201 lines
7.9 KiB
Markdown
201 lines
7.9 KiB
Markdown
# Minimal Light Client
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**Notice**: This document is a work-in-progress for researchers and implementers.
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## Table of contents
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<!-- TOC -->
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<!-- START doctoc generated TOC please keep comment here to allow auto update -->
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<!-- DON'T EDIT THIS SECTION, INSTEAD RE-RUN doctoc TO UPDATE -->
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- [Introduction](#introduction)
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- [Constants](#constants)
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- [Configuration](#configuration)
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- [Misc](#misc)
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- [Time parameters](#time-parameters)
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- [Containers](#containers)
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- [`LightClientSnapshot`](#lightclientsnapshot)
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- [`LightClientUpdate`](#lightclientupdate)
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- [`LightClientStore`](#lightclientstore)
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- [Helper functions](#helper-functions)
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- [`get_subtree_index`](#get_subtree_index)
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- [Light client state updates](#light-client-state-updates)
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- [`validate_light_client_update`](#validate_light_client_update)
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- [`apply_light_client_update`](#apply_light_client_update)
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- [`process_light_client_update`](#process_light_client_update)
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<!-- END doctoc generated TOC please keep comment here to allow auto update -->
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<!-- /TOC -->
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## Introduction
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Eth2 is designed to be light client friendly for constrained environments to
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access Eth2 with reasonable safety and liveness.
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Such environments include resource-constrained devices (e.g. phones for trust-minimised wallets)
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and metered VMs (e.g. blockchain VMs for cross-chain bridges).
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This document suggests a minimal light client design for the beacon chain that
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uses sync committees introduced in [this beacon chain extension](./beacon-chain.md).
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## Constants
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| Name | Value |
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| - | - |
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| `FINALIZED_ROOT_INDEX` | `get_generalized_index(BeaconState, 'finalized_checkpoint', 'root')` |
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| `NEXT_SYNC_COMMITTEE_INDEX` | `get_generalized_index(BeaconState, 'next_sync_committee')` |
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## Configuration
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### Misc
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| Name | Value |
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| - | - |
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| `MIN_SYNC_COMMITTEE_PARTICIPANTS` | `1` |
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| `MAX_VALID_LIGHT_CLIENT_UPDATES` | `uint64(2**64 - 1)` |
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### Time parameters
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| Name | Value | Unit | Duration |
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| - | - | :-: | :-: |
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| `LIGHT_CLIENT_UPDATE_TIMEOUT` | `Slot(2**13)` | slots | ~27 hours |
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## Containers
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### `LightClientSnapshot`
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```python
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class LightClientSnapshot(Container):
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# Beacon block header
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header: BeaconBlockHeader
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# Sync committees corresponding to the header
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current_sync_committee: SyncCommittee
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next_sync_committee: SyncCommittee
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```
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### `LightClientUpdate`
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```python
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class LightClientUpdate(Container):
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# Update beacon block header
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header: BeaconBlockHeader
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# Next sync committee corresponding to the header
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next_sync_committee: SyncCommittee
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next_sync_committee_branch: Vector[Bytes32, floorlog2(NEXT_SYNC_COMMITTEE_INDEX)]
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# Finality proof for the update header
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finality_header: BeaconBlockHeader
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finality_branch: Vector[Bytes32, floorlog2(FINALIZED_ROOT_INDEX)]
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# Sync committee aggregate signature
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sync_committee_bits: Bitvector[SYNC_COMMITTEE_SIZE]
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sync_committee_signature: BLSSignature
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# Fork version for the aggregate signature
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fork_version: Version
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```
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### `LightClientStore`
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```python
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class LightClientStore(Container):
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snapshot: LightClientSnapshot
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valid_updates: List[LightClientUpdate, MAX_VALID_LIGHT_CLIENT_UPDATES]
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```
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## Helper functions
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### `get_subtree_index`
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```python
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def get_subtree_index(generalized_index: GeneralizedIndex) -> uint64:
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return uint64(generalized_index % 2**(floorlog2(generalized_index)))
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```
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## Light client state updates
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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.
