diff --git a/AllTests-mainnet.md b/AllTests-mainnet.md
index bf687235b..7813d8ef3 100644
--- a/AllTests-mainnet.md
+++ b/AllTests-mainnet.md
@@ -101,9 +101,10 @@ OK: 1/1 Fail: 0/1 Skip: 0/1
 ```diff
 + iterating words                                                                            OK
 + overlaps                                                                                   OK
-+ roundtrips                                                                                 OK
++ roundtrips BitArray                                                                        OK
++ roundtrips BitSeq                                                                          OK
 ```
-OK: 3/3 Fail: 0/3 Skip: 0/3
+OK: 4/4 Fail: 0/4 Skip: 0/4
 ## Block pool processing [Preset: mainnet]
 ```diff
 + Adding the same block twice returns a Duplicate error [Preset: mainnet]                    OK
@@ -318,4 +319,4 @@ OK: 2/2 Fail: 0/2 Skip: 0/2
 OK: 1/1 Fail: 0/1 Skip: 0/1
 
 ---TOTAL---
-OK: 175/184 Fail: 0/184 Skip: 9/184
+OK: 176/185 Fail: 0/185 Skip: 9/185
diff --git a/beacon_chain/beacon_node_types.nim b/beacon_chain/beacon_node_types.nim
index 5e93c0562..dc506de4b 100644
--- a/beacon_chain/beacon_node_types.nim
+++ b/beacon_chain/beacon_node_types.nim
@@ -135,11 +135,11 @@ type
 
   AttestationSubnets* = object
     enabled*: bool
-    stabilitySubnets*: seq[tuple[subnet: uint8, expiration: Epoch]]
+    stabilitySubnets*: seq[tuple[subnet_id: SubnetId, expiration: Epoch]]
     nextCycleEpoch*: Epoch
 
     # These encode states in per-subnet state machines
-    subscribedSubnets*: set[uint8]
+    subscribedSubnets*: BitArray[ATTESTATION_SUBNET_COUNT]
     subscribeSlot*: array[ATTESTATION_SUBNET_COUNT, Slot]
     unsubscribeSlot*: array[ATTESTATION_SUBNET_COUNT, Slot]
 
diff --git a/beacon_chain/consensus_object_pools/spec_cache.nim b/beacon_chain/consensus_object_pools/spec_cache.nim
index 52c90a0d3..9660d93ac 100644
--- a/beacon_chain/consensus_object_pools/spec_cache.nim
+++ b/beacon_chain/consensus_object_pools/spec_cache.nim
@@ -24,6 +24,10 @@ func count_active_validators*(epochInfo: EpochRef): uint64 =
 func get_committee_count_per_slot*(epochInfo: EpochRef): uint64 =
   get_committee_count_per_slot(count_active_validators(epochInfo))
 
+iterator get_committee_indices*(epochRef: EpochRef): CommitteeIndex =
+  for i in 0'u64..<get_committee_count_per_slot(epochRef):
+    yield CommitteeIndex(i)
+
 # https://github.com/ethereum/eth2.0-specs/blob/v1.0.1/specs/phase0/beacon-chain.md#get_beacon_committee
 iterator get_beacon_committee*(
     epochRef: EpochRef, slot: Slot, index: CommitteeIndex): ValidatorIndex =
@@ -225,7 +229,7 @@ iterator get_committee_assignments*(
     epochRef: EpochRef, epoch: Epoch, validator_indices: IntSet):
     tuple[validatorIndices: IntSet,
       committeeIndex: CommitteeIndex,
-      subnetIndex: uint8, slot: Slot] =
+      subnet_id: SubnetId, slot: Slot] =
   let
     committees_per_slot = get_committee_count_per_slot(epochRef)
     start_slot = compute_start_slot_at_epoch(epoch)
@@ -240,5 +244,5 @@ iterator get_committee_assignments*(
       if includedIndices.len > 0:
         yield (
           includedIndices, idx,
-          compute_subnet_for_attestation(committees_per_slot, slot, idx).uint8,
+          compute_subnet_for_attestation(committees_per_slot, slot, idx),
           slot)
diff --git a/beacon_chain/gossip_processing/eth2_processor.nim b/beacon_chain/gossip_processing/eth2_processor.nim
index 5f62e992b..6422957a2 100644
--- a/beacon_chain/gossip_processing/eth2_processor.nim
+++ b/beacon_chain/gossip_processing/eth2_processor.nim
@@ -187,11 +187,11 @@ proc checkForPotentialDoppelganger(
 proc attestationValidator*(
     self: ref Eth2Processor,
     attestation: Attestation,
-    attestation_subnet: uint64,
-    checksExpensive: bool = true): Future[ValidationResult] {.async.} =
+    subnet_id: SubnetId,
+    checkSignature: bool = true): Future[ValidationResult] {.async.} =
   logScope:
     attestation = shortLog(attestation)
-    attestation_subnet
+    subnet_id
 
   let
     wallTime = self.getWallTime()
@@ -209,8 +209,7 @@ proc attestationValidator*(
 
   # Now proceed to validation
   let v = await self.attestationPool.validateAttestation(
-      self.batchCrypto,
-      attestation, wallTime, attestation_subnet, checksExpensive)
+      self.batchCrypto, attestation, wallTime, subnet_id, checkSignature)
   if v.isErr():
     debug "Dropping attestation", err = v.error()
     return v.error[0]
diff --git a/beacon_chain/gossip_processing/gossip_validation.nim b/beacon_chain/gossip_processing/gossip_validation.nim
index 4d334d4a5..1071413d4 100644
--- a/beacon_chain/gossip_processing/gossip_validation.nim
+++ b/beacon_chain/gossip_processing/gossip_validation.nim
@@ -140,14 +140,14 @@ func check_aggregation_count(
 
 func check_attestation_subnet(
     epochRef: EpochRef, attestation: Attestation,
-    attestation_subnet: uint64): Result[void, (ValidationResult, cstring)] =
+    subnet_id: SubnetId): Result[void, (ValidationResult, cstring)] =
   let
     expectedSubnet =
       compute_subnet_for_attestation(
         get_committee_count_per_slot(epochRef),
         attestation.data.slot, attestation.data.index.CommitteeIndex)
 
-  if expectedSubnet != attestation_subnet:
+  if expectedSubnet != subnet_id:
     return err((ValidationResult.Reject, cstring(
       "Attestation not on the correct subnet")))
 
@@ -162,7 +162,7 @@ proc validateAttestation*(
     batchCrypto: ref BatchCrypto,
     attestation: Attestation,
     wallTime: BeaconTime,
-    attestation_subnet: uint64, checksExpensive: bool):
+    subnet_id: SubnetId, checkSignature: bool):
     Future[Result[tuple[attesting_index: ValidatorIndex, sig: CookedSig],
       (ValidationResult, cstring)]] {.async.} =
   # Some of the checks below have been reordered compared to the spec, to
@@ -232,7 +232,7 @@ proc validateAttestation*(
   # attestation.data.target.epoch), which may be pre-computed along with the
   # committee information for the signature check.
   block:
-    let v = check_attestation_subnet(epochRef, attestation, attestation_subnet) # [REJECT]
+    let v = check_attestation_subnet(epochRef, attestation, subnet_id) # [REJECT]
     if v.isErr():
       return err(v.error)
 
@@ -271,14 +271,6 @@ proc validateAttestation*(
     return err((ValidationResult.Ignore, cstring(
       "Validator has already voted in epoch")))
 
-  if not checksExpensive:
-    # Only sendAttestation, which discards result, doesn't use checksExpensive
-    # TODO this means that (a) this becomes an "expensive" check and (b) it is
-    # doing in-principle unnecessary work, since this should be known from the
-    # attestation creation.
