Added light client syncing protocol
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# Minimal Light Client Design
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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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- [Custom types](#custom-types)
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- [Constants](#constants)
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- [Containers](#containers)
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- [`LightClientUpdate`](#lightclientupdate)
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- [Helpers](#helpers)
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- [`LightClientMemory`](#lightclientmemory)
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- [Light client state updates](#light-client-state-updates)
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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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Ethereum 2.0 is designed to be light client friendly. This allows low-resource clients such as mobile phones to access Ethereum 2.0 with reasonable safety and liveness. It also facilitates the development of "bridges" to external blockchains. This document suggests a minimal light client design for the beacon chain that uses the concept of "sync committees" introduced in [./beacon-chain.md](the the light-client-friendliness beacon chain extension).
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## Custom types
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We define the following Python custom types for type hinting and readability:
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| Name | SSZ equivalent | Description |
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| - | - | - |
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## Constants
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| Name | Value |
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| - | - |
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| `SYNC_COMMITTEES_GENERALIZED_INDEX` | `GeneralizedIndexConcat(GeneralizedIndex(BeaconBlock, 'state_root'), GeneralizedIndex(BeaconState, 'current_sync_committee'))` |
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| `FORK_GENERALIZED_INDEX` | `GeneralizedIndexConcat(GeneralizedIndex(BeaconBlock, 'state_root'), GeneralizedIndex(BeaconState, 'fork'))` |
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| `BEACON_CHAIN_ROOT_IN_SHARD_BLOCK_HEADER_DEPTH` | `4` |
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| `BEACON_CHAIN_ROOT_IN_SHARD_BLOCK_HEADER_INDEX` | **TBD** |
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| `PERIOD_COMMITTEE_ROOT_IN_BEACON_STATE_DEPTH` | `5` |
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| `PERIOD_COMMITTEE_ROOT_IN_BEACON_STATE_INDEX` | **TBD** |
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## Containers
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### `LightClientUpdate`
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```python
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class LightClientUpdate(Container):
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# Updated beacon header (and authenticating branch)
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header: BeaconBlockHeader
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# Sync committee signature to that header
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aggregation_bits: Bitlist[MAX_SYNC_COMMITTEE_SIZE]
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signature: BLSSignature
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header_branch: Vector[Bytes32, BEACON_CHAIN_ROOT_IN_SHARD_BLOCK_HEADER_DEPTH]
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# Updates fork version
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new_fork: Fork
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fork_branch: Vector[Bytes32, log_2(FORK_GENERALIZED_INDEX)]
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# Updated period committee (and authenticating branch)
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new_current_sync_committee: SyncCommittee
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new_next_sync_committee: SyncCommittee
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sync_committee_branch: Vector[Bytes32, log_2(SYNC_COMMITTEES_GENERALIZED_INDEX)]
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```
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## Helpers
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### `LightClientMemory`
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```python
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class LightClientMemory(Container):
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# Beacon header which is not expected to revert
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header: BeaconBlockHeader
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# Fork version data
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fork_version: Version
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# period committees corresponding to the beacon 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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## Light client state updates
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The state of a light client is stored in a `memory` object of type `LightClientMemory`. To advance its state a light client requests an `update` object of type `LightClientUpdate` from the network by sending a request containing `(memory.shard, memory.header.slot, slot_range_end)`. It calls `validate_update(memory, update)` on each update that it receives in response. If `sum(update.aggregate_bits) * 3 > len(update.aggregate_bits) * 2` for any valid update, it accepts that update immediately; otherwise, it waits around for some time and then finally calls `update_memory(memory, update)` on the valid update with the highest `sum(update.aggregate_bits)`.
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#### `validate_update`
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```python
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def validate_update(memory: LightClientMemory, update: LightClientUpdate) -> bool:
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# Verify the update does not skip a period
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current_period = compute_epoch_at_slot(memory.header.slot) // EPOCHS_PER_SYNC_COMMITTEE_PERIOD
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new_epoch = compute_epoch_of_shard_slot(update.header.slot)
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new_period = new_epoch // EPOCHS_PER_SYNC_COMMITTEE_PERIOD
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assert new_period in (current_period, current_period + 1)
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# Verify that it actually updates to a newer slot
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assert update.header.slot > memory.header.slot
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# Convenience as independent variable for convenience
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committee = memory.current_sync_committee if new_period == current_period else memory.next_sync_committee
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assert len(update.aggregation_bits) == len(committee)
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# Verify signature
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active_pubkeys = [p for (bit, p) in zip(update.aggregation_bits, committee.pubkeys) if bit]
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domain = compute_domain(DOMAIN_SYNC_COMMITTEE, memory.version)
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signing_root = compute_signing_root(update.header, domain)
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assert bls.FastAggregateVerify(pubkeys, signing_root, update.signature)
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# Verify Merkle branches of new info
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assert is_valid_merkle_branch(
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leaf=hash_tree_root(update.new_fork),
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branch=update.fork_branch,
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depth=log2(FORK_GENERALIZED_INDEX),
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index=FORK_GENERALIZED_INDEX % 2**log2(FORK_GENERALIZED_INDEX),
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root=hash_tree_root(update.header),
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)
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assert is_valid_merkle_branch(
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leaf=hash_tree_root(update.current_sync_committee),
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branch=update.sync_committee_branch,
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depth=log2(SYNC_COMMITTEES_GENERALIZED_INDEX),
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index=SYNC_COMMITTEES_GENERALIZED_INDEX % 2**log2(SYNC_COMMITTEES_GENERALIZED_INDEX),
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root=hash_tree_root(update.header),
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)
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# Verify consistency of committees
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if new_period == current_period:
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assert update.new_current_sync_committee == memory.current_sync_committee
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assert update.new_next_sync_committee == memory.next_sync_committee
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else:
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assert update.new_current_sync_committee == memory.next_sync_committee
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return True
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```
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#### `update_memory`
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```
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def update_memory(memory: LightClientMemory, update: LightClientUpdate) -> None:
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memory.header = update.header
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epoch = compute_epoch_at_slot(update.header.slot)
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memory.fork_version = update.new_fork.previous_version if epoch < update.new_fork.epoch else update.new_fork.current_version
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memory.current_sync_committee = update.new_current_sync_committee
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memory.next_sync_committee == update.new_next_sync_committee
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```
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