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#### `validate_light_client_update`
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```python
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def validate_light_client_update(snapshot: LightClientSnapshot, update: LightClientUpdate,
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genesis_validators_root: Root) -> None:
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# Verify update slot is larger than snapshot slot
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assert update.header.slot > snapshot.header.slot
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# Verify update does not skip a sync committee period
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snapshot_period = compute_epoch_at_slot(snapshot.header.slot) // EPOCHS_PER_SYNC_COMMITTEE_PERIOD
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update_period = compute_epoch_at_slot(update.header.slot) // EPOCHS_PER_SYNC_COMMITTEE_PERIOD
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assert update_period in (snapshot_period, snapshot_period + 1)
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# Verify update header root is the finalized root of the finality header, if specified
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if update.finality_header == BeaconBlockHeader():
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signed_header = update.header
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assert update.finality_branch == [Bytes32() for _ in range(floorlog2(FINALIZED_ROOT_INDEX))]
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else:
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signed_header = update.finality_header
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assert is_valid_merkle_branch(
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leaf=hash_tree_root(update.header),
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branch=update.finality_branch,
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depth=floorlog2(FINALIZED_ROOT_INDEX),
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index=get_subtree_index(FINALIZED_ROOT_INDEX),
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root=update.finality_header.state_root,
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)
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# Verify update next sync committee if the update period incremented
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if update_period == snapshot_period:
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sync_committee = snapshot.current_sync_committee
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assert update.next_sync_committee_branch == [Bytes32() for _ in range(floorlog2(NEXT_SYNC_COMMITTEE_INDEX))]
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else:
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sync_committee = snapshot.next_sync_committee
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assert is_valid_merkle_branch(
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leaf=hash_tree_root(update.next_sync_committee),
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branch=update.next_sync_committee_branch,
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depth=floorlog2(NEXT_SYNC_COMMITTEE_INDEX),
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index=get_subtree_index(NEXT_SYNC_COMMITTEE_INDEX),
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root=update.header.state_root,
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)
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# Verify sync committee has sufficient participants
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assert sum(update.sync_committee_bits) >= MIN_SYNC_COMMITTEE_PARTICIPANTS
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# Verify sync committee aggregate signature
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participant_pubkeys = [pubkey for (bit, pubkey) in zip(update.sync_committee_bits, sync_committee.pubkeys) if bit]
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domain = compute_domain(DOMAIN_SYNC_COMMITTEE, update.fork_version, genesis_validators_root)
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signing_root = compute_signing_root(signed_header, domain)
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assert bls.FastAggregateVerify(participant_pubkeys, signing_root, update.sync_committee_signature)
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```
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#### `apply_light_client_update`
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```python
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def apply_light_client_update(snapshot: LightClientSnapshot, update: LightClientUpdate) -> None:
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snapshot_period = compute_epoch_at_slot(snapshot.header.slot) // EPOCHS_PER_SYNC_COMMITTEE_PERIOD
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update_period = compute_epoch_at_slot(update.header.slot) // EPOCHS_PER_SYNC_COMMITTEE_PERIOD
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if update_period == snapshot_period + 1:
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snapshot.current_sync_committee = snapshot.next_sync_committee
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snapshot.next_sync_committee = update.next_sync_committee
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snapshot.header = update.header
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```
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#### `process_light_client_update`
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```python
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def process_light_client_update(store: LightClientStore, update: LightClientUpdate, current_slot: Slot,
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genesis_validators_root: Root) -> None:
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validate_light_client_update(store.snapshot, update, genesis_validators_root)
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store.valid_updates.append(update)
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if (
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sum(update.sync_committee_bits) * 3 > len(update.sync_committee_bits) * 2
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and update.finality_header != BeaconBlockHeader()
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):
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# Apply update if (1) 2/3 quorum is reached and (2) we have a finality proof.
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# Note that (2) means that the current light client design needs finality.
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# It may be changed to re-organizable light client design. See the on-going issue eth2.0-specs#2182.
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apply_light_client_update(store.snapshot, update)
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store.valid_updates = []
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elif current_slot > store.snapshot.header.slot + LIGHT_CLIENT_UPDATE_TIMEOUT:
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# Forced best update when the update timeout has elapsed
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apply_light_client_update(store.snapshot,
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max(store.valid_updates, key=lambda update: sum(update.sync_committee_bits)))
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store.valid_updates = []
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```
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