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Comment by dragontamer | Hacker News Reader

51% attackers mine in secret.

Basically, you spend your coins today, while controlling a 51% share. When everyone else's 49% hash-power creates 98 blocks, your 51% share will create 102 blocks.

But secretly. That's the key. Now that your chain is +4 ahead (or wait even longer and become +10 ahead), you can spend your coins on the public chain. Then, you publish your 102 alternative blocks (which barely adds any hash power to the chain), and spend your coins again on the 2nd chain.

A 51% is the end-all-be-all for a cryptocoin. If you're vulnerable to the 51% attack, the coin is effectively worthless.

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The only way to beat a 51% attack is to build more hash power than the attackers. IE: Prevent the attackers from getting to 51%.

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If only we had a currency that was immune to 51% attacks because a government was willing to use force to preserve its value, and therefore doesn't need a massive use of energy for its proof of work.

We call it a 51% attack an attack, but from the perspective of PoW it's always about the chain with the most work. Anyone is just as valid as anyone else to propose blocks. That's the point of Bitcoin: a way to always figure what the truth is, and make it as expensive as possible for people to attack/change this truth.

The only problem here is that PoW only knows one cost: hashing, and due to macro shifts in mining hardware this can sometimes go down a lot more than the value of a coin (which makes the cost of this attack worth it).

As soon as you start introducing measures to subvert this attack you are subverting either decentralization or stability. If you for example program clients to not accept reorgs deeper than 10 blocks (for example) you simply introduce new attack vectors that can split the network (into following different chains - which is even scarier than 51% attacks).

If every x blocks you snapshot the chain and force everyone to follow that snapshot you just centralized the chain, etc.

Splitting the network causes it to become a marketing problem.

Due to probabilistic nature of most PoW protocols, you don't actually need to have the chain with the most work. Attacker can keep trying till they are are lucky, with less than 50% hashpower.

In the same way you can endlessly throw money at the craps table until you get lucky and bankrupt the casino?

In the same vein, I've been yelling at the monitor over the "news":

> The Milky Way could crash into another galaxy way sooner than we thought

And the chance of successfully doing so is incredibly low, and meanwhile you're out all the computational resources you spent on your private chain that ends up being thrown out.

You'd be better off simply placing big bets at a casino

Estonia was planning (and announced) something along these lines .. but the EU basically told them no.

https://www.bloomberg.com/news/articles/2018-06-01/estonia-c...

I think OP was being facetious and was referring to normal currencies.

Maintaining that force also costs a lot of energy. Abnd such a currency doesn't do the same things as crypto currencies.

When a president tells the governor of the central bank not to raise interest rates despite the bank board's best professional judgement, is that considered an attack?

And then the governor of the central bank doesn't because they don't take their orders from the executive branch, because they're a private entity? That's actually the whole reason they're a private entity -- to separate monetary policy from politics. Them being a private entity was what Satoshi was mad about, and yet here it is, saving our asses. What if Satoshi was ... wrong about everything?

Ad hominem. It's an opinion that some "policy" is "saving our asses". BItcoin isn't undergoing a 51% attack, currently. You don't know Satoshi's identity.

Private entities still practice consensus. Just because one can't inspect or participate in the consensus doesn't mean it doesn't exist.

That’s all irrelevant to my point; I didn’t say bitcoin was being attacked. The “saving our asses” comment was that if the executive branch telling the fed how to monetary policy represents an attack then the private nature of the federal reserve represents a defense, one that’s working, or colloquially “saving our asses.” This effective structure is one bitcoin maximalists decry, and yet, it continues to work effectively. Further it’s not a stretch to attach that idea to Satoshi given the content of the genesis block. It’s not an ad hominem attack to suggest someone is wrong — even that someone’s entire thesis is wrong — the definition is to attack the attributes of someone instead of their argument, I’m refuting their argument.

It doesn’t matter who Satoshi is, I don’t need to know to opine on their ideas. That private entities practice consensus is fine (after all isn’t that just any human interaction?) and further I have no problem with consensus just the sheer wastefulness and intentional anti-efficiency of bitcoin as-is, the fact it’s waste isn’t sufficient to prevent an attack, it’s fiscal policy (inflation is fine, you’re not supposed to hold currencies) and the idea that somehow bitcoins dreadful wealth inequality will save us from the present dreadful wealth inequality. Not to mention the idea we should just defer monetary policy to a bunch of developers who hack on protocol instead of a group of trained economists. Or the idea of scarcity when anyone can and does fork coins to double the number outstanding every few weeks. Or it’s user hostility. Or it’s defenselessness to market manipulation. History won’t look kindly on this mess in a few years.

Or wait for many more confirms that the attack is impractical.

I'm not sure that is possible. If a 51% attack for 1-week costs $1-million USD to pull off (hypothetical round numbers against a hypothetical coin)... then how many confirms do you wait for?

If you wait for 1008 confirms (for a transaction of $1 Million USD), then that doesn't stop the attack at all. After all, the attacker simply has to perform $2 Million USD worth of double-spends during that timeframe to make a profit. (Not necessarily with you, but across the entire blockchain).

After waiting 1008 confirms (1-week for a 10 minute/block coin), the attacker simply replaces those 1008 blocks with his 1049 secret-blocks and makes his chain the longest. That means that ALL spends for the past week are nullified, allowing the attacker to double-spend his coins on the new blockchain.

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In effect, waiting for 1000-confirms (or whatever) is closing the barn door after the horse has bolted. It doesn't protect your transaction... it protects the NEXT GUY's transaction.

In any case, the attacker gets to double-spend his money, which probably includes a large exchange (where you can turn coins into US Dollars easily).

If the attacker has 50+% of mining power for the entire duration then yes it wouldn’t work as the longer things stretch out the further ahead the attacker gets (statistically).

However it the attacker has less than 50% they can still wage an attack. With 10% of mining power I’ve got a 1% chance of mining consecutive blocks. It dwindles quickly even for larger percentages and a large number of confirms makes that angle impractical.

> However it the attacker has less than 50% they can still wage an attack. With 10% of mining power I’ve got a 1% chance of mining consecutive blocks. It dwindles quickly even for larger percentages and a large number of confirms makes that angle impractical.

ETC's blockchain just had over 100+ blocks change, according to the tweet.

Even with 40% of the mining power, the probability of such an event (40% mining power, 100 consecutive blocks) is 0.000000000000000000000000000000000000016%. That's a very unlikely hypothesis you're presenting.

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