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Merge pull request #1 from bjornwgnr/patch-1

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fix typo
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Jeff Burdges
2018-09-19 11:48:09 +02:00
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Polkadot_Finality_Gadget_v9000.md
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@@ -279,7 +279,7 @@ In exchange for this, we get the property that we do not need to pay attention t
### Why have a primary?
We only need the primary for liveness. We need some form of coordination to defeat the repeated vote splitting attack. The idea behind that attack is that if we are in a situation where almost 2/3 of voters vote for something an the rest vote for another, then the Byzantine voters can control when we see a supermajority for something. If they can carefully time this, they may be able to split the next vote. Without the primary, they could do this for prevotes, getting a supermajority for a block $B$ late, then split precommits so we don't see that it is impossible fo thre to be a supermajority for $B$ until late. If $B$ is not the best block given the last finalised block but $B'$ with the same block number, they could stop either from being finalised like this even if the (unknown) fraction of Byzantine players is small.
We only need the primary for liveness. We need some form of coordination to defeat the repeated vote splitting attack. The idea behind that attack is that if we are in a situation where almost 2/3 of voters vote for something and the rest vote for another, then the Byzantine voters can control when we see a supermajority for something. If they can carefully time this, they may be able to split the next vote. Without the primary, they could do this for prevotes, getting a supermajority for a block $B$ late, then split precommits so we don't see that it is impossible fo thre to be a supermajority for $B$ until late. If $B$ is not the best block given the last finalised block but $B'$ with the same block number, they could stop either from being finalised like this even if the (unknown) fraction of Byzantine players is small.
When the network is well-behaved, an honest primary can defeat this attack by deciding how much we should agree on. We could also use a common coin for the same thing, where poeple would prevote for either the best chain containing $E_{r-1,v}​$ or $g(V_{r-1,v})​$ depending on the common coin. With on-chain voting, it is possible that we could use probabilistic finality of the block production mechanism - that if we don't finalise a block and always build on the best hain containing the last finalised block then not only will the best chain eventually converge, but if a block is behind the head of the best chain, then with positive probability, it will eventually be in the best chain everyone sees.