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Fix sync protocol next_sync_committee_branch verification and remove active header #2829

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Fix sync protocol next_sync_committee_branch verification and remove active header #2829

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1744814
`next_sync_committee_branch` validation bugfix and remove "active hea…
hwwhww Feb 10, 2022
03a0f28
`next_sync_committee_branch` does not have to be empty
hwwhww Feb 10, 2022
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71 changes: 31 additions & 40 deletions specs/altair/sync-protocol.md
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Original file line number Diff line number Diff line change
Expand Up @@ -17,12 +17,12 @@
- [`LightClientStore`](#lightclientstore)
- [Helper functions](#helper-functions)
- [`get_subtree_index`](#get_subtree_index)
- [`get_active_header`](#get_active_header)
- [`get_safety_threshold`](#get_safety_threshold)
- [`update_sync_committees_from_update`](#update_sync_committees_from_update)
- [`update_new_finalized_header`](#update_new_finalized_header)
- [Light client state updates](#light-client-state-updates)
- [`process_slot_for_light_client_store`](#process_slot_for_light_client_store)
- [`validate_light_client_update`](#validate_light_client_update)
- [`apply_light_client_update`](#apply_light_client_update)
- [`process_light_client_update`](#process_light_client_update)

<!-- END doctoc generated TOC please keep comment here to allow auto update -->
Expand Down Expand Up @@ -62,7 +62,7 @@ uses sync committees introduced in [this beacon chain extension](./beacon-chain.
class LightClientUpdate(Container):
# The beacon block header that is attested to by the sync committee
attested_header: BeaconBlockHeader
# Next sync committee corresponding to the active header
# Next sync committee corresponding to `attested_header`
next_sync_committee: SyncCommittee
next_sync_committee_branch: Vector[Bytes32, floorlog2(NEXT_SYNC_COMMITTEE_INDEX)]
# The finalized beacon block header attested to by Merkle branch
Expand Down Expand Up @@ -102,19 +102,6 @@ def get_subtree_index(generalized_index: GeneralizedIndex) -> uint64:
return uint64(generalized_index % 2**(floorlog2(generalized_index)))
```

### `get_active_header`

```python
def get_active_header(update: LightClientUpdate) -> BeaconBlockHeader:
# The "active header" is the header that the update is trying to convince us
# to accept. If a finalized header is present, it's the finalized header,
# otherwise it's the attested header
if update.finalized_header != BeaconBlockHeader():
return update.finalized_header
else:
return update.attested_header
```

### `get_safety_threshold`

```python
Expand All @@ -125,6 +112,27 @@ def get_safety_threshold(store: LightClientStore) -> uint64:
) // 2
```

### `update_sync_committees_from_update`

```python
def update_sync_committees_from_update(store: LightClientStore, update: LightClientUpdate) -> None:
finalized_period = compute_sync_committee_period(compute_epoch_at_slot(store.finalized_header.slot))
update_period = compute_sync_committee_period(compute_epoch_at_slot(update.attested_header.slot))
if update_period == finalized_period + 1:
store.current_sync_committee = store.next_sync_committee
store.next_sync_committee = update.next_sync_committee
```

### `update_new_finalized_header`

```python
def update_new_finalized_header(store: LightClientStore, header: BeaconBlockHeader) -> None:
store.finalized_header = header
store.best_valid_update = None
if store.finalized_header.slot > store.optimistic_header.slot:
store.optimistic_header = store.finalized_header
```

## Light client state updates

A light client maintains its state in a `store` object of type `LightClientStore` and receives `update` objects of type `LightClientUpdate`. Every `update` triggers `process_light_client_update(store, update, current_slot, genesis_validators_root)` where `current_slot` is the current slot based on a local clock. `process_slot_for_light_client_store` is triggered every time the current slot increments.
Expand All @@ -141,8 +149,8 @@ def process_slot_for_light_client_store(store: LightClientStore, current_slot: S
and store.best_valid_update is not None
):
# Forced best update when the update timeout has elapsed
apply_light_client_update(store, store.best_valid_update)
store.best_valid_update = None
update_sync_committees_from_update(store, store.best_valid_update)
update_new_finalized_header(store, store.best_valid_update.attested_header)
```

#### `validate_light_client_update`
Expand All @@ -153,12 +161,11 @@ def validate_light_client_update(store: LightClientStore,
current_slot: Slot,
genesis_validators_root: Root) -> None:
# Verify update slot is larger than slot of current best finalized header
active_header = get_active_header(update)
assert current_slot >= active_header.slot > store.finalized_header.slot
assert current_slot >= update.attested_header.slot >= store.finalized_header.slot

# Verify update does not skip a sync committee period
finalized_period = compute_sync_committee_period(compute_epoch_at_slot(store.finalized_header.slot))
update_period = compute_sync_committee_period(compute_epoch_at_slot(active_header.slot))
update_period = compute_sync_committee_period(compute_epoch_at_slot(update.attested_header.slot))
assert update_period in (finalized_period, finalized_period + 1)

# Verify that the `finalized_header`, if present, actually is the finalized header saved in the
Expand All @@ -177,15 +184,14 @@ def validate_light_client_update(store: LightClientStore,
# Verify update next sync committee if the update period incremented
if update_period == finalized_period:
sync_committee = store.current_sync_committee
assert update.next_sync_committee_branch == [Bytes32() for _ in range(floorlog2(NEXT_SYNC_COMMITTEE_INDEX))]
else:
sync_committee = store.next_sync_committee
assert is_valid_merkle_branch(
leaf=hash_tree_root(update.next_sync_committee),
branch=update.next_sync_committee_branch,
depth=floorlog2(NEXT_SYNC_COMMITTEE_INDEX),
index=get_subtree_index(NEXT_SYNC_COMMITTEE_INDEX),
root=active_header.state_root,
root=update.attested_header.state_root,
)

sync_aggregate = update.sync_aggregate
Expand All @@ -203,21 +209,6 @@ def validate_light_client_update(store: LightClientStore,
assert bls.FastAggregateVerify(participant_pubkeys, signing_root, sync_aggregate.sync_committee_signature)
```

#### `apply_light_client_update`

```python
def apply_light_client_update(store: LightClientStore, update: LightClientUpdate) -> None:
active_header = get_active_header(update)
finalized_period = compute_sync_committee_period(compute_epoch_at_slot(store.finalized_header.slot))
update_period = compute_sync_committee_period(compute_epoch_at_slot(active_header.slot))
if update_period == finalized_period + 1:
store.current_sync_committee = store.next_sync_committee
store.next_sync_committee = update.next_sync_committee
store.finalized_header = active_header
if store.finalized_header.slot > store.optimistic_header.slot:
store.optimistic_header = store.finalized_header
```

#### `process_light_client_update`

```python
Expand Down Expand Up @@ -255,6 +246,6 @@ def process_light_client_update(store: LightClientStore,
and update.finalized_header != BeaconBlockHeader()
):
# Normal update through 2/3 threshold
apply_light_client_update(store, update)
store.best_valid_update = None
update_sync_committees_from_update(store, update)
update_new_finalized_header(store, update.finalized_header)
```