7.7 KiB
7.7 KiB
Altair -- Minimal Light Client
Notice: This document is a work-in-progress for researchers and implementers.
Table of contents
Introduction
The beacon chain is designed to be light client friendly for constrained environments to access Ethereum with reasonable safety and liveness. Such environments include resource-constrained devices (e.g. phones for trust-minimised wallets) and metered VMs (e.g. blockchain VMs for cross-chain bridges).
This document suggests a minimal light client design for the beacon chain that uses sync committees introduced in this beacon chain extension.
Constants
| Name | Value |
|---|---|
FINALIZED_ROOT_INDEX |
get_generalized_index(BeaconState, 'finalized_checkpoint', 'root') |
NEXT_SYNC_COMMITTEE_INDEX |
get_generalized_index(BeaconState, 'next_sync_committee') |
Preset
Misc
| Name | Value |
|---|---|
MIN_SYNC_COMMITTEE_PARTICIPANTS |
1 |
Containers
LightClientSnapshot
class LightClientSnapshot(Container):
# Beacon block header
header: BeaconBlockHeader
# Sync committees corresponding to the header
current_sync_committee: SyncCommittee
next_sync_committee: SyncCommittee
LightClientUpdate
class LightClientUpdate(Container):
# Update beacon block header
header: BeaconBlockHeader
# Next sync committee corresponding to the header
next_sync_committee: SyncCommittee
next_sync_committee_branch: Vector[Bytes32, floorlog2(NEXT_SYNC_COMMITTEE_INDEX)]
# Finality proof for the update header
finality_header: BeaconBlockHeader
finality_branch: Vector[Bytes32, floorlog2(FINALIZED_ROOT_INDEX)]
# Sync committee aggregate signature
sync_committee_bits: Bitvector[SYNC_COMMITTEE_SIZE]
sync_committee_signature: BLSSignature
# Fork version for the aggregate signature
fork_version: Version
LightClientStore
@dataclass
class LightClientStore(object):
snapshot: LightClientSnapshot
valid_updates: Set[LightClientUpdate]
Helper functions
get_subtree_index
def get_subtree_index(generalized_index: GeneralizedIndex) -> uint64:
return uint64(generalized_index % 2**(floorlog2(generalized_index)))
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) where current_slot is the current slot based on some local clock.
validate_light_client_update
def validate_light_client_update(snapshot: LightClientSnapshot,
update: LightClientUpdate,
genesis_validators_root: Root) -> None:
# Verify update slot is larger than snapshot slot
assert update.header.slot > snapshot.header.slot
# Verify update does not skip a sync committee period
snapshot_period = compute_epoch_at_slot(snapshot.header.slot) // EPOCHS_PER_SYNC_COMMITTEE_PERIOD
update_period = compute_epoch_at_slot(update.header.slot) // EPOCHS_PER_SYNC_COMMITTEE_PERIOD
assert update_period in (snapshot_period, snapshot_period + 1)
# Verify update header root is the finalized root of the finality header, if specified
if update.finality_header == BeaconBlockHeader():
signed_header = update.header
assert update.finality_branch == [Bytes32() for _ in range(floorlog2(FINALIZED_ROOT_INDEX))]
else:
signed_header = update.finality_header
assert is_valid_merkle_branch(
leaf=hash_tree_root(update.header),
branch=update.finality_branch,
depth=floorlog2(FINALIZED_ROOT_INDEX),
index=get_subtree_index(FINALIZED_ROOT_INDEX),
root=update.finality_header.state_root,
)
# Verify update next sync committee if the update period incremented
if update_period == snapshot_period:
sync_committee = snapshot.current_sync_committee
assert update.next_sync_committee_branch == [Bytes32() for _ in range(floorlog2(NEXT_SYNC_COMMITTEE_INDEX))]
else:
sync_committee = snapshot.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=update.header.state_root,
)
# Verify sync committee has sufficient participants
assert sum(update.sync_committee_bits) >= MIN_SYNC_COMMITTEE_PARTICIPANTS
# Verify sync committee aggregate signature
participant_pubkeys = [pubkey for (bit, pubkey) in zip(update.sync_committee_bits, sync_committee.pubkeys) if bit]
domain = compute_domain(DOMAIN_SYNC_COMMITTEE, update.fork_version, genesis_validators_root)
signing_root = compute_signing_root(signed_header, domain)
assert bls.FastAggregateVerify(participant_pubkeys, signing_root, update.sync_committee_signature)
apply_light_client_update
def apply_light_client_update(snapshot: LightClientSnapshot, update: LightClientUpdate) -> None:
snapshot_period = compute_epoch_at_slot(snapshot.header.slot) // EPOCHS_PER_SYNC_COMMITTEE_PERIOD
update_period = compute_epoch_at_slot(update.header.slot) // EPOCHS_PER_SYNC_COMMITTEE_PERIOD
if update_period == snapshot_period + 1:
snapshot.current_sync_committee = snapshot.next_sync_committee
snapshot.next_sync_committee = update.next_sync_committee
snapshot.header = update.header
process_light_client_update
def process_light_client_update(store: LightClientStore, update: LightClientUpdate, current_slot: Slot,
genesis_validators_root: Root) -> None:
validate_light_client_update(store.snapshot, update, genesis_validators_root)
store.valid_updates.add(update)
update_timeout = SLOTS_PER_EPOCH * EPOCHS_PER_SYNC_COMMITTEE_PERIOD
if (
sum(update.sync_committee_bits) * 3 >= len(update.sync_committee_bits) * 2
and update.finality_header != BeaconBlockHeader()
):
# Apply update if (1) 2/3 quorum is reached and (2) we have a finality proof.
# Note that (2) means that the current light client design needs finality.
# It may be changed to re-organizable light client design. See the on-going issue consensus-specs#2182.
apply_light_client_update(store.snapshot, update)
store.valid_updates = set()
elif current_slot > store.snapshot.header.slot + update_timeout:
# Forced best update when the update timeout has elapsed
apply_light_client_update(store.snapshot,
max(store.valid_updates, key=lambda update: sum(update.sync_committee_bits)))
store.valid_updates = set()