diff --git a/beacon_chain/attestation_pool.nim b/beacon_chain/attestation_pool.nim
index bf55724ec..7a426989a 100644
--- a/beacon_chain/attestation_pool.nim
+++ b/beacon_chain/attestation_pool.nim
@@ -27,7 +27,7 @@ proc combine*(tgt: var Attestation, src: Attestation, flags: UpdateFlags) =
     tgt.aggregation_bitfield.combine(src.aggregation_bitfield)
 
     if skipValidation notin flags:
-      tgt.aggregate_signature.combine(src.aggregate_signature)
+      tgt.signature.combine(src.signature)
 
 proc validate(
     state: BeaconState, attestation: Attestation, flags: UpdateFlags): bool =
@@ -38,7 +38,7 @@ proc validate(
 
   # TODO half of this stuff is from beaconstate.validateAttestation - merge?
 
-  let attestationSlot = attestation.data.slot
+  let attestationSlot = get_attestation_slot(state, attestation)
 
   if attestationSlot < state.finalized_epoch.get_epoch_start_slot():
     debug "Old attestation",
@@ -71,7 +71,7 @@ proc validate(
   ## the rest; turns into expensive NOP until then.
   if skipValidation notin flags:
     let
-      participants = get_attestation_participants(
+      participants = get_attesting_indices_seq(
         state, attestation.data, attestation.aggregation_bitfield)
 
       ## TODO when the custody_bitfield assertion-to-emptiness disappears do this
@@ -97,9 +97,9 @@ proc validate(
           hash_tree_root(AttestationDataAndCustodyBit(
             data: attestation.data, custody_bit: true)),
         ],
-        attestation.aggregate_signature,
+        attestation.signature,
         get_domain(state, DOMAIN_ATTESTATION,
-                   slot_to_epoch(attestation.data.slot)),
+                   slot_to_epoch(get_attestation_slot(state, attestation))),
       ):
       notice "Invalid signature", participants
       return false
@@ -174,14 +174,14 @@ proc add*(pool: var AttestationPool,
   # TODO inefficient data structures..
 
   let
-    attestationSlot = attestation.data.slot
+    attestationSlot = get_attestation_slot(state, attestation)
     idx = pool.slotIndex(state, attestationSlot)
     slotData = addr pool.slots[idx]
     validation = Validation(
       aggregation_bitfield: attestation.aggregation_bitfield,
       custody_bitfield: attestation.custody_bitfield,
-      aggregate_signature: attestation.aggregate_signature)
-    participants = get_attestation_participants(
+      aggregate_signature: attestation.signature)
+    participants = get_attesting_indices_seq(
       state, attestation.data, validation.aggregation_bitfield)
 
   var found = false
@@ -196,7 +196,7 @@ proc add*(pool: var AttestationPool,
           #      sets by virtue of not overlapping with some other attestation
           #      and therefore being useful after all?
           debug "Ignoring subset attestation",
-            existingParticipants = get_attestation_participants(
+            existingParticipants = get_attesting_indices_seq(
               state, a.data, v.aggregation_bitfield),
             newParticipants = participants
           found = true
@@ -209,7 +209,7 @@ proc add*(pool: var AttestationPool,
           if it.aggregation_bitfield.isSubsetOf(
               validation.aggregation_bitfield):
             debug "Removing subset attestation",
-              existingParticipants = get_attestation_participants(
+              existingParticipants = get_attesting_indices_seq(
                 state, a.data, it.aggregation_bitfield),
               newParticipants = participants
             false
@@ -287,7 +287,7 @@ proc getAttestationsForBlock*(
         aggregation_bitfield: a.validations[0].aggregation_bitfield,
         data: a.data,
         custody_bitfield: a.validations[0].custody_bitfield,
-        aggregate_signature: a.validations[0].aggregate_signature
+        signature: a.validations[0].aggregate_signature
       )
 
     # TODO what's going on here is that when producing a block, we need to
@@ -312,7 +312,7 @@ proc getAttestationsForBlock*(
         attestation.aggregation_bitfield.combine(
           v.aggregation_bitfield)
         attestation.custody_bitfield.combine(v.custody_bitfield)
-        attestation.aggregate_signature.combine(v.aggregate_signature)
+        attestation.signature.combine(v.aggregate_signature)
 
     result.add(attestation)
 
@@ -324,7 +324,8 @@ proc resolve*(pool: var AttestationPool, state: BeaconState) =
   var resolved: seq[Attestation]
 
   for k, v in pool.unresolved.mpairs():
-    if v.tries > 8 or v.attestation.data.slot < pool.startingSlot:
+    let attestation_slot = get_attestation_slot(state, v.attestation)
+    if v.tries > 8 or attestation_slot < pool.startingSlot:
       done.add(k)
     else:
       if pool.blockPool.get(k).isSome():
diff --git a/beacon_chain/beacon_node.nim b/beacon_chain/beacon_node.nim
index dc6fbd0c8..dcf8778ad 100644
--- a/beacon_chain/beacon_node.nim
+++ b/beacon_chain/beacon_node.nim
@@ -314,7 +314,7 @@ proc sendAttestation(node: BeaconNode,
 
   var attestation = Attestation(
     data: attestationData,
-    aggregate_signature: validatorSignature,
+    signature: validatorSignature,
     aggregation_bitfield: aggregationBitfield,
     # Stub in phase0
     custody_bitfield: BitField.init(committeeLen)
@@ -371,7 +371,9 @@ proc proposeBlock(node: BeaconNode,
     var tmpState = hashedState
 
     let ok = updateState(tmpState, newBlock, {skipValidation})
-    doAssert ok # TODO: err, could this fail somehow?
+    # TODO only enable in fast-fail debugging situations
+    # otherwise, bad attestations can bring down network
+    # doAssert ok # TODO: err, could this fail somehow?
 
     newBlock.state_root = tmpState.root
 
@@ -405,7 +407,7 @@ proc onAttestation(node: BeaconNode, attestation: Attestation) =
   # we're on, or that it follows the rules of the protocol
   debug "Attestation received",
     attestationData = shortLog(attestation.data),
-    signature = shortLog(attestation.aggregate_signature)
+    signature = shortLog(attestation.signature)
 
   # TODO seems reasonable to use the latest head state here.. needs thinking
   #      though - maybe we should use the state from the block pointed to by
diff --git a/beacon_chain/spec/beaconstate.nim b/beacon_chain/spec/beaconstate.nim
index c73105fab..1796114b8 100644
--- a/beacon_chain/spec/beaconstate.nim
+++ b/beacon_chain/spec/beaconstate.nim
@@ -6,12 +6,12 @@
 # at your option. This file may not be copied, modified, or distributed except according to those terms.
 
 import
-  algorithm, chronicles, math, options, sequtils,
+  algorithm, chronicles, collections/sets, math, options, sequtils,
   ../extras, ../ssz, ../beacon_node_types,
   ./bitfield, ./crypto, ./datatypes, ./digest, ./helpers, ./validator,
   tables
 
-# https://github.com/ethereum/eth2.0-specs/blob/v0.6.0/specs/core/0_beacon-chain.md#verify_merkle_branch
+# https://github.com/ethereum/eth2.0-specs/blob/v0.6.3/specs/core/0_beacon-chain.md#verify_merkle_branch
 func verify_merkle_branch(leaf: Eth2Digest, proof: openarray[Eth2Digest], depth: uint64, index: uint64, root: Eth2Digest): bool =
   ## Verify that the given ``leaf`` is on the merkle branch ``proof``
   ## starting with the given ``root``.
@@ -153,7 +153,7 @@ func initiate_validator_exit*(state: var BeaconState,
   validator.withdrawable_epoch =
     validator.exit_epoch + MIN_VALIDATOR_WITHDRAWABILITY_DELAY
 
-# https://github.com/ethereum/eth2.0-specs/blob/v0.6.1/specs/core/0_beacon-chain.md#slash_validator
+# https://github.com/ethereum/eth2.0-specs/blob/v0.6.3/specs/core/0_beacon-chain.md#slash_validator
 func slash_validator*(state: var BeaconState, slashed_index: ValidatorIndex) =
   # Slash the validator with index ``index``.
   let current_epoch = get_current_epoch(state)
@@ -168,6 +168,7 @@ func slash_validator*(state: var BeaconState, slashed_index: ValidatorIndex) =
 
   let
     proposer_index = get_beacon_proposer_index(state)
+    # Spec has whistleblower_index as optional param, but it's never used.
     whistleblower_index = proposer_index
     whistleblowing_reward = slashed_balance div WHISTLEBLOWING_REWARD_QUOTIENT
     proposer_reward = whistleblowing_reward div PROPOSER_REWARD_QUOTIENT
@@ -176,8 +177,8 @@ func slash_validator*(state: var BeaconState, slashed_index: ValidatorIndex) =
     state, whistleblower_index, whistleblowing_reward - proposer_reward)
   decrease_balance(state, slashed_index, whistleblowing_reward)
 
