Remove fork_version from LightClientUpdate

The `fork_version` field in `LightClientUpdate` can be derived from the
`update.signature_slot` value by consulting the locally configured fork
schedule. The light client already needs access to the fork schedule to
determine the `GeneralizedIndex` values used for merkle proofs, and the
memory layouts of the structures (including `LightClientUpdate`). The
`fork_version` itself is network dependent and doesn't reveal that info.

specs/altair/beacon-chain.md

@@ -28,6 +28,7 @@
 - [Helper functions](#helper-functions)
   - [Crypto](#crypto)
   - [Misc](#misc-1)
+    - [`compute_fork_version`](#compute_fork_version)
     - [`add_flag`](#add_flag)
     - [`has_flag`](#has_flag)
   - [Beacon state accessors](#beacon-state-accessors)
@@ -226,6 +227,18 @@ the functionality described in those documents.
 
 ### Misc
 
+#### `compute_fork_version`
+
+```python
+def compute_fork_version(epoch: Epoch) -> Version:
+    """
+    Return the fork version at the given ``epoch``.
+    """
+    if epoch >= ALTAIR_FORK_EPOCH:
+        return ALTAIR_FORK_VERSION
+    return GENESIS_FORK_VERSION
+```
+
 #### `add_flag`
 
 ```python

specs/altair/sync-protocol.md

@@ -71,8 +71,6 @@ class LightClientUpdate(Container):
     finality_branch: Vector[Bytes32, floorlog2(FINALIZED_ROOT_INDEX)]
     # Sync committee aggregate signature
     sync_aggregate: SyncAggregate
-    # Fork version for the aggregate signature
-    fork_version: Version
     # Slot at which the aggregate signature was created (untrusted)
     signature_slot: Slot
 ```
@@ -211,7 +209,8 @@ def validate_light_client_update(store: LightClientStore,
         pubkey for (bit, pubkey) in zip(sync_aggregate.sync_committee_bits, sync_committee.pubkeys)
         if bit
     ]
-    domain = compute_domain(DOMAIN_SYNC_COMMITTEE, update.fork_version, genesis_validators_root)
+    fork_version = compute_fork_version(compute_epoch_at_slot(update.signature_slot))
+    domain = compute_domain(DOMAIN_SYNC_COMMITTEE, fork_version, genesis_validators_root)
     signing_root = compute_signing_root(update.attested_header, domain)
     assert bls.FastAggregateVerify(participant_pubkeys, signing_root, sync_aggregate.sync_committee_signature)
 ```

specs/bellatrix/beacon-chain.md

@@ -29,6 +29,7 @@
     - [`is_execution_enabled`](#is_execution_enabled)
   - [Misc](#misc)
     - [`compute_timestamp_at_slot`](#compute_timestamp_at_slot)
+    - [Modified `compute_fork_version`](#modified-compute_fork_version)
   - [Beacon state accessors](#beacon-state-accessors)
     - [Modified `get_inactivity_penalty_deltas`](#modified-get_inactivity_penalty_deltas)
   - [Beacon state mutators](#beacon-state-mutators)
@@ -242,6 +243,20 @@ def compute_timestamp_at_slot(state: BeaconState, slot: Slot) -> uint64:
     return uint64(state.genesis_time + slots_since_genesis * SECONDS_PER_SLOT)
 ```
 
+#### Modified `compute_fork_version`
+
+```python
+def compute_fork_version(epoch: Epoch) -> Version:
+    """
+    Return the fork version at the given ``epoch``.
+    """
+    if epoch >= BELLATRIX_FORK_EPOCH:
+        return BELLATRIX_FORK_VERSION
+    if epoch >= ALTAIR_FORK_EPOCH:
+        return ALTAIR_FORK_VERSION
+    return GENESIS_FORK_VERSION
+```
+
 ### Beacon state accessors
 
 #### Modified `get_inactivity_penalty_deltas`

specs/capella/beacon-chain.md

@@ -34,6 +34,8 @@
     - [`has_eth1_withdrawal_credential`](#has_eth1_withdrawal_credential)
     - [`is_fully_withdrawable_validator`](#is_fully_withdrawable_validator)
     - [`is_partially_withdrawable_validator`](#is_partially_withdrawable_validator)
+  - [Misc](#misc-1)
+    - [Modified `compute_fork_version`](#modified-compute_fork_version)
 - [Beacon chain state transition function](#beacon-chain-state-transition-function)
   - [Epoch processing](#epoch-processing)
     - [Full withdrawals](#full-withdrawals)
@@ -318,6 +320,24 @@ def is_partially_withdrawable_validator(validator: Validator, balance: Gwei) ->
     return has_eth1_withdrawal_credential(validator) and has_max_effective_balance and has_excess_balance
 ```
 
+### Misc
+
+#### Modified `compute_fork_version`
+
+```python
+def compute_fork_version(epoch: Epoch) -> Version:
+    """
+    Return the fork version at the given ``epoch``.
+    """
+    if epoch >= CAPELLA_FORK_EPOCH:
+        return CAPELLA_FORK_VERSION
+    if epoch >= BELLATRIX_FORK_EPOCH:
+        return BELLATRIX_FORK_VERSION
+    if epoch >= ALTAIR_FORK_EPOCH:
+        return ALTAIR_FORK_VERSION
+    return GENESIS_FORK_VERSION
+```
+
 ## Beacon chain state transition function
 
 ### Epoch processing