Does Casper FFG in phase 0 take its checkpoints from the current ETH 1.0 POW chain?
I'm reading some conflicting material on this. The Casper FFG paper describes taking a checkpoint at every 50 POW blocks - has this changed? Blocks on POW chain are created every 12 - 15 seconds, so 50 blocks is ~ 60 - 75 seconds, but it will take 2 checkpoints ( 120 - 150 seconds) to finalize a transaction. So is this the case? Will it take 2 minutes for my transaction to finalize with Casper FFG POS?
{1} First hybrid Casper FFG paper: https://arxiv.org/abs/1710.09437
{2} New hybrid Casper FFG overview paper: https://arxiv.org/abs/1903.04205
{3} EIP1001 (also specifies constants): https://github.com/ethereum/EIPs/blob/master/EIPS/eip-1011.md
Thank you for this interesting questions, during the creation of the answer I have learned a lot.
1. Does Casper FFG in phase 0 take its checkpoints from the current ETH 1.0 POW chain?
yes section 2.0 first sentence, in first phase it will be a hybrid chain, during which casper operates on the old POW chain {1}.
Within Ethereum, the proposal mechanism will initially be the existing proof of work chain, making the first version of Casper a hybrid PoW/PoS system. In future versions the PoW proposal mechanism will be replaced with something more efficient. For example, we can imagine converting the block proposal into a some kind of PoS round-robin block signing scheme.
2. The Casper FFG paper describes taking a checkpoint at every 50 POW blocks - has this changed?
Yes it has changed, in the initial paper they started of with 100 blocks, then EIP 1011 got released that suggested 50 block {3}.
EPOCH_LENGTH: 50 blocks
Since then one epoch, the time between checkpoints, equals 50 blocks. It is also stated that way in the new paper that delivers an overview of Casper FFG {2}.
l ∈ N denotes the epoch length: an epoch is defined as the contiguous sequence of blocks between two checkpoints, including the first but not the latter. Block 0 (which is also a checkpoint) denotes the genesis block. We will assume l = 50 thoughout this paper
3. Does it take 2 checkpoints ( 120 - 150 seconds) to finalize a transaction?
Note that each checkpoint only becomes "unrevertable" after it became finalized. Prequisites for that are that the checkpoint has a supermajority link (>2/3 POS votes based on staking amount) from a justified ancestor checkpoint (the first justified checkpoint is the genesis block) and additionally has a supermajority link to a direct child checkpoint. (The supermajority link to the ancestor checkpoint and the votes for the child checkpoint must be included at latest in the block associated with the child checkpoint.){1}
As far as I can tell, in the best case scenario this takes 2 epochs. In this case it will take 50-99 blocks (with EPOCH_LENGTH = 50 blocks) to finalize a checkpoint. The recent paper does support this statement {2}:
If correctly voting validators control more than 2/3 of the stake, then finalisation and hence, liveness are immediate.
They also include a graph in the paper, which plots how many epochs it will take in respect of the ratio of malicious voters. Just to give you some numbers, α denotes the honest voter ratio (in terms of staking amount) {2}:
for α = 0.33, 0,49, and 0.51, the number of epochs need for α-strong validators to resume finalisation is 3733, 2698, and 2546 respectively.
4. Will it take 2 minutes for my transaction to finalize with Casper FFG POS?
As long as Casper FFG operates on a hybrid PoW/PoS chain, it will take slightly more than the usual confirmation time. I think the confirmation time lies around 10-50 blocks, depending on the level of certainty the receiver of the funds wants to have. With hybrid Casper FFG, it will take 2 epochs = 100 blocks (with EPOCH_LENGTH = 50 blocks) {2}:
Finally, despite increasing security, the checkpoint mechanism does not reduce confirmation times (2 epochs = 100 blocks).
I think this is based on the fact that PoW forks of 50 or more blocks are extremely improbable (practically impossible).
If 1 block takes 14 seconds on average to mine, confirmation time will be around 100 * 14 seconds ≈ 23 minutes.
