Is the 51% Attack Still a Threat?

Quick question here, not an engineer, just a legal guy. So, about that 51% attack thing. If someone has over 50% hash rate, they can mess with blocks, right? But now, nodes and miners are kinda separate. If a miner tries some double spend stuff, good nodes will reject it. So does that mean it won’t really work?

In theory, yeah, a 51% attack can happen. But, check this out, you could even pull it off with less than 50% hash rate. Just need some luck on your side.

Nah, can’t agree here. A dishonest miner can technically do a 51% attack, but if it’s not over 50%, their chances drop big time. The longest chain wins, and if they succeed, it’s still valid.

Sure, double spends can still happen. Like, if a 51% attacker sends 100 BTC to an exchange, waits for 6 confirmations, then rewrites those blocks so the deposit vanishes. Crazy stuff. P.S. Look up that BTG 51% attack.

Bitcoin runs on consensus rules. Adding a rule to reject blocks based on some origins would mess with its neutrality. You can't just start censoring blocks, ya know?

Exactly, nodes just follow the longest chain that complies with their rules. The 51% attack doesn’t magically create extra Bitcoin for the attacker.

To sum up, if an attacker has more than 50% hash rate, they can roll the chain back and upend honest miners. But luck matters, especially with older blocks. And yeah, ABCbits pointed that out.

Gotcha. But with miners and nodes now working separately, nodes can reject blocks if older ones get messed with. It’s like Bitcoin Cash’s fork. Nodes don’t care about the hashrate anymore, just the consensus.

Nodes don’t vote so it’s not about agreement. They just follow the chain that meets their criteria. It’s really user-dependent to decide which chain is ‘valid’.

They could follow any chain, but realistically, they just go with the one that has the most work put into it. Hash power details aren’t something nodes consider since they can’t see it.

Disagree. What happened was that Bitcoin nodes ignored Bitcoin Cash blocks because they were invalid. Hashrate didn’t factor in. It was all about the protocol rules.

Honestly, a 51% attack is just plain dumb. You’d have to spend loads to set up mining operations, then what’s the point? Just to mess things up for yourself? If you do it, it’ll probably get fixed pretty quick.

Right, Bitcoin nodes have never mined blocks. That's the first thing to understand about them.

Just to clarify, in a 51% attack, the attacker changes the ‘tip’ of the chain, not random blocks down the line. If even one bit changes in a block, that whole chain goes invalid.

An attacker with over 50% can swap out confirmed blocks since they’ll eventually outpace the honest miners with their longer chain.

C'mon, the 51% attack is pretty much done for. Who's got the energy to coordinate that across all those mining pools? Too much hassle these days. Back in the day, maybe. But now Bitcoin is a beast.

An illustration would help explain this better. The attacker would start changing the chain from the tip down, not just any random block.

Oops, I misunderstood you. I thought ‘tip’ only meant the latest block.

Those early days ten years ago? They had better chances to attack Bitcoin. But as you said, those chances got missed.

Yeah, there were reports of mining pools holding loads of hash power. Deepbit, Ghash.io...but no one did a 51% attack even then.