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Cost of a 51% attack for different cryptocurrencies?

crypto51.app

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Re: Cost of a 51% attack for different cryptocurrencies?

#11

Post by pat2man »

skitout wrote: post #8
pat2man wrote: post #4

Since it took a while for me to understand this. A 51% attack doesn’t let you steal money from anyone. It essentially lets you block all transactions from making it to the blockchain. Nodes will still verify all transactions and ignore transactions that are invalid. Edit: you can also create multiple forks and switch between them. External viewers will see both forks and if they don’t or can’t handle the difference t…

Or they can double spend some crypto, that is generally the main goal here

As far as the block chain is concerned they cannot. No single branch can have a double spend. But of course with deep reorgs they can defraud exchanges or other parties who end up on the wrong branch.

Re: Cost of a 51% attack for different cryptocurrencies?

#12

Post by pat2man »

7777fps wrote: post #9
pat2man wrote: post #4

Since it took a while for me to understand this. A 51% attack doesn’t let you steal money from anyone. It essentially lets you block all transactions from making it to the blockchain. Nodes will still verify all transactions and ignore transactions that are invalid. Edit: you can also create multiple forks and switch between them. External viewers will see both forks and if they don’t or can’t handle the difference t…

If you can hash faster than everyone else combined you can re-write history. So you could make a transaction and then re-write that transaction out of history allowing you to spend that money again. When you make a valid transaction, you have the necessary details for both that valid transaction and no transaction at all. Just because you can't make invalid transactions doesn't mean you can't effectively steal. This…

It’s an eventually consistent system, so you can defraud people who don’t take that into account. But internally it will always be consistent.

Re: Cost of a 51% attack for different cryptocurrencies?

#13

Post by flir »

TekMol wrote: post #10
pat2man wrote: post #7

Earlier quoted context omitted.

Only of those blocks are accepted by the chain in the future. Zero transaction blocks or blocks with invalid transactions could easily be ignore by the next miner.

Well, the reason for an attack is to reverse transactions. For that to have any value, the blocks must be accepted by the chain in the future. And no, a miner can not "easily" ignore previous blocks. And blocks with invalid transactions get ignored by everyone anyhow.

Nah, the reason is to split the head of the chain for a period of time, and do different things in each chain. Then when one of the heads is declared "true", you gain advantage (typically you spend the same coins twice, once in each head).

Re: Cost of a 51% attack for different cryptocurrencies?

#14

Post by skitout »

pat2man wrote: post #11
skitout wrote: post #8

Earlier quoted context omitted.

Or they can double spend some crypto, that is generally the main goal here

As far as the block chain is concerned they cannot. No single branch can have a double spend. But of course with deep reorgs they can defraud exchanges or other parties who end up on the wrong branch.

As far as the block chain is concerned they cannot... when they are not under attack. The attack can rewrite part of the history to double spend some crypto

Re: Cost of a 51% attack for different cryptocurrencies?

#16

Post by pat2man »

skitout wrote: post #14
pat2man wrote: post #11

Earlier quoted context omitted.

As far as the block chain is concerned they cannot. No single branch can have a double spend. But of course with deep reorgs they can defraud exchanges or other parties who end up on the wrong branch.

As far as the block chain is concerned they cannot... when they are not under attack. The attack can rewrite part of the history to double spend some crypto

They cannot rewrite history. But they can create multiple histories and switch between them.

Re: Cost of a 51% attack for different cryptocurrencies?

#17

Post by yodsanklai »

pat2man wrote: post #4

Since it took a while for me to understand this. A 51% attack doesn’t let you steal money from anyone. It essentially lets you block all transactions from making it to the blockchain. Nodes will still verify all transactions and ignore transactions that are invalid. Edit: you can also create multiple forks and switch between them. External viewers will see both forks and if they don’t or can’t handle the difference t…

My understanding is that the goal is double-spending. The attacker will make a transaction to a victim. The victim would be convinced it owns the coins, as the transaction appears in the chain. The attacker will then start mining a new chain (starting at a block before the transaction) which does not contain the transaction to the victim. If the attacker has enough computing power, it will be able to make the new chain the "official" one, invalidating the first chain.

Re: Cost of a 51% attack for different cryptocurrencies?

#18

Post by greenshackle2 »

flir wrote: post #13
TekMol wrote: post #10

Earlier quoted context omitted.

Well, the reason for an attack is to reverse transactions. For that to have any value, the blocks must be accepted by the chain in the future. And no, a miner can not "easily" ignore previous blocks. And blocks with invalid transactions get ignored by everyone anyhow.

Nah, the reason is to split the head of the chain for a period of time, and do different things in each chain. Then when one of the heads is declared "true", you gain advantage (typically you spend the same coins twice, once in each head).

You say "nah", but what you are describing is reversing a transaction, I don't see how that is disagreement.

From the POV of the recipient, when the split branch becomes the "non-true" branch, it looks like they got the money but then it disappeared.

Re: Cost of a 51% attack for different cryptocurrencies?

#19

Post by yangl1996 »

pat2man wrote: post #4

Since it took a while for me to understand this. A 51% attack doesn’t let you steal money from anyone. It essentially lets you block all transactions from making it to the blockchain. Nodes will still verify all transactions and ignore transactions that are invalid. Edit: you can also create multiple forks and switch between them. External viewers will see both forks and if they don’t or can’t handle the difference t…

It does, however, allows "double spending" attacks. In such attacks the attacker first spends a coin to buy some real goods from the victim. She then launches a 51% attack on the blockchain and "rewrites" the ledger to remove the transaction to the victim. Now the attacker gets back her money, plus the goods from the victim.

Re: Cost of a 51% attack for different cryptocurrencies?

#20

Post by lordnacho »

pat2man wrote: post #4

Since it took a while for me to understand this. A 51% attack doesn’t let you steal money from anyone. It essentially lets you block all transactions from making it to the blockchain. Nodes will still verify all transactions and ignore transactions that are invalid. Edit: you can also create multiple forks and switch between them. External viewers will see both forks and if they don’t or can’t handle the difference t…

> A 51% attack doesn’t let you steal money from anyone.

1. You deposit BTC at an exchange. The exchange credits you the amount in their non-BTC ledger.

2. You send off a chain of blocks overwriting the the original deposit so that you never did it.

3. You fill in the form to withdraw your credited amount from the exchange.

Now you have 2x the coins.

Of course there are a LOT of details to this that I won't get into, and a number of mitigations for the exchange. But that's the basic outline.

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