why does people keep comparing mining with supercomputers? Supercomputers have petabytes of hard disks and terabytes of ram memory, totally useless for mining. Supercomputers are a network of thousands of cores, totally useless for mining. And so on.

CPU and nvidia mining so. No wonder they would epic fail at such attack!

If it ceases.

Government can print money at will, hundreds of billions of dollars printed at will. They "pay"? Electricity is not "free"? Lol. They spend so many billions every year for war and now they can't even spare 20-30 millions for a 51% attack?

Every block created during the 51% attack obviously can't be trusted. So you must revert to a backup blockchain. And yes, they can be reverted

As other said, the "longest" blockchain is the one with the highest difficulty, not the one with more blocks. 1 difficulty 100 block is worth 5 difficulty 20 blocks

Oh didn't know that. I thought that only the block height was relevant.

Except that there is no need for your blockcahin to become "the longest chain".  It is only necessary to have the chain with the highest total difficulty.  If your blocks are created at 10 minutes per block, but the difficulty of your blocks are four times higher than the difficulty of the blocks in "honest network", then your blockchain can/will replace the network blocks when you've created one-fourth as many blocks as the "honest network"

The attacker can then "slow down" allow difficulty to fall and mine again but even that won't be necessary.  Remember what makes the chain the "longest" the total combined difficulty.  So a very far miner will build a longer chain eventually.  It might have a lower block height but it will be "longer".

I don't know if there is a stat for the total combined blockchain difficulty (sum of all difficulties of all blocks in the blockchain) but I would guestimate it at ~250 billion combined difficulty (roughly 100,000 blocks in last 2 years @ avg difficult of 2 million and a much smaller amount prior to that).

Still it will require a lot of hashpower, far more than 51%.  Also as time goes on the blockchain contains more and more "stored difficulty" especially years and years of high difficulty (ASIC generated) blocks the amount of hashpower required to rewrite a significant portion of the blockchain becomes prohibitive.

Also I would point out the reference client (and many others) use checkpointing which means that one can't rewrite the blockchain prior to the last checkpoint even with infinite amount of hashpower.

51% attack is a credible threat but much like the attack on napster spawned better protected p2p networks and eventually bittorrent a 51% attack would simply pawn new cryptocurrencies hardened against brute force attacks.  

What's your point? They can already 51% the network, since the items they have produced can do more than the total network hashrate.

In practice it's impossible to rewrite 1 year of blockchain faster than the honest network in hope that will become the longest chain and replace the legit one. Even if you're hashing your blockchain offline you must obey the rules of the protocol which means that the difficulty will update each 2016 blocks (up to a maximum of 4 times the previous difficulty if i'm not mistaken). Even if you're using processing power orders of magnitude higher than the honest network the difficulty will catch up with your hashrate leaving you producing blocks at the same average (around 10 min each) and your blockchain will never ever catch up.

This is fun, working with high performance comuting i could not resist to throw around some fast numbers.



Here is a rough estimate of the hashing capacity of the fastest (publicly known) supercomputers today. This does not mean that they
are the best publicly known systems for Bitcoin mining... The exact specification of those cpus and cards are not on the Bitcoin hardware list but a rough estimate using slightly older(worse) equipment makes for a pessimistic estimate.

Remember today that the total network hashrate is: 22 776 GH or 22 TH
The today fastest supercomputer (top500.org) has approximately 300 000 opteron cores and 19 000 tesla cards.
rank

Top 500 rank	Name	cpu	gpu	GH/s
1	Titan system at Oak Ridge	300000	19000	3177GH/s
2	Sequoia - BlueGene/Q	1572864	0	5662GH/s
8	Tianhe-1A	186368		2656GH/s

So no 51% attack there, Notable is that only two of the top 500 machines are using amd cards (largest one has 4*56 7970)

perhaps the agent of an organization that sold a competing currency

The thing i keep in mine is motive. Who would spend huge sums of money to destroy the value of the bitcoins they are forging? It is not rational as it would cost money and produce little/nothing for the attacker. 

It becomes instantly useful again the moment the attack ceases.

That would require a lot more than 51% to do in any reasonable timeframe. For example, to rewrite 1 year of transaction history within 1 year would require 67% of the hashing power. That's a whole year of continuous mining with double the hashing power of the rest of the network before you can even begin your attack. And that's assuming no checkpoints. With checkpoints, such an attack is completely and utterly impossible.

The attack will stop if legitimate miners can upgrade their equipment faster than the government does (which the government has to constantly pay for). Also, I didn't know the government got free electricity.

Double spends can't be fixed by reverting to a back-up blockchain, but apart from that (which is probably not a major problem anyway, since nobody will trust any transactions that occur during the attack), nothing will need fixing anyway. As disruptive as a 51% attack would be, the disruption is only temporary (though even a temporary disruption might still be economically disastrous).

Avalon already owns the few hundred thousand dollar price range


They can easily prevent all transactions from happening with 51%.