Question regarding newly received block(s) that try to forking the blockchain

[anistasas]

Hi everyone,

Sorry if this is already asked somewhere else. I'm thinking about what should I do if there are some miners broadcasting and sending me their blocks intentionally trying to fork at some points of the blockchain in the past. In other words, what are the usual criteria to verify whether I should accept this received block(s) as valid.

So I did some research but I'm not sured whether this covers everything. Here goes the process in brief:

- Check basic block format and transaction format; e.g. readable contents?
- Check timestamp; i.e. no later than median timestamp of the previously received 11 blocks and no faster than network-adjusted time + 2hrs
- Check that it didn't refer to block in the past beyond checkpoint

I know that forking to rewrite the blockchain is nearly impossible since you have to compete with a very large number of other miners. But assuming that there is a quantum computer out there doing mining, what can prevent him from doing so?

Any comments are welcomed.

Thanks

[ranochigo]

Assuming all the rules are conformed to, Bitcoin Core only consider the largest number of proof of work as valid. You need to take into account that the deeper blocks you want to modify, the more power you need. You just need only 51% of the network's hashing power. A power plant might be able to power it if you got the resources.

Just to clear things up, quantum computers are expensive to run and there isn't any proof that they are much faster than an ASIC. Anyway, it isn't hard to tell if an attack occurred, everyone can see the sudden block reorg and merchants can be notified to shut their system down.

[anistasas]

Thanks for your reply, Ranochigo. As you said, I agree that this issue might never ever happen in practical, and a single quantum computer might not be faster than all ASICs out there too. In sum, Bitcoin is pretty much secure and there are a handful of attacks that someone could do.