-    return ok((validator_index, attestation.signature.load.get().CookedSig))
-
-  # The signature of attestation is valid.
   block:
     # First pass - without cryptography
     let v = is_valid_indexed_attestation(
@@ -287,29 +279,38 @@ proc validateAttestation*(
     if v.isErr():
       return err((ValidationResult.Reject, v.error))
 
-  # Buffer crypto checks
-  let deferredCrypto = batchCrypto
-                        .scheduleAttestationCheck(
-                          fork, genesis_validators_root, epochRef,
-                          attestation
-                        )
-  if deferredCrypto.isNone():
-    return err((ValidationResult.Reject,
-                cstring("validateAttestation: crypto sanity checks failure")))
+  let sig =
+    if checkSignature:
+      # Attestation signatures are batch-verified
+      let deferredCrypto = batchCrypto
+                            .scheduleAttestationCheck(
+                              fork, genesis_validators_root, epochRef,
+                              attestation
+                            )
+      if deferredCrypto.isNone():
+        return err((ValidationResult.Reject,
+                    cstring("validateAttestation: crypto sanity checks failure")))
 
-  # Await the crypto check
-  let
-    (cryptoFut, sig) = deferredCrypto.get()
+      # Await the crypto check
+      let
+        (cryptoFut, sig) = deferredCrypto.get()
 
-  var x = (await cryptoFut)
-  case x
-  of BatchResult.Invalid:
-    return err((ValidationResult.Reject, cstring("validateAttestation: invalid signature")))
-  of BatchResult.Timeout:
-    beacon_attestations_dropped_queue_full.inc()
-    return err((ValidationResult.Ignore, cstring("validateAttestation: timeout checking signature")))
-  of BatchResult.Valid:
-    discard # keep going only in this case
+      var x = (await cryptoFut)
+      case x
+      of BatchResult.Invalid:
+        return err((ValidationResult.Reject, cstring("validateAttestation: invalid signature")))
+      of BatchResult.Timeout:
+        beacon_attestations_dropped_queue_full.inc()
+        return err((ValidationResult.Ignore, cstring("validateAttestation: timeout checking signature")))
+      of BatchResult.Valid:
+        sig # keep going only in this case
+    else:
+      let sig = attestation.signature.load()
+      if not sig.isSome():
+        return err((
+          ValidationResult.Ignore,
+          cstring("validateAttestation: unable to load signature")))
+      sig.get()
 
   # Only valid attestations go in the list, which keeps validator_index
   # in range
diff --git a/beacon_chain/networking/eth2_network.nim b/beacon_chain/networking/eth2_network.nim
index 5b24c1dfb..4849c83dc 100644
--- a/beacon_chain/networking/eth2_network.nim
+++ b/beacon_chain/networking/eth2_network.nim
@@ -31,7 +31,7 @@ import
   libp2p/stream/connection,
   libp2p/utils/semaphore,
   eth/[keys, async_utils], eth/p2p/p2p_protocol_dsl,
-  eth/net/nat, eth/p2p/discoveryv5/[enr, node],
+  eth/net/nat, eth/p2p/discoveryv5/[enr, node, random2],
   ".."/[
     version, conf,
     ssz/ssz_serialization, beacon_clock],
@@ -1763,3 +1763,50 @@ proc broadcast*(node: Eth2Node, topic: string, msg: auto) =
     traceMessage(futSnappy, gossipId(uncompressed, true))
   except IOError as exc:
     raiseAssert exc.msg # TODO in-memory compression shouldn't fail
+
+proc subscribeAttestationSubnets*(
+    node: Eth2Node, subnets: BitArray[ATTESTATION_SUBNET_COUNT]) {.
+    raises: [Defect, CatchableError].} =
+  # https://github.com/ethereum/eth2.0-specs/blob/v1.0.1/specs/phase0/p2p-interface.md#attestations-and-aggregation
+  # nimbus won't score attestation subnets for now, we just rely on block and aggregate which are more stabe and reliable
+
+  for subnet_id, enabled in subnets:
+    if enabled:
+      node.subscribe(getAttestationTopic(
+        node.forkID.fork_digest, SubnetId(subnet_id)), TopicParams.init()) # don't score attestation subnets for now
+
+proc unsubscribeAttestationSubnets*(
+    node: Eth2Node, subnets: BitArray[ATTESTATION_SUBNET_COUNT]) {.
+    raises: [Defect, CatchableError].} =
+  # https://github.com/ethereum/eth2.0-specs/blob/v1.0.1/specs/phase0/p2p-interface.md#attestations-and-aggregation
+  # nimbus won't score attestation subnets for now, we just rely on block and aggregate which are more stabe and reliable
+
+  for subnet_id, enabled in subnets:
+    if enabled:
+      node.unsubscribe(getAttestationTopic(
+        node.forkID.fork_digest, SubnetId(subnet_id)))
+
+proc updateStabilitySubnetMetadata*(
+    node: Eth2Node, attnets: BitArray[ATTESTATION_SUBNET_COUNT]) =
+  # https://github.com/ethereum/eth2.0-specs/blob/v1.0.1/specs/phase0/p2p-interface.md#metadata
+  node.metadata.seq_number += 1
+  node.metadata.attnets = attnets
+
+  # https://github.com/ethereum/eth2.0-specs/blob/v1.0.1/specs/phase0/validator.md#phase-0-attestation-subnet-stability
+  # https://github.com/ethereum/eth2.0-specs/blob/v1.0.1/specs/phase0/p2p-interface.md#attestation-subnet-bitfield
+  let res = node.discovery.updateRecord(
+    {"attnets": SSZ.encode(node.metadata.attnets)})
+  if res.isErr():
+    # This should not occur in this scenario as the private key would always
+    # be the correct one and the ENR will not increase in size.
+    warn "Failed to update record on subnet cycle", error = res.error
+  else:
+    debug "Stability subnets changed; updated ENR attnets", attnets
+
+# https://github.com/ethereum/eth2.0-specs/blob/v1.0.1/specs/phase0/validator.md#phase-0-attestation-subnet-stability
+func getStabilitySubnetLength*(node: Eth2Node): uint64 =
+  EPOCHS_PER_RANDOM_SUBNET_SUBSCRIPTION +
+    node.rng[].rand(EPOCHS_PER_RANDOM_SUBNET_SUBSCRIPTION.int).uint64
+
+func getRandomSubnetId*(node: Eth2Node): SubnetId =
+  node.rng[].rand(ATTESTATION_SUBNET_COUNT - 1).SubnetId
diff --git a/beacon_chain/nimbus_beacon_node.nim b/beacon_chain/nimbus_beacon_node.nim
index 8e09f1e30..286653bfc 100644
--- a/beacon_chain/nimbus_beacon_node.nim
+++ b/beacon_chain/nimbus_beacon_node.nim
@@ -384,8 +384,8 @@ proc init*(T: type BeaconNode,
             getVoluntaryExitsTopic(enrForkId.fork_digest),
             getAggregateAndProofsTopic(enrForkId.fork_digest)
           ]
-      for subnet in 0'u64 ..< ATTESTATION_SUBNET_COUNT:
-        topics &= getAttestationTopic(enrForkId.fork_digest, subnet)
+      for subnet_id in 0'u64 ..< ATTESTATION_SUBNET_COUNT:
+        topics &= getAttestationTopic(enrForkId.fork_digest, SubnetId(subnet_id))
       topics)
 
   if node.config.inProcessValidators:
@@ -426,34 +426,9 @@ func verifyFinalization(node: BeaconNode, slot: Slot) =
     # finalization occurs every slot, to 4 slots vs scheduledSlot.
     doAssert finalizedEpoch + 4 >= epoch
 
-proc installAttestationSubnetHandlers(node: BeaconNode, subnets: set[uint8]) {.
-    raises: [Defect, CatchableError].} =
-  # https://github.com/ethereum/eth2.0-specs/blob/v1.0.1/specs/phase0/p2p-interface.md#attestations-and-aggregation
-  # nimbus won't score attestation subnets for now, we just rely on block and aggregate which are more stabe and reliable
-  for subnet in subnets:
-    node.network.subscribe(getAttestationTopic(node.forkDigest, subnet), TopicParams.init()) # don't score attestation subnets for now
-
-proc updateStabilitySubnetMetadata(
-    node: BeaconNode, stabilitySubnets: set[uint8]) =
-  # https://github.com/ethereum/eth2.0-specs/blob/v1.0.1/specs/phase0/p2p-interface.md#metadata
-  node.network.metadata.seq_number += 1
-  for subnet in 0'u8 ..< ATTESTATION_SUBNET_COUNT:
-    node.network.metadata.attnets[subnet] = (subnet in stabilitySubnets)
-
-  # https://github.com/ethereum/eth2.0-specs/blob/v1.0.1/specs/phase0/validator.md#phase-0-attestation-subnet-stability
-  # https://github.com/ethereum/eth2.0-specs/blob/v1.0.1/specs/phase0/p2p-interface.md#attestation-subnet-bitfield
-  let res = node.network.discovery.updateRecord(
-    {"attnets": SSZ.encode(node.network.metadata.attnets)})
-  if res.isErr():
-    # This should not occur in this scenario as the private key would always
-    # be the correct one and the ENR will not increase in size.
