Check correct fork version in LC sync protocol

- Sync committee is determined by signature_slot
- Signature fork version is determined by max(signature_slot, 1) - 1
- Attested block fork version can be anything < signature_slot

Old logic incorrectly derived signature fork version from signature_slot
and did not subtract a slot. Extended tests to check this edge case.

specs/altair/light-client/sync-protocol.md

@@ -387,7 +387,8 @@ def validate_light_client_update(store: LightClientStore,
         pubkey for (bit, pubkey) in zip(sync_aggregate.sync_committee_bits, sync_committee.pubkeys)
         if bit
     ]
-    fork_version = compute_fork_version(compute_epoch_at_slot(update.signature_slot))
+    fork_version_slot = max(update.signature_slot, 1) - 1
+    fork_version = compute_fork_version(compute_epoch_at_slot(fork_version_slot))
     domain = compute_domain(DOMAIN_SYNC_COMMITTEE, fork_version, genesis_validators_root)
     signing_root = compute_signing_root(update.attested_header.beacon, domain)
     assert bls.FastAggregateVerify(participant_pubkeys, signing_root, sync_aggregate.sync_committee_signature)

tests/core/pyspec/eth2spec/test/altair/light_client/test_sync.py

@@ -668,10 +668,9 @@ def run_test_single_fork(spec, phases, state, fork):
     # Upgrade to post-fork spec, attested block is still before the fork
     attested_block = block.copy()
     attested_state = state.copy()
-    state, _ = do_fork(state, spec, phases[fork], fork_epoch, with_block=False)
+    sync_aggregate, _ = get_sync_aggregate(phases[fork], state)
+    state, block = do_fork(state, spec, phases[fork], fork_epoch, sync_aggregate=sync_aggregate)
     spec = phases[fork]
-    sync_aggregate, _ = get_sync_aggregate(spec, state)
-    block = state_transition_with_full_block(spec, state, True, True, sync_aggregate=sync_aggregate)
     yield from emit_update(test, spec, state, block, attested_state, attested_block, finalized_block, phases=phases)
     assert test.store.finalized_header.beacon.slot == finalized_state.slot
     assert test.store.next_sync_committee == finalized_state.next_sync_committee
@@ -755,18 +754,16 @@ def run_test_multi_fork(spec, phases, state, fork_1, fork_2):
     # ..., attested is from `fork_1`, ...
     fork_1_epoch = getattr(phases[fork_1].config, fork_1.upper() + '_FORK_EPOCH')
     transition_to(spec, state, spec.compute_start_slot_at_epoch(fork_1_epoch) - 1)
-    state, _ = do_fork(state, spec, phases[fork_1], fork_1_epoch, with_block=False)
+    state, attested_block = do_fork(state, spec, phases[fork_1], fork_1_epoch)
     spec = phases[fork_1]
-    attested_block = state_transition_with_full_block(spec, state, True, True)
     attested_state = state.copy()
 
     # ..., and signature is from `fork_2`
     fork_2_epoch = getattr(phases[fork_2].config, fork_2.upper() + '_FORK_EPOCH')
     transition_to(spec, state, spec.compute_start_slot_at_epoch(fork_2_epoch) - 1)
-    state, _ = do_fork(state, spec, phases[fork_2], fork_2_epoch, with_block=False)
+    sync_aggregate, _ = get_sync_aggregate(phases[fork_2], state)
+    state, block = do_fork(state, spec, phases[fork_2], fork_2_epoch, sync_aggregate=sync_aggregate)
     spec = phases[fork_2]
-    sync_aggregate, _ = get_sync_aggregate(spec, state)
-    block = state_transition_with_full_block(spec, state, True, True, sync_aggregate=sync_aggregate)
 
     # Check that update applies
     yield from emit_update(test, spec, state, block, attested_state, attested_block, finalized_block, phases=phases)

tests/core/pyspec/eth2spec/test/helpers/fork_transition.py

@@ -47,6 +47,7 @@ def _set_operations_by_dict(block, operation_dict):
 
 def _state_transition_and_sign_block_at_slot(spec,
                                              state,
+                                             sync_aggregate=None,
                                              operation_dict=None):
     """
     Cribbed from ``transition_unsigned_block`` helper
@@ -61,6 +62,8 @@ def _state_transition_and_sign_block_at_slot(spec,
     Thus use dict to pass operations.
     """
     block = build_empty_block(spec, state)
+    if sync_aggregate is not None:
+        block.body.sync_aggregate = sync_aggregate
 
     if operation_dict:
         _set_operations_by_dict(block, operation_dict)
@@ -141,7 +144,7 @@ def state_transition_across_slots_with_ignoring_proposers(spec,
             next_slot(spec, state)
 
 
-def do_fork(state, spec, post_spec, fork_epoch, with_block=True, operation_dict=None):
+def do_fork(state, spec, post_spec, fork_epoch, with_block=True, sync_aggregate=None, operation_dict=None):
     spec.process_slots(state, state.slot + 1)
 
     assert state.slot % spec.SLOTS_PER_EPOCH == 0
@@ -172,7 +175,12 @@ def do_fork(state, spec, post_spec, fork_epoch, with_block=True, operation_dict=
         assert state.fork.current_version == post_spec.config.DENEB_FORK_VERSION
 
     if with_block:
-        return state, _state_transition_and_sign_block_at_slot(post_spec, state, operation_dict=operation_dict)
+        return state, _state_transition_and_sign_block_at_slot(
+            post_spec,
+            state,
+            sync_aggregate=sync_aggregate,
+            operation_dict=operation_dict,
+        )
     else:
         return state, None
 

tests/core/pyspec/eth2spec/test/helpers/light_client.py

@@ -31,7 +31,7 @@ def get_sync_aggregate(spec, state, num_participants=None, signature_slot=None):
     sync_committee_signature = compute_aggregate_sync_committee_signature(
         spec,
         signature_state,
-        signature_slot,
+        max(signature_slot, 1) - 1,
         committee_indices[:num_participants],
     )
     sync_aggregate = spec.SyncAggregate(