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I was reading there was a recent 51% attack to Ethereum. Basically, hackers rented enough computer power to get 51% of the mining power, and insert false transactions into the blockchain.

I wonder if such attacks are possible in a Proof of Work type of blockchain, as Tezos. I would assume that requires hackers to own 51% of the coins. According to tzscan, the current market cap of all the tokens is $346,558,758, which is not an insane amount (although, to be fair most of tokens are not engaged in sale orders, as they are standing in baking services; and even if someone wanted to buy a massive amount, this would raise the price and make the transaction much more expensive). Still, compared with PoS type of blockchains, it seems to me that PoW are more vulnerable to 51% attacks. Is this the case? Is there some mechanism by which the blockchain can protect itself from such attacks?

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At the present time about 80% of the tezos tokens are actively used to bake/stake so the circulating supply you can buy on exchange is quite low.

Therefore it would not even possible currently for an attacker to purchase 51% of tezos token and become a "majority holder".

Now the consensus protocol itself would not allow a 51% holder to double spend just like that because block baking rights are allocated long in advance (7 cycles or 21 days roughly) and to create a reorg you need to show a succession of block that have higher fitness than the current chain so if you look at the chain now you could already tell if some baker has been given rights to bake multiple blocks in succession.

In general to achieve statistical finality in tezos, you can wait 30 confirmations before you tx has enough confirmations to be ultra unlikely to be reorged away.

So you see that to attack the network the way you describe would not only require a large stake but also a large stake for a long time and that's not easy.

You can also find more information about finality in tezos here.

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