-    warn "Failed to update record on subnet cycle", error = res.error
-  else:
-    debug "Stability subnets changed; updated ENR attnets", stabilitySubnets
-
-func getStabilitySubnets(stabilitySubnets: auto): set[uint8] =
+func getStabilitySubnets(stabilitySubnets: auto): BitArray[ATTESTATION_SUBNET_COUNT] =
   for subnetInfo in stabilitySubnets:
-    result.incl subnetInfo.subnet
+    result[subnetInfo.subnet_id.int] = true
 
 proc getAttachedValidators(node: BeaconNode):
     Table[ValidatorIndex, AttachedValidator] =
@@ -503,7 +478,7 @@ proc updateSubscriptionSchedule(node: BeaconNode, epoch: Epoch) {.async.} =
   # The relevant bitmaps are 32 bits each.
   static: doAssert SLOTS_PER_EPOCH <= 32
 
-  for (validatorIndices, committeeIndex, subnetIndex, slot) in
+  for (validatorIndices, committeeIndex, subnet_id, slot) in
       get_committee_assignments(epochRef, epoch, validatorIndices):
 
     doAssert compute_epoch_at_slot(slot) == epoch
@@ -530,43 +505,36 @@ proc updateSubscriptionSchedule(node: BeaconNode, epoch: Epoch) {.async.} =
         get_beacon_committee_len(epochRef, slot, committeeIndex), slot):
       continue
 
-    node.attestationSubnets.unsubscribeSlot[subnetIndex] =
-      max(slot + 1, node.attestationSubnets.unsubscribeSlot[subnetIndex])
-    if subnetIndex notin node.attestationSubnets.subscribedSubnets:
+    node.attestationSubnets.unsubscribeSlot[subnet_id.uint64] =
+      max(slot + 1, node.attestationSubnets.unsubscribeSlot[subnet_id.uint64])
+    if node.attestationSubnets.subscribedSubnets[subnet_id.uint64]:
       const SUBNET_SUBSCRIPTION_LEAD_TIME_SLOTS = 34
 
-      node.attestationSubnets.subscribeSlot[subnetIndex] =
+      node.attestationSubnets.subscribeSlot[subnet_id.uint64] =
         # Queue upcoming subscription potentially earlier
         # SLOTS_PER_EPOCH emulates one boundary condition of the per-epoch
         # cycling mechanism timing buffers
         min(
           slot - min(slot.uint64, SUBNET_SUBSCRIPTION_LEAD_TIME_SLOTS),
-          node.attestationSubnets.subscribeSlot[subnetIndex])
+          node.attestationSubnets.subscribeSlot[subnet_id.uint64])
 
-# https://github.com/ethereum/eth2.0-specs/blob/v1.0.1/specs/phase0/validator.md#phase-0-attestation-subnet-stability
-func getStabilitySubnetLength(node: BeaconNode): uint64 =
-  EPOCHS_PER_RANDOM_SUBNET_SUBSCRIPTION +
-    node.network.rng[].rand(EPOCHS_PER_RANDOM_SUBNET_SUBSCRIPTION.int).uint64
-
-func updateStabilitySubnets(node: BeaconNode, slot: Slot): set[uint8] =
+func updateStabilitySubnets(node: BeaconNode, slot: Slot): BitArray[ATTESTATION_SUBNET_COUNT] =
   # Equivalent to wallSlot by cycleAttestationSubnets(), especially
   # since it'll try to run early in epochs, avoiding race conditions.
-  static: doAssert ATTESTATION_SUBNET_COUNT <= high(uint8)
   let epoch = slot.epoch
 
   # https://github.com/ethereum/eth2.0-specs/blob/v1.0.1/specs/phase0/validator.md#phase-0-attestation-subnet-stability
-  for i in 0 ..< node.attestationSubnets.stabilitySubnets.len:
-    if epoch >= node.attestationSubnets.stabilitySubnets[i].expiration:
-      node.attestationSubnets.stabilitySubnets[i].subnet =
-        node.network.rng[].rand(ATTESTATION_SUBNET_COUNT - 1).uint8
-      node.attestationSubnets.stabilitySubnets[i].expiration =
-        epoch + node.getStabilitySubnetLength()
+  for ss in node.attestationSubnets.stabilitySubnets.mitems():
+    if epoch >= ss.expiration:
+      ss.subnet_id = node.network.getRandomSubnetId()
+      ss.expiration = epoch + node.network.getStabilitySubnetLength()
 
-    result.incl node.attestationSubnets.stabilitySubnets[i].subnet
+    result[ss.subnet_id.int] = true
 
 proc cycleAttestationSubnetsPerEpoch(
-    node: BeaconNode, wallSlot: Slot, prevStabilitySubnets: set[uint8]):
-    Future[set[uint8]] {.async.} =
+    node: BeaconNode, wallSlot: Slot,
+    prevStabilitySubnets: BitArray[ATTESTATION_SUBNET_COUNT]):
+    Future[BitArray[ATTESTATION_SUBNET_COUNT]] {.async.} =
   # Per-epoch portion of subnet cycling: updating stability subnets and
   # calculating future attestation subnets.
 
@@ -595,12 +563,12 @@ proc cycleAttestationSubnetsPerEpoch(
 
   let stabilitySubnets = node.updateStabilitySubnets(wallSlot)
 
-  if  not node.config.subscribeAllSubnets and
+  if not node.config.subscribeAllSubnets and
       stabilitySubnets != prevStabilitySubnets:
