2 ms·

I didn't mean to compare speeds for comparing difficulty for two different currencies, I agree that's largely meaningless. Rather, the key probabilities I woul

I didn't mean to compare speeds for comparing difficulty for two different currencies, I agree that's largely meaningless. Rather, the key probabilities I would like to compute would the probability of chain rewriting (I don't like this term tbh; if someone forks off a new chain they haven't rewritten the one your block is in, rather they have "convinced" others to abandon it) within the next T days given an adversary with more compute than the Bitcoin network by a fraction f, for various T and f. Can't I approximate that with 1) the lead that my current chain has over the next competing chain, 2) the average time it's taking to mine a new block given the current compute? Ill confess I'm not super familiar with the intricate details of crypto, I may just be missing something obvious.

modeless 5y ago
You measure the "lead" of a chain by the amount of work it proves, not by the number of blocks or speed of blocks. If you based the lead solely on block number then an attacker's chain could manipulate the difficulty adjustment to make the attack easier. > Can't I approximate that with 1) the lead that my current chain has over the next competing chain An attacker could keep their chain secret until the attack is sprung. You cannot know how far ahead or behind you are relative to a secret chain. What you know is the amount of work an attacker would have to do, by summing the work proved by all the blocks above and including yours. Once that value gets high enough, you can be pretty confident that nobody is going to rewrite your transaction. The probability that you would have to calculate is the probability that an attacker who has that much compute exists.
- omegalulw 5y ago
  Thanks! This is good food for further reading, I had no idea miners could influence difficulty this way. I had thought, for Bitcoin, difficulty depends on how many coins mined till now so I (implicitly) assumed that small parts of the chain would have similar difficulty.

modeless 5y ago

You measure the "lead" of a chain by the amount of work it proves, not by the number of blocks or speed of blocks. If you based the lead solely on block number then an attacker's chain could manipulate the difficulty adjustment to make the attack easier. > Can't I approximate that with 1) the lead that my current chain has over the next competing chain An attacker could keep their chain secret until the attack is sprung. You cannot know how far ahead or behind you are relative to a secret chain. What you know is the amount of work an attacker would have to do, by summing the work proved by all the blocks above and including yours. Once that value gets high enough, you can be pretty confident that nobody is going to rewrite your transaction. The probability that you would have to calculate is the probability that an attacker who has that much compute exists.

omegalulw 5y ago
Thanks! This is good food for further reading, I had no idea miners could influence difficulty this way. I had thought, for Bitcoin, difficulty depends on how many coins mined till now so I (implicitly) assumed that small parts of the chain would have similar difficulty.