diff --git a/specs/bls_signature.md b/specs/bls_signature.md
index 8e0b76c6b..75827982b 100644
--- a/specs/bls_signature.md
+++ b/specs/bls_signature.md
@@ -106,11 +106,11 @@ def modular_squareroot(value: int) -> int:
 
 ### `bls_aggregate_pubkeys`
 
-Let `bls_aggregate_pubkeys(pubkeys: [uint384]) -> uint384` return `pubkeys[0] + .... + pubkeys[len(pubkeys)-1]`, where `+` is the elliptic curve addition operation over the G1 curve.
+Let `bls_aggregate_pubkeys(pubkeys: List[Bytes48]) -> Bytes48` return `pubkeys[0] + .... + pubkeys[len(pubkeys)-1]`, where `+` is the elliptic curve addition operation over the G1 curve.
 
 ### `bls_aggregate_signatures`
 
-Let `bls_aggregate_signatures(signatures: [[uint384]]) -> [uint384]` return `signatures[0] + .... + signatures[len(signatures)-1]`, where `+` is the elliptic curve addition operation over the G2 curve.
+Let `bls_aggregate_signatures(signatures: Tuple[Bytes48, Bytes48]) -> Tuple[Bytes48, Bytes48]` return `signatures[0] + .... + signatures[len(signatures)-1]`, where `+` is the elliptic curve addition operation over the G2 curve.
 
 ## Signature verification
 
@@ -124,7 +124,7 @@ g = Fq2(g_x, g_y)
 
 ### `bls_verify`
 
-Let `bls_verify(pubkey: uint384, message: bytes32, signature: [uint384], domain: uint64) -> bool`:
+Let `bls_verify(pubkey: Bytes48, message: Bytes32, signature: Tuple[Bytes48, Bytes48], domain: uint64) -> bool`:
 
 * Verify that `pubkey` is a valid G1 point.
 * Verify that `signature` is a valid G2 point.
@@ -132,7 +132,7 @@ Let `bls_verify(pubkey: uint384, message: bytes32, signature: [uint384], domain:
 
 ### `bls_verify_multiple`
 
-Let `bls_verify_multiple(pubkeys: [uint384], messages: [bytes32], signature: [uint384], domain: uint64) -> bool`:
+Let `bls_verify_multiple(pubkeys: List[Bytes48], messages: List[Bytes32], signature: Tuple[Bytes48, Bytes48], domain: uint64) -> bool`:
 