     # In subscribeAllSubnets mode, this only gets set once, at initial subnet
     # attestation handler creation, since they're all considered as stability
     # subnets in that case.
-    node.updateStabilitySubnetMetadata(stabilitySubnets)
+    node.network.updateStabilitySubnetMetadata(stabilitySubnets)
 
   return stabilitySubnets
 
@@ -610,36 +578,34 @@ proc cycleAttestationSubnets(node: BeaconNode, wallSlot: Slot) {.async.} =
 
   let prevSubscribedSubnets = node.attestationSubnets.subscribedSubnets
 
-  for i in 0'u8 ..< ATTESTATION_SUBNET_COUNT:
-    if i in node.attestationSubnets.subscribedSubnets:
+  for i in 0..<node.attestationSubnets.subscribedSubnets.len():
+    if node.attestationSubnets.subscribedSubnets[i]:
       if wallSlot >= node.attestationSubnets.unsubscribeSlot[i]:
-        node.attestationSubnets.subscribedSubnets.excl i
+        node.attestationSubnets.subscribedSubnets[i] = false
     else:
       if wallSlot >= node.attestationSubnets.subscribeSlot[i]:
-        node.attestationSubnets.subscribedSubnets.incl i
+        node.attestationSubnets.subscribedSubnets[i] = true
 
   let
     prevStabilitySubnets =
-      getStabilitySubnets(node.attestationSubnets.stabilitySubnets)
+      node.attestationSubnets.stabilitySubnets.getStabilitySubnets()
     stabilitySubnets =
       await node.cycleAttestationSubnetsPerEpoch(wallSlot, prevStabilitySubnets)
 
   # Accounting specific to non-stability subnets
-  for expiringSubnet in
-      prevSubscribedSubnets - node.attestationSubnets.subscribedSubnets:
-    node.attestationSubnets.subscribeSlot[expiringSubnet] = FAR_FUTURE_SLOT
+  for i, enabled in
+      (prevSubscribedSubnets - node.attestationSubnets.subscribedSubnets):
+    if enabled:
+      node.attestationSubnets.subscribeSlot[i] = FAR_FUTURE_SLOT
 
   let
     prevAllSubnets = prevSubscribedSubnets + prevStabilitySubnets
     allSubnets = node.attestationSubnets.subscribedSubnets + stabilitySubnets
-    unsubscribedSubnets = prevAllSubnets - allSubnets
-    subscribedSubnets = allSubnets - prevAllSubnets
+    unsubscribeSubnets = prevAllSubnets - allSubnets
+    subscribeSubnets = allSubnets - prevAllSubnets
 
-  for subnet in unsubscribedSubnets:
-    node.network.unsubscribe(
-      getAttestationTopic(node.forkDigest, subnet))
-
-  node.installAttestationSubnetHandlers(subscribedSubnets)
+  node.network.unsubscribeAttestationSubnets(unsubscribeSubnets)
+  node.network.subscribeAttestationSubnets(subscribeSubnets)
 
   debug "Attestation subnets",
     expiringSubnets =
@@ -652,10 +618,10 @@ proc cycleAttestationSubnets(node: BeaconNode, wallSlot: Slot) {.async.} =
     num_stability_subnets = node.attestationSubnets.stabilitySubnets.len,
     expiring_stability_subnets = prevStabilitySubnets - stabilitySubnets,
     new_stability_subnets = stabilitySubnets - prevStabilitySubnets,
-    subscribedSubnets,
-    unsubscribedSubnets
+    subscribeSubnets,
+    unsubscribeSubnets
 
-proc getInitialAttestationSubnets(node: BeaconNode): Table[uint8, Slot] =
+proc getInitialAttestationSubnets(node: BeaconNode): Table[SubnetId, Slot] =
   let
     wallEpoch = node.beaconClock.now().slotOrZero().epoch
     validatorIndices = toIntSet(toSeq(node.getAttachedValidators().keys()))
@@ -665,12 +631,12 @@ proc getInitialAttestationSubnets(node: BeaconNode): Table[uint8, Slot] =
     # https://github.com/nim-lang/Nim/issues/16217 are fixed, in
     # Nimbus's Nim, use (_, _, subnetIndex, slot).
     let epochRef = node.chainDag.getEpochRef(node.chainDag.head, epoch)
-    for (_, ci, subnetIndex, slot) in get_committee_assignments(
+    for (_, ci, subnet_id, slot) in get_committee_assignments(
         epochRef, epoch, validatorIndices):
-      result.withValue(subnetIndex, v) do:
+      result.withValue(subnet_id, v) do:
         v[] = max(v[], slot + 1)
       do:
-        result[subnetIndex] = slot + 1
+        result[subnet_id] = slot + 1
 
   # Either wallEpoch is 0, in which case it might be pre-genesis, but we only
   # care about the already-known first two epochs of attestations, or it's in
@@ -680,7 +646,7 @@ proc getInitialAttestationSubnets(node: BeaconNode): Table[uint8, Slot] =
   mergeAttestationSubnets(wallEpoch)
   mergeAttestationSubnets(wallEpoch + 1)
 
-proc getAttestationSubnetHandlers(node: BeaconNode) {.
+proc subscribeAttestationSubnetHandlers(node: BeaconNode) {.
   raises: [Defect, CatchableError].} =
   # https://github.com/ethereum/eth2.0-specs/blob/v1.0.1/specs/phase0/validator.md#phase-0-attestation-subnet-stability
   # TODO:
@@ -688,53 +654,46 @@ proc getAttestationSubnetHandlers(node: BeaconNode) {.
   # - Restarting the node with a presistent netkey
   # - When going from synced -> syncing -> synced state
 
-  template getAllAttestationSubnets(): Table[uint8, Slot] =
-    var subnets: Table[uint8, Slot]
-    for i in 0'u8 ..< ATTESTATION_SUBNET_COUNT:
-      subnets[i] = FAR_FUTURE_SLOT
-    subnets
+  if node.config.subscribeAllSubnets:
+    # In all-subnets mode, we create a stability subnet subscription for every
+    # subnet - this will be propagated in the attnets ENR entry
+    node.attestationSubnets.stabilitySubnets.setLen(ATTESTATION_SUBNET_COUNT)
+    for i, ss in node.attestationSubnets.stabilitySubnets.mpairs():
+      ss.subnet_id = SubnetId(i)
+      ss.expiration = FAR_FUTURE_EPOCH
+  else:
+    let wallEpoch = node.beaconClock.now().slotOrZero().epoch
+
+    # TODO make length dynamic when validator-client-based validators join and leave
+    # In normal mode, there's one subnet subscription per validator, changing
+    # randomly over time
+    node.attestationSubnets.stabilitySubnets.setLen(
+      node.attachedValidators[].count)
+    for i, ss in node.attestationSubnets.stabilitySubnets.mpairs():
+      ss.subnet_id = node.network.getRandomSubnetId()
+      ss.expiration = wallEpoch + node.network.getStabilitySubnetLength()
+
+  let initialStabilitySubnets =
+    node.attestationSubnets.stabilitySubnets.getStabilitySubnets()
+  node.network.updateStabilitySubnetMetadata(initialStabilitySubnets)
 
   let
-    initialSubnets =
-      if node.config.subscribeAllSubnets:
-        getAllAttestationSubnets()
-      else:
-        node.getInitialAttestationSubnets()
-    wallEpoch = node.beaconClock.now().slotOrZero().epoch
-
-  var initialStabilitySubnets: set[uint8]
-
-  node.attestationSubnets.stabilitySubnets.setLen(
-    node.attachedValidators[].count)
-  for i in 0 ..< node.attachedValidators[].count:
-    node.attestationSubnets.stabilitySubnets[i] = (
-      subnet: node.network.rng[].rand(ATTESTATION_SUBNET_COUNT - 1).uint8,
-      expiration: wallEpoch + node.getStabilitySubnetLength())
-    initialStabilitySubnets.incl(
-      node.attestationSubnets.stabilitySubnets[i].subnet)
-
-  node.updateStabilitySubnetMetadata(
-    if node.config.subscribeAllSubnets:
-      {0'u8 .. (ATTESTATION_SUBNET_COUNT - 1)}
-    else:
-      node.attestationSubnets.stabilitySubnets.getStabilitySubnets)
-
+    initialSubnets = node.getInitialAttestationSubnets()
   for i in 0'u8 ..< ATTESTATION_SUBNET_COUNT:
-    if i in initialSubnets:
-      node.attestationSubnets.subscribedSubnets.incl i
+    if SubnetId(i) in initialSubnets:
+      node.attestationSubnets.subscribedSubnets[i] = true
       node.attestationSubnets.unsubscribeSlot[i] =
-        try: initialSubnets[i] except KeyError: raiseAssert "checked with in"
+        try: initialSubnets[SubnetId(i)] except KeyError: raiseAssert "checked with in"
     else:
-      node.attestationSubnets.subscribedSubnets.excl i
+      node.attestationSubnets.subscribedSubnets[i] = false
       node.attestationSubnets.subscribeSlot[i] = FAR_FUTURE_SLOT
 
   node.attestationSubnets.enabled = true
 
   debug "Initial attestation subnets subscribed",
      initialSubnets,
-     initialStabilitySubnets,
-     wallEpoch
-  node.installAttestationSubnetHandlers(
+     initialStabilitySubnets
+  node.network.subscribeAttestationSubnets(
     node.attestationSubnets.subscribedSubnets + initialStabilitySubnets)
 
 proc addMessageHandlers(node: BeaconNode) {.raises: [Defect, CatchableError].} =
@@ -792,7 +751,7 @@ proc addMessageHandlers(node: BeaconNode) {.raises: [Defect, CatchableError].} =
   node.network.subscribe(getProposerSlashingsTopic(node.forkDigest), basicParams)
   node.network.subscribe(getVoluntaryExitsTopic(node.forkDigest), basicParams)
   node.network.subscribe(getAggregateAndProofsTopic(node.forkDigest), aggregateTopicParams, enableTopicMetrics = true)
-  node.getAttestationSubnetHandlers()
+  node.subscribeAttestationSubnetHandlers()
 
 func getTopicSubscriptionEnabled(node: BeaconNode): bool =
   node.attestationSubnets.enabled
@@ -807,8 +766,9 @@ proc removeMessageHandlers(node: BeaconNode) {.raises: [Defect, CatchableError].