-# https://github.com/ethereum/eth2.0-specs/blob/v0.5.1/specs/core/0_beacon-chain.md#get_temporary_block_header
-func get_temporary_block_header*(blck: BeaconBlock): BeaconBlockHeader =
+# https://github.com/ethereum/eth2.0-specs/blob/v0.5.1/specs/core/0_beacon-chain.md#on-genesis
+func get_temporary_block_header(blck: BeaconBlock): BeaconBlockHeader =
   ## Return the block header corresponding to a block with ``state_root`` set
   ## to ``ZERO_HASH``.
   BeaconBlockHeader(
@@ -186,10 +187,9 @@ func get_temporary_block_header*(blck: BeaconBlock): BeaconBlockHeader =
     state_root: ZERO_HASH,
     block_body_root: hash_tree_root(blck.body),
     # signing_root(block) is used for block id purposes so signature is a stub
-    signature: EMPTY_SIGNATURE,
+    signature: ValidatorSig(),
   )
 
-# https://github.com/ethereum/eth2.0-specs/blob/v0.5.1/specs/core/0_beacon-chain.md#on-genesis
 func get_empty_block*(): BeaconBlock =
   # Nim default values fill this in mostly correctly.
   BeaconBlock(slot: GENESIS_SLOT)
@@ -283,7 +283,7 @@ func get_initial_beacon_block*(state: BeaconState): BeaconBlock =
     # initialized to default values.
   )
 
-# https://github.com/ethereum/eth2.0-specs/blob/v0.6.3/specs/core/0_beacon-chain.md#get_attestation_slot
+# https://github.com/ethereum/eth2.0-specs/blob/v0.6.2/specs/core/0_beacon-chain.md#get_attestation_slot
 func get_attestation_slot*(state: BeaconState,
     attestation: Attestation|PendingAttestation,
     committee_count: uint64): Slot =
@@ -291,7 +291,10 @@ func get_attestation_slot*(state: BeaconState,
     epoch = attestation.data.target_epoch
     offset = (attestation.data.shard + SHARD_COUNT -
       get_epoch_start_shard(state, epoch)) mod SHARD_COUNT
-  result = get_epoch_start_slot(epoch) + offset div (committee_count div SLOTS_PER_EPOCH)
+
+  # TODO re-instate once double-check correct conditions in attestation pool
+  #get_epoch_start_slot(epoch) + offset div (committee_count div SLOTS_PER_EPOCH)
+  attestation.data.slot
 
 # This is the slower (O(n)), spec-compatible signature.
 func get_attestation_slot*(state: BeaconState,
@@ -314,81 +317,6 @@ func get_block_root*(state: BeaconState, epoch: Epoch): Eth2Digest =
   # Return the block root at a recent ``epoch``.
   get_block_root_at_slot(state, get_epoch_start_slot(epoch))
 
-# https://github.com/ethereum/eth2.0-specs/blob/v0.5.1/specs/core/0_beacon-chain.md#get_attestation_participants
-func get_attestation_participants*(state: BeaconState,
-                                   attestation_data: AttestationData,
-                                   bitfield: BitField): seq[ValidatorIndex] =
-  ## Return the participant indices at for the ``attestation_data`` and
-  ## ``bitfield``.
-  ## Attestation participants in the attestation data are called out in a
-  ## bit field that corresponds to the committee of the shard at the time;
-  ## this function converts it to list of indices in to BeaconState.validators
-  ##
-  ## Returns empty list if the shard is not found
-  ## Return the participant indices at for the ``attestation_data`` and ``bitfield``.
-  ##
-  # TODO Linear search through shard list? borderline ok, it's a small list
-  # TODO iterator candidate
-
-  # Find the committee in the list with the desired shard
-  let crosslink_committees = get_crosslink_committees_at_slot(
-    state, attestation_data.slot)
-
-  doAssert anyIt(
-    crosslink_committees,
-    it[1] == attestation_data.shard)
-  let crosslink_committee = mapIt(
-    filterIt(crosslink_committees, it.shard == attestation_data.shard),
-    it.committee)[0]
-
-  # TODO this and other attestation-based fields need validation so we don't
-  #      crash on a malicious attestation!
-  doAssert verify_bitfield(bitfield, len(crosslink_committee))
-
-  # Find the participating attesters in the committee
-  result = @[]
-  for i, validator_index in crosslink_committee:
-    let aggregation_bit = get_bitfield_bit(bitfield, i)
-    if aggregation_bit:
-      result.add(validator_index)
-
-iterator get_attestation_participants_cached*(state: BeaconState,
-                                   attestation_data: AttestationData,
-                                   bitfield: BitField,
-                                   cache: var StateCache): ValidatorIndex =
-  ## Return the participant indices at for the ``attestation_data`` and
-  ## ``bitfield``.
-  ## Attestation participants in the attestation data are called out in a
-  ## bit field that corresponds to the committee of the shard at the time;
-  ## this function converts it to list of indices in to BeaconState.validators
-  ##
-  ## Returns empty list if the shard is not found
-  ## Return the participant indices at for the ``attestation_data`` and ``bitfield``.
-  ##
-  # TODO Linear search through shard list? borderline ok, it's a small list
-  # TODO iterator candidate
-
-  # Find the committee in the list with the desired shard
-  # let crosslink_committees = get_crosslink_committees_at_slot_cached(
-  #   state, attestation_data.slot, false, crosslink_committees_cached)
-
-  var found = false
-  for crosslink_committee in get_crosslink_committees_at_slot_cached(
-      state, attestation_data.slot, cache):
-    if crosslink_committee.shard == attestation_data.shard:
-      # TODO this and other attestation-based fields need validation so we don't
-      #      crash on a malicious attestation!
-      doAssert verify_bitfield(bitfield, len(crosslink_committee.committee))
-
-      # Find the participating attesters in the committee
-      for i, validator_index in crosslink_committee.committee:
-        let aggregation_bit = get_bitfield_bit(bitfield, i)
-        if aggregation_bit:
-          yield validator_index
-      found = true
-      break
-  doAssert found, "Couldn't find crosslink committee"
-
 # https://github.com/ethereum/eth2.0-specs/blob/v0.6.3/specs/core/0_beacon-chain.md#get_total_balance
 func get_total_balance*(state: BeaconState, validators: auto): Gwei =
   # Return the combined effective balance of an array of ``validators``.
@@ -432,9 +360,133 @@ func process_registry_updates*(state: var BeaconState) =
       validator.activation_epoch =
         get_delayed_activation_exit_epoch(get_current_epoch(state))
 
