diff --git a/chain/actors/policy/policy.go b/chain/actors/policy/policy.go
index 8606119f0..f29da9802 100644
--- a/chain/actors/policy/policy.go
+++ b/chain/actors/policy/policy.go
@@ -97,3 +97,13 @@ func DealProviderCollateralBounds(
 		panic("unsupported network version")
 	}
 }
+
+// Sets the challenge window and scales the proving period to match (such that
+// there are always 48 challenge windows in a proving period).
+func SetWPoStChallengeWindow(period abi.ChainEpoch) {
+	miner0.WPoStChallengeWindow = period
+	miner0.WPoStProvingPeriod = period * abi.ChainEpoch(miner0.WPoStPeriodDeadlines)
+
+	miner2.WPoStChallengeWindow = period
+	miner2.WPoStProvingPeriod = period * abi.ChainEpoch(miner2.WPoStPeriodDeadlines)
+}
diff --git a/chain/actors/policy/policy_test.go b/chain/actors/policy/policy_test.go
index 8d6aac09f..62e7f8964 100644
--- a/chain/actors/policy/policy_test.go
+++ b/chain/actors/policy/policy_test.go
@@ -43,5 +43,8 @@ func TestAssumptions(t *testing.T) {
 	require.EqualValues(t, miner0.SupportedProofTypes, miner2.SupportedProofTypes)
 	require.Equal(t, miner0.PreCommitChallengeDelay, miner2.PreCommitChallengeDelay)
 	require.Equal(t, miner0.ChainFinality, miner2.ChainFinality)
+	require.Equal(t, miner0.WPoStChallengeWindow, miner2.WPoStChallengeWindow)
+	require.Equal(t, miner0.WPoStProvingPeriod, miner2.WPoStProvingPeriod)
+	require.Equal(t, miner0.WPoStPeriodDeadlines, miner2.WPoStPeriodDeadlines)
 	require.True(t, verifreg0.MinVerifiedDealSize.Equals(verifreg2.MinVerifiedDealSize))
 }
diff --git a/extern/oni b/extern/oni
index 9a0d5cd73..a5b57068d 160000
--- a/extern/oni
+++ b/extern/oni
@@ -1 +1 @@
-Subproject commit 9a0d5cd739de77b357589ac1fc8b756ed27299be
+Subproject commit a5b57068d4c2ee0fda76f053edf9825613d1fe9b