eth2.0-specs/specs/lightclient/beacon-chain.md

Ethereum 2.0 Light Client Support

Table of contents

• Introduction
• Constants
• Configuration
  • Constants
  • Misc
  • Time parameters
  • Domain types
• Containers
  • Extended containers
    • BeaconBlockBody
    • BeaconState
  • New containers
    • SyncCommittee
• Helper functions
  • Predicates
    • eth2_fast_aggregate_verify
  • Beacon state accessors
    • get_sync_committee_indices
    • get_sync_committee
  • Block processing
    • Sync committee processing
  • Epoch processing
    • Components of attestation deltas
    • Final updates

Introduction

This is a standalone beacon chain patch adding light client support via sync committees.

Constants

Name	Value
BASE_REWARDS_PER_EPOCH	uint64(5)

Configuration

Constants

Name	Value
G2_POINT_AT_INFINITY	BLSSignature(b'\xc0' + b'\x00' * 95)

Misc

Name	Value
SYNC_COMMITTEE_SIZE	uint64(2**10) (= 1024)
SYNC_COMMITTEE_PUBKEY_AGGREGATES_SIZE	uint64(2**6) (= 64)

Time parameters

Name	Value	Unit	Duration
EPOCHS_PER_SYNC_COMMITTEE_PERIOD	Epoch(2**8) (= 256)	epochs	~27 hours

Domain types

Name	Value
DOMAIN_SYNC_COMMITTEE	DomainType('0x07000000')

Containers

Extended containers

Note: Extended SSZ containers inherit all fields from the parent in the original order and append any additional fields to the end.

BeaconBlockBody

class BeaconBlockBody(phase0.BeaconBlockBody):
    # Sync committee aggregate signature
    sync_committee_bits: Bitvector[SYNC_COMMITTEE_SIZE]
    sync_committee_signature: BLSSignature

BeaconState

class BeaconState(phase0.BeaconState):
    # Sync committees
    current_sync_committee: SyncCommittee
    next_sync_committee: SyncCommittee

New containers

SyncCommittee

class SyncCommittee(Container):
    pubkeys: Vector[BLSPubkey, SYNC_COMMITTEE_SIZE]
    pubkey_aggregates: Vector[BLSPubkey, SYNC_COMMITTEE_SIZE // SYNC_COMMITTEE_PUBKEY_AGGREGATES_SIZE]

Helper functions

Predicates

eth2_fast_aggregate_verify

def eth2_fast_aggregate_verify(pubkeys: Sequence[BLSPubkey], message: Bytes32, signature: BLSSignature) -> bool:
    """
    Wrapper to ``bls.FastAggregateVerify`` accepting the ``G2_POINT_AT_INFINITY`` signature when ``pubkeys`` is empty.
    """
    if len(pubkeys) == 0 and signature == G2_POINT_AT_INFINITY:
        return True
    return bls.FastAggregateVerify(pubkeys, message, signature)

Beacon state accessors

get_sync_committee_indices

def get_sync_committee_indices(state: BeaconState, epoch: Epoch) -> Sequence[ValidatorIndex]:
    """
    Return the sync committee indices for a given state and epoch.
    """ 
    MAX_RANDOM_BYTE = 2**8 - 1
    base_epoch = Epoch((max(epoch // EPOCHS_PER_SYNC_COMMITTEE_PERIOD, 1) - 1) * EPOCHS_PER_SYNC_COMMITTEE_PERIOD)
    active_validator_indices = get_active_validator_indices(state, base_epoch)
    active_validator_count = uint64(len(active_validator_indices))
    seed = get_seed(state, base_epoch, DOMAIN_SYNC_COMMITTEE)
    i = 0
    sync_committee_indices: List[ValidatorIndex] = []
    while len(sync_committee_indices) < SYNC_COMMITTEE_SIZE:
        shuffled_index = compute_shuffled_index(uint64(i % active_validator_count), active_validator_count, seed)
        candidate_index = active_validator_indices[shuffled_index]
        random_byte = hash(seed + uint_to_bytes(uint64(i // 32)))[i % 32]
        effective_balance = state.validators[candidate_index].effective_balance
        if effective_balance * MAX_RANDOM_BYTE >= MAX_EFFECTIVE_BALANCE * random_byte:
            sync_committee_indices.append(candidate_index)
        i += 1
    return sync_committee_indices