-# https://github.com/ethereum/eth2.0-specs/blob/v0.5.1/specs/core/0_beacon-chain.md#attestations
+# https://github.com/ethereum/eth2.0-specs/blob/v0.6.3/specs/core/0_beacon-chain.md#verify_indexed_attestation
+func verify_indexed_attestation*(
+    state: BeaconState, indexed_attestation: IndexedAttestation): bool =
+  # Verify validity of ``indexed_attestation`` fields.
+
+  let
+    custody_bit_0_indices = indexed_attestation.custody_bit_0_indices
+    custody_bit_1_indices = indexed_attestation.custody_bit_1_indices
+
+  # Ensure no duplicate indices across custody bits
+  if len(intersection(toSet(custody_bit_0_indices), toSet(custody_bit_1_indices))) != 0:
+     return false
+
+  if len(custody_bit_1_indices) > 0:  # [TO BE REMOVED IN PHASE 1]
+    return false
+
+  let combined_len = len(custody_bit_0_indices) + len(custody_bit_1_indices)
+  if not (1 <= combined_len and combined_len <= MAX_INDICES_PER_ATTESTATION):
+    return false
+
+  if custody_bit_0_indices != sorted(custody_bit_0_indices, system.cmp):
+    return false
+
+  if custody_bit_1_indices != sorted(custody_bit_1_indices, system.cmp):
+    return false
+
+  bls_verify_multiple(
+    @[
+      bls_aggregate_pubkeys(
+        mapIt(custody_bit_0_indices, state.validator_registry[it.int].pubkey)),
+      bls_aggregate_pubkeys(
+        mapIt(custody_bit_1_indices, state.validator_registry[it.int].pubkey)),
+    ],
+    @[
+      hash_tree_root(AttestationDataAndCustodyBit(
+        data: indexed_attestation.data, custody_bit: false)),
+      hash_tree_root(AttestationDataAndCustodyBit(
+        data: indexed_attestation.data, custody_bit: true)),
+    ],
+    indexed_attestation.signature,
+    get_domain(
+      state,
+      DOMAIN_ATTESTATION,
+      indexed_attestation.data.target_epoch
+    ),
+  )
+
+# https://github.com/ethereum/eth2.0-specs/blob/v0.6.3/specs/core/0_beacon-chain.md#get_attesting_indices
+func get_attesting_indices*(state: BeaconState,
+                            attestation_data: AttestationData,
+                            bitfield: BitField): HashSet[ValidatorIndex] =
+  ## Return the sorted attesting indices corresponding to ``attestation_data``
+  ## and ``bitfield``.
+  ## The spec goes through a lot of hoops to sort things, and sometimes
+  ## constructs sets from the results here. The basic idea is to always
+  ## just keep it in a HashSet, which seems to suffice. If needed, it's
+  ## possible to follow the spec more literally.
+  result = initSet[ValidatorIndex]()
+  let committee =
+    get_crosslink_committee(state, attestation_data.target_epoch,
+      attestation_data.shard)
+  doAssert verify_bitfield(bitfield, len(committee))
+  for i, index in committee:
+    if get_bitfield_bit(bitfield, i):
+      result.incl index
+
+func get_attesting_indices_seq*(
+    state: BeaconState, attestation_data: AttestationData, bitfield: BitField):
+    seq[ValidatorIndex] =
+  toSeq(items(get_attesting_indices(state, attestation_data, bitfield)))
+
+# TODO legacy function name; rename, reimplement caching if useful, blob/v0.6.2
+iterator get_attestation_participants_cached*(
+    state: BeaconState, attestation_data: AttestationData, bitfield: BitField,
+    cache: var StateCache): ValidatorIndex =
+  for participant in get_attesting_indices(state, attestation_data, bitfield):
+    yield participant
+
+# https://github.com/ethereum/eth2.0-specs/blob/v0.6.3/specs/core/0_beacon-chain.md#convert_to_indexed
+func convert_to_indexed(state: BeaconState, attestation: Attestation): IndexedAttestation =
+  # Convert ``attestation`` to (almost) indexed-verifiable form.
+  let
+    attesting_indices =
+      get_attesting_indices(
+        state, attestation.data, attestation.aggregation_bitfield)
+    custody_bit_1_indices =
+      get_attesting_indices(
+        state, attestation.data, attestation.custody_bitfield)
+
+    ## TODO quadratic, .items, but first-class iterators, etc
+    ## filterIt can't work on HashSets directly because it is
+    ## assuming int-indexable thing to extract type, because,
+    ## like lots of other things in sequtils, it's a template
+    ## which doesn't otherwise care about the type system. It
+    ## is a mess. Just write the for-loop, etc, I guess, is a
+    ## reasonable reaction because of the special for binding
+    ## with (non-closure, etc) iterators no other part of Nim
+    ## can access. As such, this function's doing many copies
+    ## and allocations it has no fundamental reason to do.
+    custody_bit_0_indices =
+      filterIt(toSeq(items(attesting_indices)), it notin custody_bit_1_indices)
+
+  ## TODO No fundamental reason to do so many type conversions
+  ## verify_indexed_attestation checks for sortedness but it's
+  ## entirely a local artifact, seemingly; networking uses the
+  ## Attestation data structure, which can't be unsorted. That
+  ## the conversion here otherwise needs sorting is due to the
+  ## usage of HashSet -- order only matters in one place (that
+  ## 0.6.3 highlights and explicates) except in that the spec,
+  ## for no obvious reason, verifies it. So, here goes, sort a
+  ## list just so a called function can verify it's sorted.
+  IndexedAttestation(
+    custody_bit_0_indices: sorted(
+      mapIt(custody_bit_0_indices, it.uint64), system.cmp),
+    # toSeq pointlessly constructs int-indexable copy so mapIt can infer type;
+    # see above
+    custody_bit_1_indices:
+      sorted(mapIt(toSeq(items(custody_bit_1_indices)), it.uint64),
+        system.cmp),
+    data: attestation.data,
+    signature: attestation.signature,
+  )
+
+# https://github.com/ethereum/eth2.0-specs/blob/v0.6.3/specs/core/0_beacon-chain.md#attestations
 proc checkAttestation*(
     state: BeaconState, attestation: Attestation, flags: UpdateFlags): bool =
+  ## Process ``Attestation`` operation.
   ## Check that an attestation follows the rules of being included in the state
   ## at the current slot. When acting as a proposer, the same rules need to
   ## be followed!
@@ -443,142 +495,45 @@ proc checkAttestation*(
     if nextSlot in flags: state.slot + 1
     else: state.slot
 
-  # Can't submit attestations that are too far in history (or in prehistory)
-  if not (attestation.data.slot >= GENESIS_SLOT):
-    warn("Attestation predates genesis slot",
-      attestation_slot = attestation.data.slot,
-      state_slot = humaneSlotNum(stateSlot))
-    return
-
-  if not (stateSlot <= attestation.data.slot + SLOTS_PER_EPOCH):
-    warn("Attestation too old",
-      attestation_slot = humaneSlotNum(attestation.data.slot),
-      state_slot = humaneSlotNum(stateSlot))
-    return
-
-  # Can't submit attestations too quickly
-  if not (
-      attestation.data.slot + MIN_ATTESTATION_INCLUSION_DELAY <= stateSlot):
+  let attestation_slot = get_attestation_slot(state, attestation)
+  if not (attestation_slot + MIN_ATTESTATION_INCLUSION_DELAY <= stateSlot):
     warn("Attestation too new",
-      attestation_slot = humaneSlotNum(attestation.data.slot),
+      attestation_slot = humaneSlotNum(attestation_slot),
       state_slot = humaneSlotNum(stateSlot))
     return
 
-  # # Verify that the justified epoch and root is correct
-  if slot_to_epoch(attestation.data.slot) >= stateSlot.slot_to_epoch():
-    # Case 1: current epoch attestations
-    if not (attestation.data.source_epoch == state.current_justified_epoch):
-      warn("Source epoch is not current justified epoch",
-        attestation_slot = humaneSlotNum(attestation.data.slot),
-        state_slot = humaneSlotNum(stateSlot))
-      return
-
-    if not (attestation.data.source_root == state.current_justified_root):
-      warn("Source root is not current justified root",
-        attestation_slot = humaneSlotNum(attestation.data.slot),
-        state_slot = humaneSlotNum(stateSlot))
-      return
-  else:
-    # Case 2: previous epoch attestations
-    if not (attestation.data.source_epoch == state.previous_justified_epoch):
-      warn("Source epoch is not previous justified epoch",
-        attestation_slot = humaneSlotNum(attestation.data.slot),
-        state_slot = humaneSlotNum(stateSlot))
-      return
-
-    if not (attestation.data.source_root == state.previous_justified_root):
-      warn("Source root is not previous justified root",
-        attestation_slot = humaneSlotNum(attestation.data.slot),
-        state_slot = humaneSlotNum(stateSlot))
-      return
-
-  # Check that the crosslink data is valid
-  let acceptable_crosslink_data = @[
-    # Case 1: Latest crosslink matches the one in the state
-    attestation.data.previous_crosslink,
-
-    # Case 2: State has already been updated, state's latest crosslink matches
-    # the crosslink the attestation is trying to create
-    Crosslink(
-      crosslink_data_root: attestation.data.crosslink_data_root,
-      epoch: slot_to_epoch(attestation.data.slot)
-    )
-  ]
-  if not (state.current_crosslinks[attestation.data.shard] in
-      acceptable_crosslink_data):
-    warn("Unexpected crosslink shard",
-      state_latest_crosslinks_attestation_data_shard =
-        state.current_crosslinks[attestation.data.shard],
-      attestation_data_previous_crosslink = attestation.data.previous_crosslink,
-      epoch = humaneEpochNum(slot_to_epoch(attestation.data.slot)),
-      actual_epoch = slot_to_epoch(attestation.data.slot),
-      crosslink_data_root = attestation.data.crosslink_data_root,
-      acceptable_crosslink_data = acceptable_crosslink_data)
+  if not (stateSlot <= attestation_slot + SLOTS_PER_EPOCH):
+    warn("Attestation too old",
+      attestation_slot = humaneSlotNum(attestation_slot),
+      state_slot = humaneSlotNum(stateSlot))
     return
 
-  # Attestation must be nonempty!
-  if not anyIt(attestation.aggregation_bitfield.bits, it != 0):
-    warn("No signature bits")
+  # Check target epoch, source epoch, source root, and source crosslink
+  let data = attestation.data
+  if not (
+    (data.target_epoch, data.source_epoch, data.source_root, data.previous_crosslink_root) ==
+      (get_current_epoch(state), state.current_justified_epoch,
+       state.current_justified_root,
+       hash_tree_root(state.current_crosslinks[data.shard])) or
+    (data.target_epoch, data.source_epoch, data.source_root, data.previous_crosslink_root) ==
+      (get_previous_epoch(state), state.previous_justified_epoch,
+       state.previous_justified_root,
+       hash_tree_root(state.previous_crosslinks[data.shard]))):
+    warn("checkAttestation: target epoch, source epoch, source root, or source crosslink invalid")
     return
 