   node.network.unsubscribe(getAttesterSlashingsTopic(node.forkDigest))
   node.network.unsubscribe(getAggregateAndProofsTopic(node.forkDigest))
 
-  for subnet in 0'u64 ..< ATTESTATION_SUBNET_COUNT:
-    node.network.unsubscribe(getAttestationTopic(node.forkDigest, subnet))
+  for subnet_id in 0'u64 ..< ATTESTATION_SUBNET_COUNT:
+    node.network.unsubscribe(
+      getAttestationTopic(node.forkDigest, SubnetId(subnet_id)))
 
 proc setupDoppelgangerDetection(node: BeaconNode, slot: Slot) =
   # When another client's already running, this is very likely to detect
@@ -1185,12 +1145,12 @@ proc installMessageValidators(node: BeaconNode) =
   # subnets are subscribed to during any given epoch.
   for it in 0'u64 ..< ATTESTATION_SUBNET_COUNT.uint64:
     closureScope:
-      let attestation_subnet = it
+      let subnet_id = SubnetId(it)
       node.network.addAsyncValidator(
-        getAttestationTopic(node.forkDigest, attestation_subnet),
+        getAttestationTopic(node.forkDigest, subnet_id),
         # This proc needs to be within closureScope; don't lift out of loop.
         proc(attestation: Attestation): Future[ValidationResult] =
-          node.processor.attestationValidator(attestation, attestation_subnet))
+          node.processor.attestationValidator(attestation, subnet_id))
 
   node.network.addAsyncValidator(
     getAggregateAndProofsTopic(node.forkDigest),
diff --git a/beacon_chain/nimbus_validator_client.nim b/beacon_chain/nimbus_validator_client.nim
index c94dc45b0..af4542040 100644
--- a/beacon_chain/nimbus_validator_client.nim
+++ b/beacon_chain/nimbus_validator_client.nim
@@ -194,9 +194,12 @@ proc onSlotStart(vc: ValidatorClient, lastSlot, scheduledSlot: Slot) {.gcsafe, a
             ad, a.committee_length.int, a.validator_committee_index,
             vc.fork, vc.beaconGenesis.genesis_validators_root)
 
-          notice "Attesting",
-            slot, public_key = a.public_key, attestation = shortLog(attestation)
-          discard await vc.client.post_v1_beacon_pool_attestations(attestation)
+          notice "Sending attestation to beacon node",
+            public_key = a.public_key, attestation = shortLog(attestation)
+          let ok = await vc.client.post_v1_beacon_pool_attestations(attestation)
+          if not ok:
+            warn "Failed to send attestation to beacon node",
+              public_key = a.public_key, attestation = shortLog(attestation)
 
           validatorToAttestationDataRoot[a.public_key] = attestation.data.hash_tree_root
         else:
diff --git a/beacon_chain/rpc/beacon_api.nim b/beacon_chain/rpc/beacon_api.nim
index 97b2a4213..2353b4837 100644
--- a/beacon_chain/rpc/beacon_api.nim
+++ b/beacon_chain/rpc/beacon_api.nim
@@ -461,8 +461,7 @@ proc installBeaconApiHandlers*(rpcServer: RpcServer, node: BeaconNode) {.
 
   rpcServer.rpc("post_v1_beacon_pool_attestations") do (
       attestation: Attestation) -> bool:
-    node.sendAttestation(attestation)
-    return true
+    return await node.sendAttestation(attestation)
 
   rpcServer.rpc("get_v1_beacon_pool_attester_slashings") do (
       ) -> seq[AttesterSlashing]:
diff --git a/beacon_chain/rpc/beacon_rest_api.nim b/beacon_chain/rpc/beacon_rest_api.nim
index 43c614cb0..09bd3ee58 100644
--- a/beacon_chain/rpc/beacon_rest_api.nim
+++ b/beacon_chain/rpc/beacon_rest_api.nim
@@ -718,15 +718,9 @@ proc installBeaconApiHandlers*(router: var RestRouter, node: BeaconNode) =
 
     var failures: seq[RestAttestationsFailureTuple]
     for atindex, attestation in attestations.pairs():
-      let wallTime = node.processor.getWallTime()
-      let res = await node.attestationPool.validateAttestation(
-        node.processor.batchCrypto, attestation, wallTime,
-        attestation.data.index, true
-      )
-      if res.isErr():
-        failures.add((index: uint64(atindex), message: $res.error()))
-      else:
-        node.sendAttestation(attestation)
+      if not await node.sendAttestation(attestation):
+        failures.add(
+          (index: uint64(atindex), message: "Attestation failed validation"))
 
     if len(failures) > 0:
       return RestApiResponse.jsonErrorList(Http400, AttestationValidationError,
diff --git a/beacon_chain/rpc/validator_api.nim b/beacon_chain/rpc/validator_api.nim
index 30508e8ae..8194008ee 100644
--- a/beacon_chain/rpc/validator_api.nim
+++ b/beacon_chain/rpc/validator_api.nim
@@ -150,21 +150,21 @@ proc installValidatorApiHandlers*(rpcServer: RpcServer, node: BeaconNode) {.
     let
       head = node.doChecksAndGetCurrentHead(epoch)
       epochRef = node.chainDag.getEpochRef(head, epoch)
-      subnet = compute_subnet_for_attestation(
-        get_committee_count_per_slot(epochRef), slot, committee_index).uint8
+      subnet_id = compute_subnet_for_attestation(
+        get_committee_count_per_slot(epochRef), slot, committee_index)
 
     # Either subnet already subscribed or not. If not, subscribe. If it is,
     # extend subscription. All one knows from the API combined with how far
     # ahead one can check for attestation schedule is that it might be used
     # for up to the end of next epoch. Therefore, arrange for subscriptions
     # to last at least that long.
-    if subnet notin node.attestationSubnets.subscribedSubnets:
+    if node.attestationSubnets.subscribedSubnets[subnet_id.uint64]:
