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Added support for updates that point to finalized ancestors

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vbuterin 2020-11-18 16:27:19 +08:00 committed by GitHub
parent 830efa496a
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1 changed files with 26 additions and 5 deletions
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specs/lightclient/sync-protocol.md
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@ -35,7 +35,8 @@ This document suggests a minimal light client design for the beacon chain that u
| Name | Value |
| - | - |
| `NEXT_SYNC_COMMITTEE_INDEX` | `IndexConcat(Index(BeaconBlock, 'state_root'), Index(BeaconState, 'next_sync_committee'))` |
| `NEXT_SYNC_COMMITTEE_INDEX` | `Index(BeaconState, 'next_sync_committee')` |
| `FINALIZED_ROOT_INDEX` | `Index(BeaconState, 'finalized_checkpoint', 'root')` |
## Configuration
@ -71,13 +72,17 @@ class LightClientSnapshot(Container):
class LightClientUpdate(Container):
# Updated snapshot
snapshot: LightClientSnapshot
# Header that the new snapshot is a finalized ancestor of
signed_header: BeaconBlockHeader
# Merkle branch proving ancestry of the header in the snapshot
ancestry_branch: Vector[Bytes32, log2(FINALIZED_ROOT_INDEX)]
# Merkle branches for the next sync committee
next_sync_committee_branch: Vector[Bytes32, log2(NEXT_SYNC_COMMITTEE_INDEX)]
# Sync committee aggregate signature
sync_committee_bits: Bitlist[MAX_SYNC_COMMITTEE_SIZE]
sync_committee_signature: BLSSignature
# Fork version for the aggregate signature
fork_version
fork_version: Version
```
#### `LightClientStore`
@ -105,6 +110,18 @@ def is_valid_light_client_update(store: LightClientStore, update: LightClientUpd
old_period = compute_epoch_at_slot(old_snapshot.header.slot) // EPOCHS_PER_SYNC_COMMITTEE_PERIOD
new_period = compute_epoch_at_slot(new_snapshot.header.slot) // EPOCHS_PER_SYNC_COMMITTEE_PERIOD
assert new_period in (old_period, old_period + 1)
# Verify relationship between signed header and ancestor header
if update.signed_header == new_snapshot.header:
assert update.ancestry_branch == [ZERO_HASH for _ in range(log2(FINALIZED_ROOT_INDEX))]
else:
assert is_valid_merkle_branch(
leaf=hash_tree_root(new_snapshot.header),
branch=update.ancestry_branch,
depth=log2(FINALIZED_ROOT_INDEX),
index=FINALIZED_ROOT_INDEX % 2**log2(FINALIZED_ROOT_INDEX),
root=update.signed_header.state_root,
)
# Verify new snapshot sync committees
if new_period == old_period:
@ -117,7 +134,7 @@ def is_valid_light_client_update(store: LightClientStore, update: LightClientUpd
branch=update.next_sync_committee_branch,
depth=log2(NEXT_SYNC_COMMITTEE_INDEX),
index=NEXT_SYNC_COMMITTEE_INDEX % 2**log2(NEXT_SYNC_COMMITTEE_INDEX),
root=hash_tree_root(new_snapshot.header),
root=new_snapshot.header.state_root,
)
# Verify sync committee bitfield length
@ -128,7 +145,7 @@ def is_valid_light_client_update(store: LightClientStore, update: LightClientUpd
# 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)
signing_root = compute_signing_root(new_snapshot.header, domain)
signing_root = compute_signing_root(update.signed_header, domain)
assert bls.FastAggregateVerify(participant_pubkeys, signing_root, update.sync_committee_signature)
return True
@ -142,7 +159,11 @@ def process_light_client_update(store: LightClientStore, update: LightClientUpda
assert is_valid_light_client_update(store, update)
valid_updates.append(update)
if sum(update.sync_committee_bits) * 3 > len(update.sync_committee_bits) * 2:
# Immediate update "happy path" requires:
# (i) 2/3 participation
# (ii) an update that refers to the finalized ancestor of a signed block, and not the signed block directly
if sum(update.sync_committee_bits) * 3 > len(update.sync_committee_bits) * 2 and update.snapshot.header != update.signed_header:
# Immediate update when quorum is reached
store.snapshot = update.snapshot
valid_updates = []