-  # Custody must be empty (to be removed in phase 1)
-  if not allIt(attestation.custody_bitfield.bits, it == 0):
-    warn("Custody bits set in phase0")
-    return
-
-  # Get the committee for the specific shard that this attestation is for
-  let crosslink_committee = mapIt(
-    filterIt(get_crosslink_committees_at_slot(state, attestation.data.slot),
-             it.shard == attestation.data.shard),
-    it.committee)[0]
-
-  # Custody bitfield must be a subset of the attestation bitfield
-  if not allIt(0 ..< len(crosslink_committee),
-      if not get_bitfield_bit(attestation.aggregation_bitfield, it):
-        not get_bitfield_bit(attestation.custody_bitfield, it)
-      else:
-        true):
-    warn("Wrong custody bits set")
-    return
-
-  # Verify aggregate signature
-  let
-    participants = get_attestation_participants(
-      state, attestation.data, attestation.aggregation_bitfield)
-
-    ## TODO when the custody_bitfield assertion-to-emptiness disappears do this
-    ## and fix the custody_bit_0_participants check to depend on it.
-    # custody_bit_1_participants = {nothing, always, because assertion above}
-    custody_bit_1_participants: seq[ValidatorIndex] = @[]
-    custody_bit_0_participants = participants
-
-  if skipValidation notin flags:
-    # Verify that aggregate_signature verifies using the group pubkey.
-    if not bls_verify_multiple(
-      @[
-        bls_aggregate_pubkeys(mapIt(custody_bit_0_participants,
-                                    state.validator_registry[it].pubkey)),
-        bls_aggregate_pubkeys(mapIt(custody_bit_1_participants,
-                                    state.validator_registry[it].pubkey)),
-      ],
-      @[
-        hash_tree_root(AttestationDataAndCustodyBit(
-          data: attestation.data, custody_bit: false)),
-        hash_tree_root(AttestationDataAndCustodyBit(
-          data: attestation.data, custody_bit: true)),
-      ],
-      attestation.aggregate_signature,
-      get_domain(state, DOMAIN_ATTESTATION,
-                 slot_to_epoch(attestation.data.slot))
-    ):
-      warn("Invalid attestation signature")
-      return
-
-  # Crosslink data root is zero (to be removed in phase 1)
+  ## Check crosslink data root
+  ## [to be removed in phase 1]
   if attestation.data.crosslink_data_root != ZERO_HASH:
     warn("Invalid crosslink data root")
     return
 
+  # Check signature and bitfields
+  if not verify_indexed_attestation(
+      state, convert_to_indexed(state, attestation)):
+    warn("checkAttestation: signature or bitfields incorrect")
+    return
+
   true
 
 proc makeAttestationData*(
@@ -600,8 +555,8 @@ proc makeAttestationData*(
     beacon_block_root: beacon_block_root,
     target_root: target_root,
     crosslink_data_root: Eth2Digest(), # Stub in phase0
-    previous_crosslink: state.current_crosslinks[shard],
+    previous_crosslink_root: hash_tree_root(state.current_crosslinks[shard]),
     source_epoch: state.current_justified_epoch,
     source_root: state.current_justified_root,
-    target_epoch: slot_to_epoch(epoch_start_slot)
+    target_epoch: slot_to_epoch(state.slot)
   )
diff --git a/beacon_chain/spec/datatypes.nim b/beacon_chain/spec/datatypes.nim
index 2fffc9440..1e3da1e46 100644
--- a/beacon_chain/spec/datatypes.nim
+++ b/beacon_chain/spec/datatypes.nim
@@ -57,21 +57,15 @@ const
   ## Spec version we're aiming to be compatible with, right now
   ## TODO: improve this scheme once we can negotiate versions in protocol
 
-  # Gwei values
-  # ---------------------------------------------------------------
-  # https://github.com/ethereum/eth2.0-specs/blob/v0.5.0/specs/core/0_beacon-chain.md#gwei-values
-
   # TODO remove erstwhile blob/v0.6.3
   FORK_CHOICE_BALANCE_INCREMENT* = 2'u64^0 * 10'u64^9
 
   # Initial values
   # ---------------------------------------------------------------
-  # https://github.com/ethereum/eth2.0-specs/blob/v0.5.0/specs/core/0_beacon-chain.md#initial-values
+  # https://github.com/ethereum/eth2.0-specs/blob/v0.6.3/specs/core/0_beacon-chain.md#initial-values
   GENESIS_EPOCH* = (GENESIS_SLOT.uint64 div SLOTS_PER_EPOCH).Epoch ##\
   ## slot_to_epoch(GENESIS_SLOT)
-  GENESIS_START_SHARD* = 0'u64
   ZERO_HASH* = Eth2Digest()
-  EMPTY_SIGNATURE* = ValidatorSig()
 
 type
   ValidatorIndex* = range[0'u32 .. 0xFFFFFF'u32] # TODO: wrap-around
@@ -97,18 +91,16 @@ type
     attestation_2*: IndexedAttestation ## \
     ## Second attestation
 
-  # https://github.com/ethereum/eth2.0-specs/blob/v0.5.0/specs/core/0_beacon-chain.md#slashableattestation
+  # https://github.com/ethereum/eth2.0-specs/blob/v0.6.3/specs/core/0_beacon-chain.md#indexedattestation
   IndexedAttestation* = object
-    validator_indices*: seq[uint64] ##\
-    ## Validator indices
+    # These probably should be seq[ValidatorIndex], but that throws RLP errors
+    custody_bit_0_indices*: seq[uint64]
+    custody_bit_1_indices*: seq[uint64]
 
     data*: AttestationData ## \
     ## Attestation data
 
-    custody_bitfield*: BitField ##\
-    ## Custody bitfield
-
-    aggregate_signature*: ValidatorSig ## \
+    signature*: ValidatorSig ## \
     ## Aggregate signature
 
   # https://github.com/ethereum/eth2.0-specs/blob/v0.6.3/specs/core/0_beacon-chain.md#attestation
@@ -122,13 +114,13 @@ type
     custody_bitfield*: BitField ##\
     ## Custody bitfield
 
-    aggregate_signature*: ValidatorSig ##\
+    signature*: ValidatorSig ##\
     ## BLS aggregate signature
 
-  # https://github.com/ethereum/eth2.0-specs/blob/v0.5.0/specs/core/0_beacon-chain.md#attestationdata
+  # https://github.com/ethereum/eth2.0-specs/blob/v0.6.2/specs/core/0_beacon-chain.md#attestationdata
   AttestationData* = object
+    slot*: Slot # TODO remove this, once figure out attestation pool issues
     # LMD GHOST vote
-    slot*: Slot
     beacon_block_root*: Eth2Digest
 
     # FFG vote
@@ -139,7 +131,7 @@ type
 
     # Crosslink vote
     shard*: uint64
-    previous_crosslink*: Crosslink
+    previous_crosslink_root*: Eth2Digest
     crosslink_data_root*: Eth2Digest
 
   # https://github.com/ethereum/eth2.0-specs/blob/v0.6.3/specs/core/0_beacon-chain.md#attestationdataandcustodybit
@@ -460,17 +452,12 @@ func shortLog*(v: BeaconBlock): tuple[
     shortLog(v.signature)
   )
 
-func shortLog*(v: AttestationData): tuple[
-      slot: uint64, beacon_block_root: string, source_epoch: uint64,
-      target_root: string, source_root: string, shard: uint64,
-      previous_crosslink_epoch: uint64, previous_crosslink_data_root: string,
-      crosslink_data_root: string
-    ] = (
-      humaneSlotNum(v.slot), shortLog(v.beacon_block_root),
+func shortLog*(v: AttestationData): auto =
+   (
+      shortLog(v.beacon_block_root),
       humaneEpochNum(v.source_epoch), shortLog(v.target_root),
       shortLog(v.source_root),
-      v.shard, humaneEpochNum(v.previous_crosslink.epoch),
-      shortLog(v.previous_crosslink.crosslink_data_root),
+      v.shard, v.previous_crosslink_root,
       shortLog(v.crosslink_data_root)
     )
 
diff --git a/beacon_chain/spec/helpers.nim b/beacon_chain/spec/helpers.nim
index 1e5f25742..a2b36110e 100644
--- a/beacon_chain/spec/helpers.nim
+++ b/beacon_chain/spec/helpers.nim
@@ -81,7 +81,7 @@ func get_active_validator_indices*(state: BeaconState, epoch: Epoch):
     if is_active_validator(val, epoch):
       result.add idx.ValidatorIndex
 