       # When to subscribe. Since it's not clear when from the API it's first
       # needed, do so immediately.
-      node.attestationSubnets.subscribeSlot[subnet] =
-        min(node.attestationSubnets.subscribeSlot[subnet], wallSlot)
+      node.attestationSubnets.subscribeSlot[subnet_id.uint64] =
+        min(node.attestationSubnets.subscribeSlot[subnet_id.uint64], wallSlot)
 
-    node.attestationSubnets.unsubscribeSlot[subnet] =
+    node.attestationSubnets.unsubscribeSlot[subnet_id.uint64] =
       max(
         compute_start_slot_at_epoch(epoch + 2),
-        node.attestationSubnets.unsubscribeSlot[subnet])
+        node.attestationSubnets.unsubscribeSlot[subnet_id.uint64])
diff --git a/beacon_chain/spec/datatypes/base.nim b/beacon_chain/spec/datatypes/base.nim
index f5d7645f7..3eb73895e 100644
--- a/beacon_chain/spec/datatypes/base.nim
+++ b/beacon_chain/spec/datatypes/base.nim
@@ -149,6 +149,10 @@ type
   # leave it at spec size
   CommitteeIndex* = distinct uint64
 
+  # The subnet id maps which gossip subscription to use to publish an
+  # attestation - it is distinct from the CommitteeIndex in particular
+  SubnetId* = distinct uint8
+
   Gwei* = uint64
 
   # https://github.com/ethereum/eth2.0-specs/blob/v1.0.1/specs/phase0/beacon-chain.md#proposerslashing
@@ -816,6 +820,18 @@ proc readValue*(reader: var JsonReader, value: var CommitteeIndex)
                {.raises: [IOError, SerializationError, Defect].} =
   value = CommitteeIndex reader.readValue(distinctBase CommitteeIndex)
 
+proc writeValue*(writer: var JsonWriter, value: SubnetId)
+                {.raises: [IOError, Defect].} =
+  writeValue(writer, distinctBase value)
+
+proc readValue*(reader: var JsonReader, value: var SubnetId)
+               {.raises: [IOError, SerializationError, Defect].} =
+  let v = reader.readValue(distinctBase SubnetId)
+  if v > ATTESTATION_SUBNET_COUNT:
+    raiseUnexpectedValue(
+      reader, "Subnet id must be <= " & $ATTESTATION_SUBNET_COUNT)
+  value = SubnetId(v)
+
 proc writeValue*(writer: var JsonWriter, value: HashList)
                 {.raises: [IOError, SerializationError, Defect].} =
   writeValue(writer, value.data)
@@ -883,6 +899,9 @@ proc `<`*(x, y: CommitteeIndex) : bool {.borrow, noSideEffect.}
 proc hash*(x: CommitteeIndex): Hash {.borrow, noSideEffect.}
 func `$`*(x: CommitteeIndex): auto = $(distinctBase(x))
 
+proc `==`*(x, y: SubnetId) : bool {.borrow, noSideEffect.}
+proc `$`*(x: SubnetId): string {.borrow, noSideEffect.}
+
 func `as`*(d: DepositData, T: type DepositMessage): T =
   T(pubkey: d.pubkey,
     withdrawal_credentials: d.withdrawal_credentials,
@@ -1138,3 +1157,4 @@ static:
   # Sanity checks - these types should be trivial enough to copy with memcpy
   doAssert supportsCopyMem(Validator)
   doAssert supportsCopyMem(Eth2Digest)
+  doAssert ATTESTATION_SUBNET_COUNT <= high(distinctBase SubnetId).int
diff --git a/beacon_chain/spec/network.nim b/beacon_chain/spec/network.nim
index a03ea2eeb..bb9fdd2a8 100644
--- a/beacon_chain/spec/network.nim
+++ b/beacon_chain/spec/network.nim
@@ -53,7 +53,7 @@ func getAggregateAndProofsTopic*(forkDigest: ForkDigest): string =
 # https://github.com/ethereum/eth2.0-specs/blob/v1.0.1/specs/phase0/validator.md#broadcast-attestation
 func compute_subnet_for_attestation*(
     committees_per_slot: uint64, slot: Slot, committee_index: CommitteeIndex):
-    uint64 =
+    SubnetId =
   # Compute the correct subnet for an attestation for Phase 0.
   # Note, this mimics expected Phase 1 behavior where attestations will be
   # mapped to their shard subnet.
@@ -62,16 +62,15 @@ func compute_subnet_for_attestation*(
     committees_since_epoch_start =
       committees_per_slot * slots_since_epoch_start
 
-  (committees_since_epoch_start + committee_index.uint64) mod
-    ATTESTATION_SUBNET_COUNT
+  SubnetId(
+    (committees_since_epoch_start + committee_index.uint64) mod
+    ATTESTATION_SUBNET_COUNT)
 
 # https://github.com/ethereum/eth2.0-specs/blob/v1.0.1/specs/phase0/validator.md#broadcast-attestation
-func getAttestationTopic*(forkDigest: ForkDigest, subnetIndex: uint64):
+func getAttestationTopic*(forkDigest: ForkDigest, subnet_id: SubnetId):
     string =
   ## For subscribing and unsubscribing to/from a subnet.
-  doAssert subnetIndex < ATTESTATION_SUBNET_COUNT
-
-  eth2Prefix(forkDigest) & "beacon_attestation_" & $subnetIndex & "/ssz"
+  eth2Prefix(forkDigest) & "beacon_attestation_" & $uint64(subnet_id) & "/ssz"
 
 func getENRForkID*(fork: Fork, genesis_validators_root: Eth2Digest): ENRForkID =
   let
diff --git a/beacon_chain/ssz/bitseqs.nim b/beacon_chain/ssz/bitseqs.nim
index 3412d76c3..823c003d8 100644
--- a/beacon_chain/ssz/bitseqs.nim
+++ b/beacon_chain/ssz/bitseqs.nim
@@ -281,3 +281,29 @@ func countZeros*(x: BitSeq): int =
 
 template bytes*(x: BitSeq): untyped =
   seq[byte](x)
+
+iterator items*(x: BitArray): bool =
+  for i in 0..<x.bits:
+    yield x[i]
+
+iterator pairs*(x: BitArray): (int, bool) =
+  for i in 0..<x.bits:
+    yield (i, x[i])
+
+func incl*(a: var BitArray, b: BitArray) =
+  # Update `a` to include the bits of `b`, as if applying `or` to each bit
+  for i in 0..<a.bytes.len:
+    a[i] = a[i] or b[i]
+
+func clear*(a: var BitArray) =
+  for b in a.bytes.mitems(): b = 0
+
+# Set operations
+func `+`*(a, b: BitArray): BitArray =
+  for i in 0..<a.bytes.len:
+    result[i] = a[i] or b[i]
+
+func `-`*(a, b: BitArray): BitArray =
+  for i in 0..<a.bytes.len:
+    result[i] = a[i] and (not b[i])
+
diff --git a/beacon_chain/validators/validator_duties.nim b/beacon_chain/validators/validator_duties.nim
index 6b3d711c6..e26c12528 100644
--- a/beacon_chain/validators/validator_duties.nim
+++ b/beacon_chain/validators/validator_duties.nim
@@ -160,19 +160,26 @@ proc isSynced*(node: BeaconNode, head: BlockRef): bool =
     true
 
 proc sendAttestation*(
-    node: BeaconNode, attestation: Attestation, num_active_validators: uint64) =
-  let subnet_index =
-    compute_subnet_for_attestation(
-      get_committee_count_per_slot(num_active_validators), attestation.data.slot,
-      attestation.data.index.CommitteeIndex)
-  node.network.broadcast(
-    getAttestationTopic(node.forkDigest, subnet_index), attestation)
+    node: BeaconNode, attestation: Attestation,
+    subnet_id: SubnetId, checkSignature: bool): Future[bool] {.async.} =
+  # Validate attestation before sending it via gossip - validation will also
+  # register the attestation with the attestation pool. Notably, although
+  # libp2p calls the data handler for any subscription on the subnet
+  # topic, it does not perform validation.
+  let ok = await node.processor.attestationValidator(
+    attestation, subnet_id, checkSignature)
 
-  # Ensure node's own broadcast attestations end up in its attestation pool
-  discard node.processor.attestationValidator(
-    attestation, subnet_index, false)
-
-  beacon_attestations_sent.inc()
+  return case ok
+    of ValidationResult.Accept:
+      node.network.broadcast(
+        getAttestationTopic(node.forkDigest, subnet_id), attestation)
+      beacon_attestations_sent.inc()
+      true
+    else:
+      notice "Produced attestation failed validation",
+        attestation = shortLog(attestation),
+        result = $ok
+      false
 
 proc sendVoluntaryExit*(node: BeaconNode, exit: SignedVoluntaryExit) =
   node.network.broadcast(getVoluntaryExitsTopic(node.forkDigest), exit)
@@ -185,18 +192,21 @@ proc sendProposerSlashing*(node: BeaconNode, slashing: ProposerSlashing) =
   node.network.broadcast(getProposerSlashingsTopic(node.forkDigest),
                          slashing)
 
-proc sendAttestation*(node: BeaconNode, attestation: Attestation) =
-  # For the validator API, which doesn't supply num_active_validators.
