diff --git a/specs/beacon-chain.md b/specs/beacon-chain.md
index 0e9266188..04664e114 100644
--- a/specs/beacon-chain.md
+++ b/specs/beacon-chain.md
@@ -119,6 +119,20 @@ An `AttestationRecord` has the following fields:
 }
 ```
 
+Here's the data signed (`AttestationSignedData`):
+
+```python
+fields = {
+    # Version of the chain
+    'version': 'int64',
+    'slot': 'int64',
+    'parent_hashes': ['hash32'],
+    'shard_id': 'int16',
+    'shard_block_hash': 'hash32',
+    'justified_slot': 'int64'
+}
+```
+
 A `SpecialRecord` has the following fields:
 
 ```python
@@ -179,6 +193,11 @@ The `CrystallizedState` has the following fields:
     'deposits_penalized_in_period': ['int32'],
     # Hash chain of validator set changes (for light clients to easily track deltas)
     'validator_set_delta_hash_chain': 'hash32'
+    # Parameters relevant to hard forks / versioning.
+    # Should be updated only by hard forks.
+    'pre_fork_version': 'int32',
+    'post_fork_version': 'int32',
+    'fork_slot_number': 'int64',
 }
 ```
 
@@ -471,7 +490,8 @@ For each one of these attestations:
 * Let `attestation_indices` be `get_shards_and_committees_for_slot(crystallized_state, slot)[x]`, choosing `x` so that `attestation_indices.shard` equals the `shard` value provided to find the set of validators that is creating this attestation record.
 * Verify that `len(attester_bitfield) == ceil_div8(len(attestation_indices))`, where `ceil_div8 = (x + 7) // 8`. Verify that bits `len(attestation_indices)....` and higher, if present (i.e. `len(attestation_indices)` is not a multiple of 8), are all zero
 * Derive a group public key by adding the public keys of all of the attesters in `attestation_indices` for whom the corresponding bit in `attester_bitfield` (the ith bit is `(attester_bitfield[i // 8] >> (7 - (i %8))) % 2`) equals 1
-* Verify that `aggregate_sig` verifies using the group pubkey generated and `hash(slot.to_bytes(8, 'big') + parent_hashes + shard + shard_block_hash + justified_slot.to_bytes(8, 'big'))` as the message.
+* Let `version = pre_fork_version if slot < fork_slot_number else post_fork_version`.
+* Verify that `aggregate_sig` verifies using the group pubkey generated and the serialized form of `AttestationSignedData(version, slot, parent_hashes, shard_id, shard_block_hash, justified_slot)` as the message.
 
 Extend the list of `AttestationRecord` objects in the `active_state` with those included in the block, ordering the new additions in the same order as they came in the block. Similarly extend the list of `SpecialRecord` objects in the `active_state` with those included in the block.