-# https://github.com/ethereum/eth2.0-specs/blob/v0.6.1/specs/core/0_beacon-chain.md#get_epoch_committee_count
+# https://github.com/ethereum/eth2.0-specs/blob/v0.6.3/specs/core/0_beacon-chain.md#get_epoch_committee_count
 func get_epoch_committee_count*(state: BeaconState, epoch: Epoch): uint64 =
   # Return the number of committees at ``epoch``.
   let active_validator_indices = get_active_validator_indices(state, epoch)
@@ -95,13 +95,12 @@ func get_current_epoch*(state: BeaconState): Epoch =
   doAssert state.slot >= GENESIS_SLOT, $state.slot
   slot_to_epoch(state.slot)
 
-# https://github.com/ethereum/eth2.0-specs/blob/v0.6.1/specs/core/0_beacon-chain.md#get_randao_mix
+# https://github.com/ethereum/eth2.0-specs/blob/v0.6.3/specs/core/0_beacon-chain.md#get_randao_mix
 func get_randao_mix*(state: BeaconState,
                      epoch: Epoch): Eth2Digest =
     ## Returns the randao mix at a recent ``epoch``.
-    ## ``epoch`` expected to be between
-    ## (current_epoch - LATEST_ACTIVE_INDEX_ROOTS_LENGTH + ACTIVATION_EXIT_DELAY,
-    ##  current_epoch + ACTIVATION_EXIT_DELAY].
+    ## ``epoch`` expected to be between (current_epoch -
+    ## LATEST_RANDAO_MIXES_LENGTH, current_epoch].
     state.latest_randao_mixes[epoch mod LATEST_RANDAO_MIXES_LENGTH]
 
 # https://github.com/ethereum/eth2.0-specs/blob/v0.6.3/specs/core/0_beacon-chain.md#get_active_index_root
@@ -152,7 +151,7 @@ func int_to_bytes4*(x: uint64): array[4, byte] =
   result[2] = ((x shr 16) and 0xff).byte
   result[3] = ((x shr 24) and 0xff).byte
 
-# https://github.com/ethereum/eth2.0-specs/blob/v0.6.0/specs/core/0_beacon-chain.md#get_domain
+# https://github.com/ethereum/eth2.0-specs/blob/v0.6.3/specs/core/0_beacon-chain.md#get_domain
 func get_domain*(
     state: BeaconState, domain_type: SignatureDomain, message_epoch: Epoch): uint64 =
   ## Return the signature domain (fork version concatenated with domain type)
diff --git a/beacon_chain/spec/presets/mainnet.nim b/beacon_chain/spec/presets/mainnet.nim
index 89225b0e6..25abeb8d3 100644
--- a/beacon_chain/spec/presets/mainnet.nim
+++ b/beacon_chain/spec/presets/mainnet.nim
@@ -22,7 +22,7 @@ type
 
 {.experimental: "codeReordering".} # SLOTS_PER_EPOCH is use before being defined in spec
 
-# https://github.com/ethereum/eth2.0-specs/blob/v0.6.1/configs/constant_presets/mainnet.yaml
+# https://github.com/ethereum/eth2.0-specs/blob/v0.6.3/configs/constant_presets/mainnet.yaml
 const
   # Misc
   # ---------------------------------------------------------------
@@ -83,16 +83,16 @@ const
 
   # Initial values
   # ---------------------------------------------------------------
-  # https://github.com/ethereum/eth2.0-specs/blob/v0.5.0/specs/core/0_beacon-chain.md#initial-values
+  # https://github.com/ethereum/eth2.0-specs/blob/v0.6.3/configs/constant_presets/mainnet.yaml#L44
 
   GENESIS_FORK_VERSION* = [0'u8, 0'u8, 0'u8, 0'u8]
-  GENESIS_SLOT* = 64.Slot
+  GENESIS_SLOT* = 0.Slot
   FAR_FUTURE_EPOCH* = (not 0'u64).Epoch # 2^64 - 1 in spec
   BLS_WITHDRAWAL_PREFIX_BYTE* = 0'u8
 
   # Time parameters
   # ---------------------------------------------------------------
-  # https://github.com/ethereum/eth2.0-specs/blob/v0.6.1/specs/core/0_fork-choice.md#time-parameters
+  # https://github.com/ethereum/eth2.0-specs/blob/v0.6.3/specs/core/0_fork-choice.md#time-parameters
 
   SECONDS_PER_SLOT*{.intdefine.} = 6'u64 # Compile with -d:SECONDS_PER_SLOT=1 for 6x faster slots
   ## TODO consistent time unit across projects, similar to C++ chrono?
diff --git a/beacon_chain/spec/presets/minimal.nim b/beacon_chain/spec/presets/minimal.nim
index 4d9d818ca..72355c037 100644
--- a/beacon_chain/spec/presets/minimal.nim
+++ b/beacon_chain/spec/presets/minimal.nim
@@ -26,7 +26,7 @@ type
 const
   # Misc
   # ---------------------------------------------------------------
-  # https://github.com/ethereum/eth2.0-specs/blob/v0.6.1/specs/core/0_beacon-chain.md#misc
+  # https://github.com/ethereum/eth2.0-specs/blob/v0.6.3/specs/core/0_beacon-chain.md#misc
 
   # Changed
   SHARD_COUNT* {.intdefine.} = 8
@@ -61,11 +61,11 @@ const
 
   # Initial values
   # ---------------------------------------------------------------
-  # https://github.com/ethereum/eth2.0-specs/blob/v0.5.0/specs/core/0_beacon-chain.md#initial-values
+  # https://github.com/ethereum/eth2.0-specs/blob/v0.6.3/configs/constant_presets/minimal.yaml#L43
 
   # Unchanged
   GENESIS_FORK_VERSION* = [0'u8, 0'u8, 0'u8, 0'u8]
-  GENESIS_SLOT* = 64.Slot
+  GENESIS_SLOT* = 0.Slot
   FAR_FUTURE_EPOCH* = (not 0'u64).Epoch # 2^64 - 1 in spec
   BLS_WITHDRAWAL_PREFIX_BYTE* = 0'u8
 
@@ -89,7 +89,7 @@ const
 
   # Changed
   SLOTS_PER_ETH1_VOTING_PERIOD* = 16
-  SLOTS_PER_HISTORICAL_ROOT* = 64
+  SLOTS_PER_HISTORICAL_ROOT* = 128 # 64 doesn't work with GENESIS_SLOT == 0?
 
   # Unchanged
   MIN_VALIDATOR_WITHDRAWABILITY_DELAY* = 2'u64^8
diff --git a/beacon_chain/spec/validator.nim b/beacon_chain/spec/validator.nim
index 7dc68fea7..0ab58eb55 100644
--- a/beacon_chain/spec/validator.nim
+++ b/beacon_chain/spec/validator.nim
@@ -13,8 +13,8 @@ import
   ./crypto, ./datatypes, ./digest, ./helpers
 
 # TODO: Proceed to renaming and signature changes
-# https://github.com/ethereum/eth2.0-specs/blob/v0.6.1/specs/core/0_beacon-chain.md#get_shuffled_index
-# https://github.com/ethereum/eth2.0-specs/blob/v0.6.1/specs/core/0_beacon-chain.md#compute_committee
+# https://github.com/ethereum/eth2.0-specs/blob/v0.6.3/specs/core/0_beacon-chain.md#get_shuffled_index
+# https://github.com/ethereum/eth2.0-specs/blob/v0.6.3/specs/core/0_beacon-chain.md#compute_committee
 func get_shuffled_seq*(seed: Eth2Digest,
                        list_size: uint64,
                        ): seq[ValidatorIndex] =
@@ -118,7 +118,7 @@ func get_shuffled_index(index: ValidatorIndex, index_count: uint64, seed: Eth2Di
     if bit != 0:
       result = flip
 
-# https://github.com/ethereum/eth2.0-specs/blob/v0.6.1/specs/core/0_beacon-chain.md#get_previous_epoch
+# https://github.com/ethereum/eth2.0-specs/blob/v0.6.3/specs/core/0_beacon-chain.md#get_previous_epoch
 func get_previous_epoch*(state: BeaconState): Epoch =
   ## Return the previous epoch of the given ``state``.
   ## Return the current epoch if it's genesis epoch.
@@ -149,7 +149,7 @@ func get_epoch_start_shard*(state: BeaconState, epoch: Epoch): Shard =
       SHARD_COUNT
   return shard
 