+proc sendAttestation*(node: BeaconNode, attestation: Attestation): Future[bool] =
+  # For the validator API, which doesn't supply the subnet id.
   let attestationBlck =
     node.chainDag.getRef(attestation.data.beacon_block_root)
   if attestationBlck.isNil:
     debug "Attempt to send attestation without corresponding block"
     return
+  let
+    epochRef = node.chainDag.getEpochRef(
+      attestationBlck, attestation.data.target.epoch)
+    subnet_id = compute_subnet_for_attestation(
+      get_committee_count_per_slot(epochRef), attestation.data.slot,
+      attestation.data.index.CommitteeIndex)
 
-  node.sendAttestation(
-    attestation,
-    count_active_validators(
-      node.chainDag.getEpochRef(attestationBlck, attestation.data.target.epoch)))
+  node.sendAttestation(attestation, subnet_id, checkSignature = true)
 
 proc createAndSendAttestation(node: BeaconNode,
                               fork: Fork,
@@ -205,31 +215,41 @@ proc createAndSendAttestation(node: BeaconNode,
                               attestationData: AttestationData,
                               committeeLen: int,
                               indexInCommittee: int,
-                              num_active_validators: uint64) {.async.} =
-  var attestation = await validator.produceAndSignAttestation(
-    attestationData, committeeLen, indexInCommittee, fork,
-    genesis_validators_root)
+                              subnet_id: SubnetId) {.async.} =
+  try:
+    var
+      attestation = await validator.produceAndSignAttestation(
+        attestationData, committeeLen, indexInCommittee, fork,
+        genesis_validators_root)
 
-  node.sendAttestation(attestation, num_active_validators)
+    let ok = await node.sendAttestation(
+      attestation, subnet_id, checkSignature = false)
+    if not ok: # Logged in sendAttestation
+      return
 
-  if node.config.dumpEnabled:
-    dump(node.config.dumpDirOutgoing, attestation.data, validator.pubKey)
+    let sent = node.beaconClock.now()
+    if node.config.dumpEnabled:
+      dump(node.config.dumpDirOutgoing, attestation.data, validator.pubKey)
 
-  let wallTime = node.beaconClock.now()
-  let deadline = attestationData.slot.toBeaconTime() +
-                 seconds(int(SECONDS_PER_SLOT div 3))
+    let wallTime = node.beaconClock.now()
+    let deadline = attestationData.slot.toBeaconTime() +
+                  seconds(int(SECONDS_PER_SLOT div 3))
 
-  let (delayStr, delaySecs) =
-    if wallTime < deadline:
-      ("-" & $(deadline - wallTime), -toFloatSeconds(deadline - wallTime))
-    else:
-      ($(wallTime - deadline), toFloatSeconds(wallTime - deadline))
+    let (delayStr, delaySecs) =
+      if wallTime < deadline:
+        ("-" & $(deadline - wallTime), -toFloatSeconds(deadline - wallTime))
+      else:
+        ($(wallTime - deadline), toFloatSeconds(wallTime - deadline))
 
-  notice "Attestation sent", attestation = shortLog(attestation),
-                             validator = shortLog(validator), delay = delayStr,
-                             indexInCommittee = indexInCommittee
+    notice "Attestation sent", attestation = shortLog(attestation),
+                              validator = shortLog(validator), delay = delayStr,
+                              indexInCommittee = indexInCommittee
 
-  beacon_attestation_sent_delay.observe(delaySecs)
+    beacon_attestation_sent_delay.observe(delaySecs)
+  except CatchableError as exc:
+    # An error could happen here when the signature task fails - we must
+    # not leak the exception because this is an asyncSpawn task
+    notice "Error sending attestation", err = exc.msg
 
 proc getBlockProposalEth1Data*(node: BeaconNode,
                                stateData: StateData): BlockProposalEth1Data =
@@ -405,10 +425,6 @@ proc handleAttestations(node: BeaconNode, head: BlockRef, slot: Slot) =
     attestationHeadRoot = shortLog(attestationHead.blck.root),
     attestationSlot = shortLog(slot)
 
-  var attestations: seq[tuple[
-    data: AttestationData, committeeLen, indexInCommittee: int,
-    validator: AttachedValidator, validator_index: ValidatorIndex]]
-
   # We need to run attestations exactly for the slot that we're attesting to.
   # In case blocks went missing, this means advancing past the latest block
   # using empty slots as fillers.
@@ -416,46 +432,42 @@ proc handleAttestations(node: BeaconNode, head: BlockRef, slot: Slot) =
   let
     epochRef = node.chainDag.getEpochRef(
       attestationHead.blck, slot.compute_epoch_at_slot())
-    committees_per_slot =
-      get_committee_count_per_slot(epochRef)
-    num_active_validators = count_active_validators(epochRef)
+    committees_per_slot = get_committee_count_per_slot(epochRef)
     fork = getStateField(node.chainDag.headState, fork)
     genesis_validators_root =
       getStateField(node.chainDag.headState, genesis_validators_root)
 
-  for committee_index in 0'u64..<committees_per_slot:
-    let committee = get_beacon_committee(
-      epochRef, slot, committee_index.CommitteeIndex)
+  for committee_index in get_committee_indices(epochRef):
+    let committee = get_beacon_committee(epochRef, slot, committee_index)
 
     for index_in_committee, validator_index in committee:
       let validator = node.getAttachedValidator(epochRef, validator_index)
-      if validator != nil:
-        let ad = makeAttestationData(
-          epochRef, attestationHead, committee_index.CommitteeIndex)
-        attestations.add(
-          (ad, committee.len, index_in_committee, validator, validator_index))
+      if validator == nil:
+        continue
 
-  for a in attestations:
-    # TODO signing_root is recomputed in produceAndSignAttestation/signAttestation just after
-    let signing_root = compute_attestation_root(
-          fork, genesis_validators_root, a.data)
-    let notSlashable = node.attachedValidators
-        .slashingProtection
-        .registerAttestation(
-          a.validator_index,
-          a.validator.pubkey,
-          a.data.source.epoch,
-          a.data.target.epoch,
-          signing_root
-        )
-    if notSlashable.isOk():
-      traceAsyncErrors createAndSendAttestation(
-        node, fork, genesis_validators_root, a.validator, a.data,
-        a.committeeLen, a.indexInCommittee, num_active_validators)
-    else:
-      warn "Slashing protection activated for attestation",
-        validator = a.validator.pubkey,
-        badVoteDetails = $notSlashable.error
+      let
+        data = makeAttestationData(epochRef, attestationHead, committee_index)
+        # TODO signing_root is recomputed in produceAndSignAttestation/signAttestation just after
+        signing_root = compute_attestation_root(
+          fork, genesis_validators_root, data)
+        registered = node.attachedValidators
+          .slashingProtection
+          .registerAttestation(
+            validator_index,
+            validator.pubkey,
+            data.source.epoch,
+            data.target.epoch,
+            signing_root)
+      if registered.isOk():
+        let subnet_id = compute_subnet_for_attestation(
+          committees_per_slot, data.slot, data.index.CommitteeIndex)
+        asyncSpawn createAndSendAttestation(
+          node, fork, genesis_validators_root, validator, data,
+          committee.len(), index_in_committee, subnet_id)
+      else:
+        warn "Slashing protection activated for attestation",
+          validator = validator.pubkey,
+          badVoteDetails = $registered.error()
 
 proc handleProposal(node: BeaconNode, head: BlockRef, slot: Slot):
     Future[BlockRef] {.async.} =
diff --git a/beacon_chain/validators/validator_pool.nim b/beacon_chain/validators/validator_pool.nim
index 446f94f92..2e9de792b 100644
--- a/beacon_chain/validators/validator_pool.nim
