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"Rolling back" the network means successfully creating a new longest chain that forks off of an arbitrary old block. The further back in time you to "rollback"

"Rolling back" the network means successfully creating a new longest chain that forks off of an arbitrary old block. The further back in time you to "rollback" to, the less probable it is for the 51% to successfully mine all those needed blocks to get up to par with the existing longest chain. Realistically speaking, this sort of attack is highly improbable.

sillysaurusx 6y ago
Indeed. See the "bitcoin snack machine" for some hard numbers: https://bitcointalk.org/index.php?topic=423.0 https://bitcointalk.org/index.php?topic=423.0 > The network nodes only accept the first version of a transaction they receive to incorporate into the block they're trying to generate. When you broadcast a transaction, if someone else broadcasts a double-spend at the same time, it's a race to propagate to the most nodes first. If one has a slight head start, it'll geometrically spread through the network faster and get most of the nodes. > A rough back-of-the-envelope example: 1 0 4 1 16 4 64 16 80% 20% > So if a double-spend has to wait even a second, it has a huge disadvantage. I'm not sure anyone has put the infrastructure into place, though.
- Thorrez 6y ago
  That has nothing to do with rolling back or 51% attacks. A racing attack and a 51% attack are different things.
  - sillysaurusx 6y ago
    Oh, you're probably right. It seemed related because the further back an adversary wanted to roll back to, the more hash power they'd need to get ahead. But it's a different category. I wonder if there has been an analysis of how much hash power you'd need relative to how many blocks in the past you'd want to roll back.
    - Thorrez 6y ago
      That analysis is actually in the original 2008 bitcoin whitepaper: https://bitcoin.org/bitcoin.pdf https://bitcoin.org/bitcoin.pdf If you have 51% of the power, you can roll back everything if you spend enough time on it. If you have 30%, you have a 4% chance of being able to catch up from 10 blocks behind, even if you spend infinite time on that specific fork.

sillysaurusx 6y ago

Indeed. See the "bitcoin snack machine" for some hard numbers: https://bitcointalk.org/index.php?topic=423.0 https://bitcointalk.org/index.php?topic=423.0 > The network nodes only accept the first version of a transaction they receive to incorporate into the block they're trying to generate. When you broadcast a transaction, if someone else broadcasts a double-spend at the same time, it's a race to propagate to the most nodes first. If one has a slight head start, it'll geometrically spread through the network faster and get most of the nodes. > A rough back-of-the-envelope example: 1 0 4 1 16 4 64 16 80% 20% > So if a double-spend has to wait even a second, it has a huge disadvantage. I'm not sure anyone has put the infrastructure into place, though.

Thorrez 6y ago
That has nothing to do with rolling back or 51% attacks. A racing attack and a 51% attack are different things.
- sillysaurusx 6y ago
  Oh, you're probably right. It seemed related because the further back an adversary wanted to roll back to, the more hash power they'd need to get ahead. But it's a different category. I wonder if there has been an analysis of how much hash power you'd need relative to how many blocks in the past you'd want to roll back.
  - Thorrez 6y ago
    That analysis is actually in the original 2008 bitcoin whitepaper: https://bitcoin.org/bitcoin.pdf https://bitcoin.org/bitcoin.pdf If you have 51% of the power, you can roll back everything if you spend enough time on it. If you have 30%, you have a 4% chance of being able to catch up from 10 blocks behind, even if you spend infinite time on that specific fork.