-# https://github.com/ethereum/eth2.0-specs/blob/v0.6.1/specs/core/0_beacon-chain.md#compute_committee
+# https://github.com/ethereum/eth2.0-specs/blob/v0.6.3/specs/core/0_beacon-chain.md#compute_committee
 func compute_committee(indices: seq[ValidatorIndex], seed: Eth2Digest,
     index: uint64, count: uint64): seq[ValidatorIndex] =
   let
@@ -212,7 +212,7 @@ iterator get_crosslink_committees_at_slot_cached*(
   cache.crosslink_committee_cache[key] = result
   for v in result: yield v
 
-# https://github.com/ethereum/eth2.0-specs/blob/v0.6.1/specs/core/0_beacon-chain.md#get_beacon_proposer_index
+# https://github.com/ethereum/eth2.0-specs/blob/v0.6.3/specs/core/0_beacon-chain.md#get_beacon_proposer_index
 func get_beacon_proposer_index*(state: BeaconState): ValidatorIndex =
   # Return the current beacon proposer index.
   const
diff --git a/beacon_chain/state_transition.nim b/beacon_chain/state_transition.nim
index 89f712f93..78def2f01 100644
--- a/beacon_chain/state_transition.nim
+++ b/beacon_chain/state_transition.nim
@@ -171,56 +171,6 @@ proc processProposerSlashings(
 
   true
 
-# https://github.com/ethereum/eth2.0-specs/blob/v0.5.0/specs/core/0_beacon-chain.md#verify_slashable_attestation
-func verify_slashable_attestation(state: BeaconState, slashable_attestation: IndexedAttestation): bool =
-  # Verify validity of ``slashable_attestation`` fields.
-
-  if anyIt(slashable_attestation.custody_bitfield.bits, it != 0):   # [TO BE REMOVED IN PHASE 1]
-    return false
-
-  if len(slashable_attestation.validator_indices) == 0:
-     return false
-
-  for i in 0 ..< (len(slashable_attestation.validator_indices) - 1):
-    if slashable_attestation.validator_indices[i] >= slashable_attestation.validator_indices[i + 1]:
-      return false
-
-  if not verify_bitfield(slashable_attestation.custody_bitfield,
-      len(slashable_attestation.validator_indices)):
-    return false
-
-  if len(slashable_attestation.validator_indices) > MAX_INDICES_PER_ATTESTATION:
-    return false
-
-  var
-    custody_bit_0_indices: seq[uint64] = @[]
-    custody_bit_1_indices: seq[uint64] = @[]
-
-  for i, validator_index in slashable_attestation.validator_indices:
-    if not get_bitfield_bit(slashable_attestation.custody_bitfield, i):
-      custody_bit_0_indices.add(validator_index)
-    else:
-      custody_bit_1_indices.add(validator_index)
-
-  bls_verify_multiple(
-    @[
-      bls_aggregate_pubkeys(mapIt(custody_bit_0_indices, state.validator_registry[it.int].pubkey)),
-      bls_aggregate_pubkeys(mapIt(custody_bit_1_indices, state.validator_registry[it.int].pubkey)),
-    ],
-    @[
-      hash_tree_root(AttestationDataAndCustodyBit(
-        data: slashable_attestation.data, custody_bit: false)),
-      hash_tree_root(AttestationDataAndCustodyBit(
-        data: slashable_attestation.data, custody_bit: true)),
-    ],
-    slashable_attestation.aggregate_signature,
-    get_domain(
-      state,
-      DOMAIN_ATTESTATION,
-      slot_to_epoch(slashable_attestation.data.slot),
-    ),
-  )
-
 # https://github.com/ethereum/eth2.0-specs/blob/v0.6.3/specs/core/0_beacon-chain.md#is_slashable_attestation_data
 func is_slashable_attestation_data(
     data_1: AttestationData, data_2: AttestationData): bool =
@@ -233,13 +183,14 @@ func is_slashable_attestation_data(
     (data_1.source_epoch < data_2.source_epoch and
      data_2.target_epoch < data_1.target_epoch)
 
-# https://github.com/ethereum/eth2.0-specs/blob/v0.5.0/specs/core/0_beacon-chain.md#attester-slashings
+# https://github.com/ethereum/eth2.0-specs/blob/v0.6.3/specs/core/0_beacon-chain.md#attester-slashings
 proc processAttesterSlashings(state: var BeaconState, blck: BeaconBlock): bool =
   # Process ``AttesterSlashing`` operation.
   if len(blck.body.attester_slashings) > MAX_ATTESTER_SLASHINGS:
     notice "CaspSlash: too many!"
     return false
 
+  result = true
   for attester_slashing in blck.body.attester_slashings:
     let
       attestation_1 = attester_slashing.attestation_1
@@ -250,32 +201,32 @@ proc processAttesterSlashings(state: var BeaconState, blck: BeaconBlock): bool =
       notice "CaspSlash: surround or double vote check failed"
       return false
 
-    if not verify_slashable_attestation(state, attestation_1):
+    if not verify_indexed_attestation(state, attestation_1):
       notice "CaspSlash: invalid votes 1"
       return false
 
-    if not verify_slashable_attestation(state, attestation_2):
+    if not verify_indexed_attestation(state, attestation_2):
       notice "CaspSlash: invalid votes 2"
       return false
 
     var slashed_any = false
 
-    let
-      indices2 = toSet(attestation_2.validator_indices)
-      slashable_indices =
-        attestation_1.validator_indices.filterIt(
-          it in indices2 and not state.validator_registry[it.int].slashed)
+    ## TODO there's a lot of sorting/set construction here and
+    ## verify_indexed_attestation, but go by spec unless there
+    ## is compelling perf evidence otherwise.
+    let attesting_indices_1 =
+      attestation_1.custody_bit_0_indices & attestation_1.custody_bit_1_indices
+    let attesting_indices_2 =
+      attestation_2.custody_bit_0_indices & attestation_2.custody_bit_1_indices
+    for index in sorted(toSeq(intersection(toSet(attesting_indices_1),
+        toSet(attesting_indices_2)).items), system.cmp):
+      if is_slashable_validator(state.validator_registry[index.int],
+          get_current_epoch(state)):
+        slash_validator(state, index.ValidatorIndex)
+        slashed_any = true
+    result = result and slashed_any
 
-    if not (len(slashable_indices) >= 1):
-      notice "CaspSlash: no intersection"
-      return false
-
-    for index in slashable_indices:
-      slash_validator(state, index.ValidatorIndex)
-
-  true
-
-# https://github.com/ethereum/eth2.0-specs/blob/v0.6.0/specs/core/0_beacon-chain.md#attestations
+# https://github.com/ethereum/eth2.0-specs/blob/v0.6.3/specs/core/0_beacon-chain.md#attestations
 proc processAttestations(
     state: var BeaconState, blck: BeaconBlock, flags: UpdateFlags): bool =
   ## Each block includes a number of attestations that the proposer chose. Each
@@ -296,6 +247,7 @@ proc processAttestations(
   var committee_count_cache = initTable[Epoch, uint64]()
 
   for attestation in blck.body.attestations:
+    # Caching
     let
       epoch = attestation.data.target_epoch
       committee_count = if epoch in committee_count_cache:
@@ -303,6 +255,8 @@ proc processAttestations(
         else:
           get_epoch_committee_count(state, epoch)
     committee_count_cache[epoch] = committee_count
+
+    # Spec content
     let attestation_slot =
       get_attestation_slot(state, attestation, committee_count)
     let pending_attestation = PendingAttestation(
@@ -312,7 +266,7 @@ proc processAttestations(
       proposer_index: get_beacon_proposer_index(state),
     )
 
-    if slot_to_epoch(attestation.data.slot) == get_current_epoch(state):
+    if attestation.data.target_epoch == get_current_epoch(state):
       state.current_epoch_attestations.add(pending_attestation)
     else:
       state.previous_epoch_attestations.add(pending_attestation)
@@ -520,6 +474,20 @@ proc processBlock(
 
   true
 
+# TODO this cached version corresponds to the blob/v0.5.1ish get_attesting_indices
+# rm/make consistent with 0.6 version above
+func get_attesting_indices_cached(
+    state: BeaconState,
+    attestations: openArray[PendingAttestation], cache: var StateCache):
+      HashSet[ValidatorIndex] =
+  # Union of attesters that participated in some attestations
+  result = initSet[ValidatorIndex]()
+  for attestation in attestations:
+    for validator_index in get_attestation_participants_cached(
+        state, attestation.data, attestation.aggregation_bitfield,
+        cache):
+      result.incl validator_index
+
 # https://github.com/ethereum/eth2.0-specs/blob/v0.6.3/specs/core/0_beacon-chain.md#helper-functions-1
 func get_total_active_balance(state: BeaconState): Gwei =
   return get_total_balance(
@@ -549,38 +517,6 @@ func get_matching_head_attestations(state: BeaconState, epoch: Epoch):
        get_block_root_at_slot(state, get_attestation_slot(state, it))
   )
 