 * Verify that each `pubkey` in `pubkeys` is a valid G1 point.
 * Verify that `signature` is a valid G2 point.
diff --git a/specs/core/0_beacon-chain.md b/specs/core/0_beacon-chain.md
index 56207a4ca..17e9ca24f 100644
--- a/specs/core/0_beacon-chain.md
+++ b/specs/core/0_beacon-chain.md
@@ -85,6 +85,7 @@
             - [`is_double_vote`](#is_double_vote)
             - [`is_surround_vote`](#is_surround_vote)
             - [`integer_squareroot`](#integer_squareroot)
+            - [`signature_to_tuple`](#signature_to_tuple)
             - [`bls_verify`](#bls_verify)
             - [`bls_verify_multiple`](#bls_verify_multiple)
             - [`bls_aggregate_pubkeys`](#bls_aggregate_pubkeys)
@@ -1112,7 +1113,7 @@ def verify_slashable_vote_data(state: BeaconState, vote_data: SlashableVoteData)
             hash_tree_root(AttestationDataAndCustodyBit(vote_data.data, False)),
             hash_tree_root(AttestationDataAndCustodyBit(vote_data.data, True)),
         ],
-        signature=vote_data.aggregate_signature,
+        signature=signature_to_tuple(vote_data.aggregate_signature),
         domain=get_domain(
             state.fork_data,
             state.slot,
@@ -1175,6 +1176,16 @@ def integer_squareroot(n: int) -> int:
     return x
 ```
 
+#### `signature_to_tuple`
+
+```python
+def signature_to_tuple(signature: Signature) -> Tuple[bytes, bytes]:
+    """
+    Convert SSZ object ``signature`` to ``tuple`` type for BLS APIs.
+    """
+    return (signature.z_1, signature.z_2)
+```
+
 #### `bls_verify`
 
 `bls_verify` is a function for verifying a BLS signature, defined in the [BLS Signature spec](https://github.com/ethereum/eth2.0-specs/blob/master/specs/bls_signature.md#bls_verify).
@@ -1308,7 +1319,7 @@ def validate_proof_of_possession(state: BeaconState,
     return bls_verify(
         pubkey=pubkey,
         message=hash_tree_root(proof_of_possession_data),
-        signature=proof_of_possession,
+        signature=signature_to_tuple(proof_of_possession),
         domain=get_domain(
             state.fork_data,
             state.slot,
@@ -1469,7 +1480,7 @@ Below are the processing steps that happen at every `block`.
 
 * Let `block_without_signature_root` be the `hash_tree_root` of `block` where `block.signature` is set to `EMPTY_SIGNATURE`.
 * Let `proposal_root = hash_tree_root(ProposalSignedData(state.slot, BEACON_CHAIN_SHARD_NUMBER, block_without_signature_root))`.
-* Verify that `bls_verify(pubkey=state.validator_registry[get_beacon_proposer_index(state, state.slot)].pubkey, message=proposal_root, signature=block.signature, domain=get_domain(state.fork_data, state.slot, DOMAIN_PROPOSAL))`.
+* Verify that `bls_verify(pubkey=state.validator_registry[get_beacon_proposer_index(state, state.slot)].pubkey, message=proposal_root, signature=signature_to_tuple(block.signature), domain=get_domain(state.fork_data, state.slot, DOMAIN_PROPOSAL))`.
 
 ### RANDAO
 
@@ -1498,8 +1509,8 @@ For each `proposer_slashing` in `block.body.proposer_slashings`:
 * Verify that `proposer_slashing.proposal_data_1.shard == proposer_slashing.proposal_data_2.shard`.
 * Verify that `proposer_slashing.proposal_data_1.block_root != proposer_slashing.proposal_data_2.block_root`.
 * Verify that `proposer.penalized_slot > state.slot`.
-* Verify that `bls_verify(pubkey=proposer.pubkey, message=hash_tree_root(proposer_slashing.proposal_data_1), signature=proposer_slashing.proposal_signature_1, domain=get_domain(state.fork_data, proposer_slashing.proposal_data_1.slot, DOMAIN_PROPOSAL))`.
-* Verify that `bls_verify(pubkey=proposer.pubkey, message=hash_tree_root(proposer_slashing.proposal_data_2), signature=proposer_slashing.proposal_signature_2, domain=get_domain(state.fork_data, proposer_slashing.proposal_data_2.slot, DOMAIN_PROPOSAL))`.
+* Verify that `bls_verify(pubkey=proposer.pubkey, message=hash_tree_root(proposer_slashing.proposal_data_1), signature=signature_to_tuple(proposer_slashing.proposal_signature_1), domain=get_domain(state.fork_data, proposer_slashing.proposal_data_1.slot, DOMAIN_PROPOSAL))`.
+* Verify that `bls_verify(pubkey=proposer.pubkey, message=hash_tree_root(proposer_slashing.proposal_data_2), signature=signature_to_tuple(proposer_slashing.proposal_signature_2), domain=get_domain(state.fork_data, proposer_slashing.proposal_data_2.slot, DOMAIN_PROPOSAL))`.
 * Run `penalize_validator(state, proposer_slashing.proposer_index)`.
 
 #### Casper slashings
@@ -1533,7 +1544,7 @@ 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(AttestationDataAndCustodyBit(attestation.data, False)), 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=signature_to_tuple(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`.
 
@@ -1583,7 +1594,7 @@ For each `exit` in `block.body.exits`:
 * Let `validator = state.validator_registry[exit.validator_index]`.
 * Verify that `validator.exit_slot > state.slot + ENTRY_EXIT_DELAY`.
 * Verify that `state.slot >= exit.slot`.
-* Verify that `bls_verify(pubkey=validator.pubkey, message=ZERO_HASH, signature=exit.signature, domain=get_domain(state.fork_data, exit.slot, DOMAIN_EXIT))`.
+* Verify that `bls_verify(pubkey=validator.pubkey, message=ZERO_HASH, signature=signature_to_tuple(exit.signature), domain=get_domain(state.fork_data, exit.slot, DOMAIN_EXIT))`.
 * Run `initiate_validator_exit(state, exit.validator_index)`.
 
 #### Custody