Describe the bug
I originally saw (in lotus code) and I read "period: 25", assuming that drand round is 25s.
Given our analysis there must be at least one drand entry per block (for the slow catchup mode to fully work). Having less than 1 drand per block allows miners to rush during catch-up on some epochs (e.g. if drand is 30 and lotus is 25, they can catch up every 5 blocks).
We do not have analysis that show that the impact of a single rush every few epochs is insecure nor that it insecure, however, we do know that one drand per round is secure.
Proposal 1: Change Filecoin blocktime: Set Filecoin block time to ≥30s
Proposal 2: Change Filecoin blocktime: Set Drand round time to ≤25s