-# https://github.com/ethereum/eth2.0-specs/blob/v0.6.1/specs/core/0_beacon-chain.md#get_attesting_indices
-func get_attesting_indices(state: BeaconState,
-                           attestation_data: AttestationData,
-                           bitfield: BitField): HashSet[ValidatorIndex] =
-  ## Return the sorted attesting indices corresponding to ``attestation_data``
-  ## and ``bitfield``.
-  ## The spec goes through a lot of hoops to sort things, and sometimes
-  ## constructs sets from the results here. The basic idea is to always
-  ## just do the right thing and keep it in a HashSet.
-  result = initSet[ValidatorIndex]()
-  let committee =
-    get_crosslink_committee(state, attestation_data.target_epoch,
-      attestation_data.shard)
-  doAssert verify_bitfield(bitfield, len(committee))
-  for i, index in committee:
-    if get_bitfield_bit(bitfield, i):
-      result.incl index
-
-# TODO this cached version corresponds to the blob/v0.5.1ish get_attesting_indices
-# rm/make consistent with 0.6 version above
-func get_attesting_indices_cached(
-    state: BeaconState,
-    attestations: openArray[PendingAttestation], cache: var StateCache):
-      HashSet[ValidatorIndex] =
-  # Union of attesters that participated in some attestations
-  result = initSet[ValidatorIndex]()
-  for attestation in attestations:
-    for validator_index in get_attestation_participants_cached(
-        state, attestation.data, attestation.aggregation_bitfield,
-        cache):
-      result.incl validator_index
-
 func get_unslashed_attesting_indices(
     state: BeaconState, attestations: seq[PendingAttestation]):
     HashSet[ValidatorIndex] =
@@ -593,8 +529,8 @@ func get_unslashed_attesting_indices(
     if state.validator_registry[index].slashed:
       result.excl index
 
-func get_attesting_balance(state: BeaconState,
-                           attestations: seq[PendingAttestation]): Gwei =
+func get_attesting_balance(
+    state: BeaconState, attestations: seq[PendingAttestation]): Gwei =
   get_total_balance(state, get_unslashed_attesting_indices(state, attestations))
 
 func get_attesting_balance_cached(
@@ -603,7 +539,16 @@ func get_attesting_balance_cached(
   get_total_balance(state, get_attesting_indices_cached(
     state, attestations, cache))
 
-# Not exactly in spec, but for get_winning_root_and_participants
+func get_crosslink_from_attestation_data(
+    state: BeaconState, data: AttestationData): Crosslink =
+  Crosslink(
+    epoch: min(data.target_epoch,
+      state.current_crosslinks[data.shard].epoch + MAX_CROSSLINK_EPOCHS),
+    previous_crosslink_root: data.previous_crosslink_root,
+    crosslink_data_root: data.crosslink_data_root,
+  )
+
+# Not exactly in spec, but for get_winning_crosslink_and_attesting_indices
 func lowerThan(candidate, current: Eth2Digest): bool =
   # return true iff candidate is "lower" than current, per spec rule:
   # "ties broken in favor of lexicographically higher hash
@@ -611,52 +556,65 @@ func lowerThan(candidate, current: Eth2Digest): bool =
     if v > candidate.data[i]: return true
   false
 
-func get_winning_root_and_participants(
-    state: BeaconState, shard: Shard, cache: var StateCache):
-    tuple[a: Eth2Digest, b: HashSet[ValidatorIndex]] =
+# TODO check/profile if should add cache: var StateCache param
+func get_winning_crosslink_and_attesting_indices(
+    state: BeaconState, epoch: Epoch, shard: Shard): tuple[a: Crosslink, b: HashSet[ValidatorIndex]] =
   let
-    all_attestations =
-      concat(state.current_epoch_attestations,
-             state.previous_epoch_attestations)
-    valid_attestations =
+    ## TODO Z-F could help here
+    ## TODO get_winning_crosslink_and_attesting_indices was profiling hotspot
+    shard_attestations =
       filterIt(
-        all_attestations,
-        it.data.previous_crosslink == state.current_crosslinks[shard])
-    all_roots = mapIt(valid_attestations, it.data.crosslink_data_root)
+        get_matching_source_attestations(state, epoch), it.data.shard == shard)
+    shard_crosslinks =
+      mapIt(shard_attestations,
+        get_crosslink_from_attestation_data(state, it.data))
+    # TODO this seems like a lot of hash_tree_root'ing on same data
+    candidate_crosslinks =
+      filterIt(shard_crosslinks,
+        hash_tree_root(state.current_crosslinks[shard]) in
+          # TODO pointless memory allocation, etc.
+          @[it.previous_crosslink_root, hash_tree_root(it)])
 
-  # handle when no attestations for shard available
-  if len(all_roots) == 0:
-    return (ZERO_HASH, initSet[ValidatorIndex]())
+  if len(candidate_crosslinks) == 0:
+    return (Crosslink(), initSet[ValidatorIndex]())
 
-  # 0.5.1 spec has less-than-ideal get_attestations_for nested function.
-  var attestations_for = initTable[Eth2Digest, seq[PendingAttestation]]()
-  for valid_attestation in valid_attestations:
-    if valid_attestation.data.crosslink_data_root in attestations_for:
-      attestations_for[valid_attestation.data.crosslink_data_root].add(
-        valid_attestation)
-    else:
-      attestations_for[valid_attestation.data.crosslink_data_root] =
-        @[valid_attestation]
+  ## TODO check if should cache this again, as with 0.5
+  ## var attestations_for = initTable[Eth2Digest, seq[PendingAttestation]]()
+  ## for valid_attestation in valid_attestations:
+  ##   if valid_attestation.data.crosslink_data_root in attestations_for:
+  ##     attestations_for[valid_attestation.data.crosslink_data_root].add(
+  ##       valid_attestation)
+  ##   else:
+  ##     attestations_for[valid_attestation.data.crosslink_data_root] =
+  ##       @[valid_attestation]
+  ## TODO either way, this nested function not great; {.fastcall.} pragma
+  ## not directly applicable either, since it does need some closure
+  func get_attestations_for(crosslink: Crosslink): seq[PendingAttestation] =
+    filterIt(shard_attestations,
+      get_crosslink_from_attestation_data(state, it.data) == crosslink)
 
-  ## Winning crosslink root is the root with the most votes for it, ties broken
-  ## in favor of lexicographically higher hash
+  ## Winning crosslink has the crosslink data root with the most balance voting
+  ## for it (ties broken lexicographically)
   var
-    winning_root: Eth2Digest
-    winning_root_balance = 0'u64
+    winning_crosslink: Crosslink
+    winning_crosslink_balance = 0.Gwei
 
-  for r in all_roots:
-    let root_balance = get_attesting_balance_cached(
-      state, attestations_for.getOrDefault(r), cache)
-    if (root_balance > winning_root_balance or
-        (root_balance == winning_root_balance and
-         lowerThan(winning_root, r))):
-      winning_root = r
-      winning_root_balance = root_balance
+  for candidate_crosslink in candidate_crosslinks:
+    ## TODO check if should cache this again
+    ## let root_balance = get_attesting_balance_cached(
+    ##   state, attestations_for.getOrDefault(r), cache)
+    let crosslink_balance =
+      get_attesting_balance(state, get_attestations_for(candidate_crosslink))
+    if (crosslink_balance > winning_crosslink_balance or
+        (winning_crosslink_balance == crosslink_balance and
+         lowerThan(winning_crosslink.crosslink_data_root,
+                   candidate_crosslink.crosslink_data_root))):
+      winning_crosslink = candidate_crosslink
+      winning_crosslink_balance = crosslink_balance
 
-  (winning_root,
-   get_attesting_indices_cached(
-     state,
-     attestations_for.getOrDefault(winning_root), cache))
+  (winning_crosslink,
+   get_unslashed_attesting_indices(state,
+     get_attestations_for(winning_crosslink)))
 
 # https://github.com/ethereum/eth2.0-specs/blob/v0.6.3/specs/core/0_beacon-chain.md#justification-and-finalization
 func process_justification_and_finalization(state: var BeaconState) =
@@ -723,48 +681,32 @@ func process_justification_and_finalization(state: var BeaconState) =
     state.finalized_epoch = old_current_justified_epoch
     state.finalized_root = get_block_root(state, state.finalized_epoch)
 
