diff --git a/specs/core/0_beacon-chain.md b/specs/core/0_beacon-chain.md
index 24251e1e2..56a9299d7 100644
--- a/specs/core/0_beacon-chain.md
+++ b/specs/core/0_beacon-chain.md
@@ -30,6 +30,7 @@
             - [Attestations](#attestations)
                 - [`Attestation`](#attestation)
                 - [`AttestationData`](#attestationdata)
+                - [`AttestationDataAndCustodyBit`](#attestationdataandcustodybit)
             - [Deposits](#deposits)
                 - [`Deposit`](#deposit)
                 - [`DepositData`](#depositdata)
@@ -105,7 +106,6 @@
         - [Helpers](#helpers)
         - [Receipt roots](#receipt-roots)
         - [Justification](#justification)
-        - [Finalization](#finalization)
         - [Crosslinks](#crosslinks)
         - [Rewards and penalties](#rewards-and-penalties)
             - [Justification and finalization](#justification-and-finalization)
@@ -340,6 +340,17 @@ Unless otherwise indicated, code appearing in `this style` is to be interpreted
 }
 ```
 
+##### `AttestationDataAndCustodyBit`
+
+```python
+{
+    # Attestation data
+    data: AttestationData,
+    # Proof of custody bit
+    poc_bit: bool,
+}
+```
+
 #### Deposits
 
 ##### `Deposit`
@@ -1038,18 +1049,15 @@ def verify_slashable_vote_data(state: BeaconState, vote_data: SlashableVoteData)
     if len(vote_data.aggregate_signature_poc_0_indices) + len(vote_data.aggregate_signature_poc_1_indices) > MAX_CASPER_VOTES:
         return False
 
-    pubs = [
-        aggregate_pubkey([state.validators[i].pubkey for i in vote_data.aggregate_signature_poc_0_indices]),
-        aggregate_pubkey([state.validators[i].pubkey for i in vote_data.aggregate_signature_poc_1_indices])
-    ]
-    vote_data_root = hash_tree_root(vote_data)
-    messages = [
-        vote_data_root + bytes1(0),
-        vote_data_root + bytes1(1)
-    ]
     return bls_verify_multiple(
-        pubkeys=pubs,
-        messages=messages,
+        pubkeys=[
+            aggregate_pubkey([state.validators[i].pubkey for i in vote_data.aggregate_signature_poc_0_indices]),
+            aggregate_pubkey([state.validators[i].pubkey for i in vote_data.aggregate_signature_poc_1_indices]),
+        ],
+        messages=[
+            hash_tree_root(AttestationDataAndCustodyBit(vote_data, False)),
+            hash_tree_root(AttestationDataAndCustodyBit(vote_data, True)),
+        ],
         signature=vote_data.aggregate_signature,
         domain=get_domain(
             state.fork_data,
@@ -1138,7 +1146,7 @@ A valid block with slot `INITIAL_SLOT_NUMBER` (a "genesis block") has the follow
         poc_challenges=[],
         poc_responses=[],
         deposits=[],
-        exits=[]
+        exits=[],
     ),
 }
 ```
@@ -1202,7 +1210,7 @@ def get_initial_beacon_state(initial_validator_deposits: List[Deposit],
             deposit=deposit.deposit_data.value,
             proof_of_possession=deposit.deposit_data.deposit_input.proof_of_possession,
             withdrawal_credentials=deposit.deposit_data.deposit_input.withdrawal_credentials,
-            randao_commitment=deposit.deposit_data.deposit_input.randao_commitment
+            randao_commitment=deposit.deposit_data.deposit_input.randao_commitment,
         )
         if get_effective_balance(state, validator_index) == MAX_DEPOSIT * GWEI_PER_ETH:
             update_validator_status(state, validator_index, ACTIVE)
@@ -1407,7 +1415,7 @@ def exit_validator(state: BeaconState,
         current_validator_registry_delta_chain_tip=state.validator_registry_delta_chain_tip,
         validator_index=index,
         pubkey=validator.pubkey,
-        flag=EXIT
+        flag=EXIT,
     )
 
     # Remove validator from persistent committees
@@ -1510,7 +1518,7 @@ For each `attestation` in `block.body.attestations`:
 * `aggregate_signature` verification:
     * Let `participants = get_attestation_participants(state, attestation.data, attestation.participation_bitfield)`.
     * Let `group_public_key = bls_aggregate_pubkeys([state.validator_registry[v].pubkey for v in participants])`.
-    * Verify that `bls_verify(pubkey=group_public_key, message=hash_tree_root(attestation.data) + bytes1(0), signature=attestation.aggregate_signature, domain=get_domain(state.fork_data, attestation.data.slot, DOMAIN_ATTESTATION))`.
+    * Verify that `bls_verify(pubkey=group_public_key, message=hash_tree_root(AttestationDataAndCustodyBit(attestation.data, False)), signature=attestation.aggregate_signature, domain=get_domain(state.fork_data, attestation.data.slot, DOMAIN_ATTESTATION))`.
 * [TO BE REMOVED IN PHASE 1] Verify that `attestation.data.shard_block_root == ZERO_HASH`.
 * Append `PendingAttestationRecord(data=attestation.data, participation_bitfield=attestation.participation_bitfield, custody_bitfield=attestation.custody_bitfield, slot_included=state.slot)` to `state.latest_attestations`.
 
@@ -1548,7 +1556,6 @@ process_deposit(
     withdrawal_credentials=deposit.deposit_data.deposit_input.withdrawal_credentials,
     poc_commitment=deposit.deposit_data.deposit_input.poc_commitment,
     randao_commitment=deposit.deposit_data.deposit_input.randao_commitment,
-    
 )
 ```