+++ b/beacon_chain/validators/validator_pool.nim
@@ -63,13 +63,9 @@ proc signWithRemoteValidator(v: AttachedValidator, data: Eth2Digest):
   v.connection.inStream.flush()
   var line = newStringOfCap(120).TaintedString
   discard v.connection.outStream.readLine(line)
-  result = ValidatorSig.fromHex(line).get()
-  # TODO this is an ugly hack to fake a delay and subsequent async reordering
-  #      for the purpose of testing the external validator delay - to be
-  #      replaced by something more sensible
-  await sleepAsync(chronos.milliseconds(1))
+  return ValidatorSig.fromHex(line).get()
 
-# TODO: Honest validator - https://github.com/ethereum/eth2.0-specs/blob/v1.0.1/specs/phase0/validator.md
+# https://github.com/ethereum/eth2.0-specs/blob/v1.0.1/specs/phase0/validator.md
 proc signBlockProposal*(v: AttachedValidator, fork: Fork,
                         genesis_validators_root: Eth2Digest, slot: Slot,
                         blockRoot: Eth2Digest): Future[ValidatorSig] {.async.} =
@@ -81,14 +77,14 @@ proc signBlockProposal*(v: AttachedValidator, fork: Fork,
     await signWithRemoteValidator(v, root)
 
 proc signAttestation*(v: AttachedValidator,
-                      attestation: AttestationData,
+                      data: AttestationData,
                       fork: Fork, genesis_validators_root: Eth2Digest):
                       Future[ValidatorSig] {.async.} =
   return if v.kind == inProcess:
     get_attestation_signature(
-      fork, genesis_validators_root, attestation, v.privKey).toValidatorSig()
+      fork, genesis_validators_root, data, v.privKey).toValidatorSig()
   else:
-    let root = compute_attestation_root(fork, genesis_validators_root, attestation)
+    let root = compute_attestation_root(fork, genesis_validators_root, data)
     await signWithRemoteValidator(v, root)
 
 proc produceAndSignAttestation*(validator: AttachedValidator,
diff --git a/ncli/inspector.nim b/ncli/inspector.nim
index acc8462dc..df18d3066 100644
--- a/ncli/inspector.nim
+++ b/ncli/inspector.nim
@@ -194,7 +194,7 @@ func getTopics(forkDigest: ForkDigest,
   of TopicFilter.Attestations:
     mapIt(
       0'u64 ..< ATTESTATION_SUBNET_COUNT.uint64,
-      getAttestationTopic(forkDigest, it) & "_snappy")
+      getAttestationTopic(forkDigest, SubnetId(it)) & "_snappy")
 
 proc loadBootFile(name: string): seq[string] =
   try:
diff --git a/tests/test_bitseqs.nim b/tests/test_bitseqs.nim
index 38c12447a..7bd8c0dd3 100644
--- a/tests/test_bitseqs.nim
+++ b/tests/test_bitseqs.nim
@@ -7,7 +7,45 @@ import
   ./testutil
 
 suite "Bit fields":
-  test "roundtrips":
+  test "roundtrips BitArray":
+    var
+      a = BitArray[100]()
+      b = BitArray[100]()
+
+    check:
+      not a[0]
+
+    a.setBit 1
+
+    check:
+      not a[0]
+      a[1]
+
+      a + b == a
+      a - b == a
+
+      b + a == a
+      b - a == b # b is empty
+
+    b.setBit 2
+
+    check:
+      (a + b)[2]
+      (b - a)[2]
+      not (b - a)[1]
+
+    a.incl(b)
+
+    check:
+      not a[0]
+      a[1]
+      a[2]
+
+    a.clear()
+    check:
+      not a[1]
+
+  test "roundtrips BitSeq":
     var
       a = BitSeq.init(100)
       b = BitSeq.init(100)
diff --git a/tests/test_gossip_validation.nim b/tests/test_gossip_validation.nim
index d62504f6d..f488a1618 100644
--- a/tests/test_gossip_validation.nim
+++ b/tests/test_gossip_validation.nim
@@ -92,13 +92,14 @@ suite "Gossip validation " & preset():
     pool[].nextAttestationEpoch.setLen(0) # reset for test
     check:
       # Wrong subnet
-      validateAttestation(pool, batchCrypto, att_1_0, beaconTime, subnet + 1, true).waitFor().isErr
+      validateAttestation(
+        pool, batchCrypto, att_1_0, beaconTime, SubnetId(subnet.uint8 + 1), true).waitFor().isErr
 
     pool[].nextAttestationEpoch.setLen(0) # reset for test
     check:
       # Too far in the future
       validateAttestation(
-        pool, batchCrypto, att_1_0, beaconTime - 1.seconds, subnet + 1, true).waitFor().isErr
+        pool, batchCrypto, att_1_0, beaconTime - 1.seconds, subnet, true).waitFor().isErr
 
     pool[].nextAttestationEpoch.setLen(0) # reset for test
     check:
@@ -106,7 +107,7 @@ suite "Gossip validation " & preset():
       validateAttestation(
         pool, batchCrypto, att_1_0,
         beaconTime - (SECONDS_PER_SLOT * SLOTS_PER_EPOCH - 1).int.seconds,
-        subnet + 1, true).waitFor().isErr
+        subnet, true).waitFor().isErr
 
     block:
       var broken = att_1_0
diff --git a/tests/test_honest_validator.nim b/tests/test_honest_validator.nim
index 7582ce620..c6f0aff96 100644
--- a/tests/test_honest_validator.nim
+++ b/tests/test_honest_validator.nim
@@ -19,45 +19,45 @@ suite "Honest validator":
 
   test "Mainnet attestation topics":
     check:
-      getAttestationTopic(forkDigest, 0) ==
+      getAttestationTopic(forkDigest, SubnetId(0)) ==
         "/eth2/00000000/beacon_attestation_0/ssz"
-      getAttestationTopic(forkDigest, 5) ==
+      getAttestationTopic(forkDigest, SubnetId(5)) ==
         "/eth2/00000000/beacon_attestation_5/ssz"
-      getAttestationTopic(forkDigest, 7) ==
+      getAttestationTopic(forkDigest, SubnetId(7)) ==
         "/eth2/00000000/beacon_attestation_7/ssz"
-      getAttestationTopic(forkDigest, 9) ==
+      getAttestationTopic(forkDigest, SubnetId(9)) ==
         "/eth2/00000000/beacon_attestation_9/ssz"
-      getAttestationTopic(forkDigest, 13) ==
+      getAttestationTopic(forkDigest, SubnetId(13)) ==
         "/eth2/00000000/beacon_attestation_13/ssz"
-      getAttestationTopic(forkDigest, 19) ==
+      getAttestationTopic(forkDigest, SubnetId(19)) ==
         "/eth2/00000000/beacon_attestation_19/ssz"
-      getAttestationTopic(forkDigest, 20) ==
+      getAttestationTopic(forkDigest, SubnetId(20)) ==
         "/eth2/00000000/beacon_attestation_20/ssz"
-      getAttestationTopic(forkDigest, 22) ==
+      getAttestationTopic(forkDigest, SubnetId(22)) ==
         "/eth2/00000000/beacon_attestation_22/ssz"
-      getAttestationTopic(forkDigest, 25) ==
+      getAttestationTopic(forkDigest, SubnetId(25)) ==
         "/eth2/00000000/beacon_attestation_25/ssz"
-      getAttestationTopic(forkDigest, 27) ==
+      getAttestationTopic(forkDigest, SubnetId(27)) ==
         "/eth2/00000000/beacon_attestation_27/ssz"
-      getAttestationTopic(forkDigest, 31) ==
+      getAttestationTopic(forkDigest, SubnetId(31)) ==
         "/eth2/00000000/beacon_attestation_31/ssz"
-      getAttestationTopic(forkDigest, 39) ==
+      getAttestationTopic(forkDigest, SubnetId(39)) ==
         "/eth2/00000000/beacon_attestation_39/ssz"
-      getAttestationTopic(forkDigest, 45) ==
+      getAttestationTopic(forkDigest, SubnetId(45)) ==
         "/eth2/00000000/beacon_attestation_45/ssz"
-      getAttestationTopic(forkDigest, 47) ==
+      getAttestationTopic(forkDigest, SubnetId(47)) ==
         "/eth2/00000000/beacon_attestation_47/ssz"
-      getAttestationTopic(forkDigest, 48) ==
+      getAttestationTopic(forkDigest, SubnetId(48)) ==
         "/eth2/00000000/beacon_attestation_48/ssz"
-      getAttestationTopic(forkDigest, 50) ==
+      getAttestationTopic(forkDigest, SubnetId(50)) ==
         "/eth2/00000000/beacon_attestation_50/ssz"
-      getAttestationTopic(forkDigest, 53) ==
+      getAttestationTopic(forkDigest, SubnetId(53)) ==
         "/eth2/00000000/beacon_attestation_53/ssz"
-      getAttestationTopic(forkDigest, 54) ==
+      getAttestationTopic(forkDigest, SubnetId(54)) ==
         "/eth2/00000000/beacon_attestation_54/ssz"
-      getAttestationTopic(forkDigest, 62) ==
+      getAttestationTopic(forkDigest, SubnetId(62)) ==
         "/eth2/00000000/beacon_attestation_62/ssz"
-      getAttestationTopic(forkDigest, 63) ==
+      getAttestationTopic(forkDigest, SubnetId(63)) ==
         "/eth2/00000000/beacon_attestation_63/ssz"
 
   test "is_aggregator":