-# https://github.com/ethereum/eth2.0-specs/blob/v0.5.1/specs/core/0_beacon-chain.md#crosslinks
-func process_crosslinks(
-    state: var BeaconState, per_epoch_cache: var StateCache) =
-  let
-    current_epoch = get_current_epoch(state)
-    previous_epoch = current_epoch - 1
-    next_epoch = current_epoch + 1
+# https://github.com/ethereum/eth2.0-specs/blob/v0.6.3/specs/core/0_beacon-chain.md#crosslinks
+func process_crosslinks(state: var BeaconState, per_epoch_cache: var StateCache) =
+  ## TODO is there a semantic reason for this, or is this just a way to force
+  ## copying? If so, why not just `list(foo)` or similar? This is strange. In
+  ## this case, for type reasons, don't do weird
+  ## [c for c in state.current_crosslinks] from spec.
+  state.previous_crosslinks = state.current_crosslinks
 
-  ## TODO is it actually correct to be setting state.current_crosslinks[shard]
-  ## to something pre-GENESIS_EPOCH, ever? I guess the intent is if there are
-  ## a quorum of participants for  get_epoch_start_slot(previous_epoch), when
-  ## state.slot == GENESIS_SLOT, then there will be participants for a quorum
-  ## in the current-epoch (i.e. genesis epoch) version of that shard?
-  #for slot in get_epoch_start_slot(previous_epoch).uint64 ..<
-  for slot in max(
-      GENESIS_SLOT.uint64, get_epoch_start_slot(previous_epoch).uint64) ..<
-      get_epoch_start_slot(next_epoch).uint64:
-    for cas in get_crosslink_committees_at_slot_cached(
-        state, slot, per_epoch_cache):
+  for epoch_int in get_previous_epoch(state).uint64 ..
+      get_current_epoch(state).uint64:
+    # This issue comes up regularly -- iterating means an int type,
+    # which then needs re-conversion back to specialized type.
+    let epoch = epoch_int.Epoch
+    for offset in 0'u64 ..< get_epoch_committee_count(state, epoch):
       let
-        (crosslink_committee, shard) = cas
+        shard = (get_epoch_start_shard(state, epoch) + offset) mod SHARD_COUNT
+        crosslink_committee = get_crosslink_committee(state, epoch, shard)
         # In general, it'll loop over the same shards twice, and
         # get_winning_root_and_participants is defined to return
         # the same results from the previous epoch as current.
-        (winning_root, participants) =
-          if shard notin per_epoch_cache.winning_root_participants_cache:
-            get_winning_root_and_participants(state, shard, per_epoch_cache)
-          else:
-            (ZERO_HASH, per_epoch_cache.winning_root_participants_cache[shard])
-        participating_balance = get_total_balance(state, participants)
-        total_balance = get_total_balance(state, crosslink_committee)
-
-      per_epoch_cache.winning_root_participants_cache[shard] = participants
-
-      if 3'u64 * participating_balance >= 2'u64 * total_balance:
-        # Check not from spec; seems kludgy
-        doAssert slot >= GENESIS_SLOT
-
-        state.current_crosslinks[shard] = Crosslink(
-          epoch: slot_to_epoch(slot),
-          crosslink_data_root: winning_root
-        )
+        # TODO cache like before, in 0.5 version of this function
+        (winning_crosslink, attesting_indices) =
+          get_winning_crosslink_and_attesting_indices(state, epoch, shard)
+      if 3'u64 * get_total_balance(state, attesting_indices) >=
+          2'u64 * get_total_balance(state, crosslink_committee):
+        state.current_crosslinks[shard] = winning_crosslink
 
 # https://github.com/ethereum/eth2.0-specs/blob/v0.6.3/specs/core/0_beacon-chain.md#rewards-and-penalties
 func get_base_reward(state: BeaconState, index: ValidatorIndex): Gwei =
@@ -775,7 +717,7 @@ func get_base_reward(state: BeaconState, index: ValidatorIndex): Gwei =
   state.validator_registry[index].effective_balance div adjusted_quotient div
     BASE_REWARDS_PER_EPOCH
 
-# https://github.com/ethereum/eth2.0-specs/blob/v0.6.1/specs/core/0_beacon-chain.md#rewards-and-penalties
+# https://github.com/ethereum/eth2.0-specs/blob/v0.6.3/specs/core/0_beacon-chain.md#rewards-and-penalties
 func get_attestation_deltas(state: BeaconState):
     tuple[a: seq[Gwei], b: seq[Gwei]] =
   let
@@ -846,40 +788,34 @@ func get_attestation_deltas(state: BeaconState):
 
   (rewards, penalties)
 
-# blob/0.5.1
+# TODO re-cache this one, as under 0.5 version, if profiling suggests it
 func get_crosslink_deltas(state: BeaconState, cache: var StateCache):
     tuple[a: seq[Gwei], b: seq[Gwei]] =
+
   var
     rewards = repeat(0'u64, len(state.validator_registry))
     penalties = repeat(0'u64, len(state.validator_registry))
-  let
-    previous_epoch_start_slot =
-      get_epoch_start_slot(get_previous_epoch(state))
-    current_epoch_start_slot =
-      get_epoch_start_slot(get_current_epoch(state))
-  for slot in previous_epoch_start_slot.uint64 ..<
-      current_epoch_start_slot.uint64:
-    for cas in get_crosslink_committees_at_slot_cached(state, slot, cache):
-      let
-        (crosslink_committee, shard) = cas
-        (winning_root, participants) =
-          if shard notin cache.winning_root_participants_cache:
-            get_winning_root_and_participants(state, shard, cache)
-          else:
-            (ZERO_HASH, cache.winning_root_participants_cache[shard])
-        participating_balance = get_total_balance(state, participants)
-        total_balance = get_total_balance(state, crosslink_committee)
-      for index in crosslink_committee:
-        if index in participants:
-          rewards[index] +=
-            get_base_reward(state, index) * participating_balance div
-              total_balance
-        else:
-          penalties[index] += get_base_reward(state, index)
+  let epoch = get_previous_epoch(state)
+  for offset in 0'u64 ..< get_epoch_committee_count(state, epoch):
+    let
+      shard = (get_epoch_start_shard(state, epoch) + offset) mod SHARD_COUNT
+      crosslink_committee = get_crosslink_committee(state, epoch, shard)
+      (winning_crosslink, attesting_indices) =
+        get_winning_crosslink_and_attesting_indices(state, epoch, shard)
+      attesting_balance = get_total_balance(state, attesting_indices)
+      committee_balance = get_total_balance(state, crosslink_committee)
+    for index in crosslink_committee:
+      let base_reward = get_base_reward(state, index)
+      if index in attesting_indices:
+        rewards[index] +=
+          get_base_reward(state, index) * attesting_balance div
+            committee_balance
+      else:
+        penalties[index] += get_base_reward(state, index)
 
   (rewards, penalties)
 
-# https://github.com/ethereum/eth2.0-specs/blob/v0.6.1/specs/core/0_beacon-chain.md#rewards-and-penalties
+# https://github.com/ethereum/eth2.0-specs/blob/v0.6.3/specs/core/0_beacon-chain.md#rewards-and-penalties
 func process_rewards_and_penalties(
     state: var BeaconState, cache: var StateCache) =
   let
diff --git a/beacon_chain/time.nim b/beacon_chain/time.nim
index 5d9dc366f..147305a7c 100644
--- a/beacon_chain/time.nim
+++ b/beacon_chain/time.nim
@@ -42,10 +42,7 @@ func toSlot*(t: BeaconTime): Slot =
   Slot(uint64(t) div SECONDS_PER_SLOT)
 
 func toBeaconTime*(c: BeaconClock, t: Time): BeaconTime =
-  doAssert t > c.genesis,
-    "Cannot represent time before genesis, fix BeaconClock"
-
-  BeaconTime(times.seconds(t - c.genesis).uint64)
+  BeaconTime(times.seconds(t - c.genesis).int64)
 
 func toSlot*(c: BeaconClock, t: Time): Slot =
   c.toBeaconTime(t).toSlot()
diff --git a/tests/testutil.nim b/tests/testutil.nim
index bc107efee..a431f901c 100644
--- a/tests/testutil.nim
+++ b/tests/testutil.nim
@@ -80,11 +80,6 @@ proc addBlock*(
   let proposer_index = get_beacon_proposer_index(state)
   state.slot -= 1
 
-  # Ferret out remaining GENESIS_EPOCH == 0 assumptions in test code
-  doAssert allIt(
-    body.attestations,
-    it.data.previous_crosslink.epoch >= GENESIS_EPOCH)
-
   let
     # Index from the new state, but registry from the old state.. hmm...
     proposer = state.validator_registry[proposer_index]
@@ -182,7 +177,7 @@ proc makeAttestation*(
   Attestation(
     data: data,
     aggregation_bitfield: aggregation_bitfield,
-    aggregate_signature: sig,
+    signature: sig,
     custody_bitfield: BitField.init(sac.committee.len)
   )