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Comment by jakecraige | Hacker News Reader

It would be helpful to expand on how many blocks can be reversed given only one hour of hash rate.

Many wallets, exchanges, etc require a minimum number of confirmations before allowing you to spend the coins to handle reorgs that happen naturally.

For example, in Bitcoin this is often around 6. With 1hr of hash rate you might be able to get 7 blocks but likely you’ll actually need more time to get enough blocks or successfully attack. Accounting for this would increase the cost substantially.

Either way, pretty neat site!

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I don't think it is particularly useful to talk about blocks reversed in this context.

In all cases, you can reverse about 1 hour worth of blocks. In practice, when wallets/exchanges/etc pick a minimum number of confirmations, they are really picking a minimum time. So, if bitcoin had double the blockrate, they wouldn't say 'well, we only require 6 blocks, so you only need ~30 minutes of an attack', but rather, 'we require ~1 hour, so you need 12 blocks of confirmation'.

Yeah it’s sort of time based assuming consistent blocks, but it would still based on confs. If the hash rate doubled then one would only require 30 minutes until the difficulty adjustment kicked in.

The main purpose of my post is that to actually pull off a successful attack you probably need more than an hour, so the numbers are making it look like it’s cheaper than it would be in practice (ignoring other factors other commenters have shared about acquiring ASICS, data centers, and such).

You can’t 51% attack Bitcoin for $611K, but your site says you can. While the site can’t be perfect, it would be helpful to make it more realistic. If a news site picks this up and doesn’t understand the nuances they’ll accidentally spread FUD and mislead a lot of people.

> If the hash rate doubled then one would only require 30 minutes until the difficulty adjustment kicked in.

Difficulty adjusts every 2016 blocks.

Yes, I’m saying that rather than needing an hour to get 6 blocks if the hash rate doubled, you’d only need 30 mins _until_ the adjustment occurs. Then it would be an hour again.

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