173 lines
7.3 KiB
Markdown
173 lines
7.3 KiB
Markdown
# 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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- [Minimal Light Client Design](#minimal-light-client-design)
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- [Table of contents](#table-of-contents)
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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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- [`get_persistent_committee_pubkeys_and_balances`](#get_persistent_committee_pubkeys_and_balances)
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- [Light client state updates](#light-client-state-updates)
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- [Data overhead](#data-overhead)
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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.
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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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| `CompactValidator` | `uint64` | compact representation of a validator for light clients |
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## Constants
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| Name | Value |
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| - | - |
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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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# Shard block root (and authenticating signature data)
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shard_block_root: Hash
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fork_version: Version
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aggregation_bits: Bitlist[MAX_VALIDATORS_PER_COMMITTEE]
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signature: BLSSignature
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# Updated beacon header (and authenticating branch)
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header: BeaconBlockHeader
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header_branch: Vector[Hash, BEACON_CHAIN_ROOT_IN_SHARD_BLOCK_HEADER_DEPTH]
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# Updated period committee (and authenticating branch)
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committee: CompactCommittee
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committee_branch: Vector[Hash, PERIOD_COMMITTEE_ROOT_IN_BEACON_STATE_DEPTH + log_2(SHARD_COUNT)]
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```
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## Helpers
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### `LightClientMemory`
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```python
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@dataclass
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class LightClientMemory(object):
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shard: Shard # Randomly initialized and retained forever
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header: BeaconBlockHeader # Beacon header which is not expected to revert
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# period committees corresponding to the beacon header
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previous_committee: CompactCommittee
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current_committee: CompactCommittee
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next_committee: CompactCommittee
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```
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### `get_persistent_committee_pubkeys_and_balances`
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```python
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def get_persistent_committee_pubkeys_and_balances(memory: LightClientMemory,
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epoch: Epoch) -> Tuple[Sequence[BLSPubkey], Sequence[uint64]]:
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"""
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Return pubkeys and balances for the persistent committee at ``epoch``.
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"""
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current_period = compute_epoch_at_slot(memory.header.slot) // EPOCHS_PER_SHARD_PERIOD
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next_period = epoch // EPOCHS_PER_SHARD_PERIOD
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assert next_period in (current_period, current_period + 1)
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if next_period == current_period:
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earlier_committee, later_committee = memory.previous_committee, memory.current_committee
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else:
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earlier_committee, later_committee = memory.current_committee, memory.next_committee
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pubkeys = []
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balances = []
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for pubkey, compact_validator in zip(earlier_committee.pubkeys, earlier_committee.compact_validators):
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index, slashed, balance = unpack_compact_validator(compact_validator)
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if epoch % EPOCHS_PER_SHARD_PERIOD < index % EPOCHS_PER_SHARD_PERIOD:
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pubkeys.append(pubkey)
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balances.append(balance)
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for pubkey, compact_validator in zip(later_committee.pubkeys, later_committee.compact_validators):
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index, slashed, balance = unpack_compact_validator(compact_validator)
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if epoch % EPOCHS_PER_SHARD_PERIOD >= index % EPOCHS_PER_SHARD_PERIOD:
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pubkeys.append(pubkey)
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balances.append(balance)
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return pubkeys, balances
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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)` and calls `update_memory(memory, update)`.
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```python
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def update_memory(memory: LightClientMemory, update: LightClientUpdate) -> None:
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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_SHARD_PERIOD
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next_epoch = compute_epoch_of_shard_slot(update.header.slot)
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next_period = next_epoch // EPOCHS_PER_SHARD_PERIOD
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assert next_period in (current_period, current_period + 1)
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# Verify update header against shard block root and header branch
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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.header_branch,
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depth=BEACON_CHAIN_ROOT_IN_SHARD_BLOCK_HEADER_DEPTH,
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index=BEACON_CHAIN_ROOT_IN_SHARD_BLOCK_HEADER_INDEX,
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root=update.shard_block_root,
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)
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# Verify persistent committee votes pass 2/3 threshold
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pubkeys, balances = get_persistent_committee_pubkeys_and_balances(memory, next_epoch)
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assert 3 * sum(filter(lambda i: update.aggregation_bits[i], balances)) > 2 * sum(balances)
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# Verify shard attestations
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pubkey = bls_aggregate_pubkeys(filter(lambda i: update.aggregation_bits[i], pubkeys))
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domain = compute_domain(DOMAIN_SHARD_ATTESTER, update.fork_version)
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assert bls_verify(pubkey, update.shard_block_root, update.signature, domain)
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# Update period committees if entering a new period
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if next_period == current_period + 1:
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assert is_valid_merkle_branch(
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leaf=hash_tree_root(update.committee),
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branch=update.committee_branch,
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depth=PERIOD_COMMITTEE_ROOT_IN_BEACON_STATE_DEPTH + log_2(SHARD_COUNT),
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index=PERIOD_COMMITTEE_ROOT_IN_BEACON_STATE_INDEX << log_2(SHARD_COUNT) + memory.shard,
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root=hash_tree_root(update.header),
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)
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memory.previous_committee = memory.current_committee
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memory.current_committee = memory.next_committee
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memory.next_committee = update.committee
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# Update header
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memory.header = update.header
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```
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## Data overhead
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Once every `EPOCHS_PER_SHARD_PERIOD` epochs (~27 hours) a light client downloads a `LightClientUpdate` object:
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* `shard_block_root`: 32 bytes
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* `fork_version`: 4 bytes
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* `aggregation_bits`: 16 bytes
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* `signature`: 96 bytes
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* `header`: 8 + 32 + 32 + 32 + 96 = 200 bytes
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* `header_branch`: 4 * 32 = 128 bytes
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* `committee`: 128 * (48 + 8) = 7,168 bytes
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* `committee_branch`: (5 + 10) * 32 = 480 bytes
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The total overhead is 8,124 bytes, or ~0.083 bytes per second. The Bitcoin SPV equivalent is 80 bytes per ~560 seconds, or ~0.143 bytes per second. Various compression optimisations (similar to [these](https://github.com/RCasatta/compressedheaders)) are possible.
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A light client can choose to update the header (without updating the committee) more frequently than once every `EPOCHS_PER_SHARD_PERIOD` epochs at a cost of 32 + 4 + 16 + 96 + 200 + 128 = 476 bytes per update.
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