get_sync_committee

def get_sync_committee(state: BeaconState, epoch: Epoch) -> SyncCommittee:
    """
    Return the sync committee for a given state and epoch.
    """
    indices = get_sync_committee_indices(state, epoch)
    validators = [state.validators[index] for index in indices]
    pubkeys = [validator.pubkey for validator in validators]
    aggregates = [
        bls.AggregatePKs(pubkeys[i:i + SYNC_COMMITTEE_PUBKEY_AGGREGATES_SIZE])
        for i in range(0, len(pubkeys), SYNC_COMMITTEE_PUBKEY_AGGREGATES_SIZE)
    ]
    return SyncCommittee(pubkeys=pubkeys, pubkey_aggregates=aggregates)

Block processing

def process_block(state: BeaconState, block: BeaconBlock) -> None:
    process_block_header(state, block)
    process_randao(state, block.body)
    process_eth1_data(state, block.body)
    process_operations(state, block.body)
    # Light client support
    process_sync_committee(state, block.body)

Sync committee processing

def process_sync_committee(state: BeaconState, body: BeaconBlockBody) -> None:
    # Verify sync committee aggregate signature signing over the previous slot block root
    previous_slot = Slot(max(int(state.slot), 1) - 1)
    committee_indices = get_sync_committee_indices(state, get_current_epoch(state))
    participant_indices = [index for index, bit in zip(committee_indices, body.sync_committee_bits) if bit]
    committee_pubkeys = state.current_sync_committee.pubkeys
    participant_pubkeys = [pubkey for pubkey, bit in zip(committee_pubkeys, body.sync_committee_bits) if bit]
    domain = get_domain(state, DOMAIN_SYNC_COMMITTEE, compute_epoch_at_slot(previous_slot))
    signing_root = compute_signing_root(get_block_root_at_slot(state, previous_slot), domain)
    assert eth2_fast_aggregate_verify(participant_pubkeys, signing_root, body.sync_committee_signature)

    # Reward sync committee participants
    proposer_reward = Gwei(0)
    active_validator_count = uint64(len(get_active_validator_indices(state, get_current_epoch(state))))
    for participant_index in participant_indices:
        base_reward = get_base_reward(state, participant_index)
        proposer_reward = get_proposer_reward(state, participant_index)
        max_participant_reward = base_reward - proposer_reward
        reward = Gwei(max_participant_reward * active_validator_count // len(committee_indices) // SLOTS_PER_EPOCH)
        increase_balance(state, participant_index, reward)
        proposer_reward += proposer_reward

    # Reward beacon proposer
    increase_balance(state, get_beacon_proposer_index(state), proposer_reward)

Epoch processing

Components of attestation deltas

Note: The function get_inactivity_penalty_deltas is modified with BASE_REWARDS_PER_EPOCH replaced by BASE_REWARDS_PER_EPOCH - 1.

def get_inactivity_penalty_deltas(state: BeaconState) -> Tuple[Sequence[Gwei], Sequence[Gwei]]:
    """
    Return inactivity reward/penalty deltas for each validator.
    """
    penalties = [Gwei(0) for _ in range(len(state.validators))]
    if is_in_inactivity_leak(state):
        matching_target_attestations = get_matching_target_attestations(state, get_previous_epoch(state))
        matching_target_attesting_indices = get_unslashed_attesting_indices(state, matching_target_attestations)
        for index in get_eligible_validator_indices(state):
            # Penalize validator so that optimal attestation performance is rewarded with one base reward per epoch
            base_reward = get_base_reward(state, index)
            penalties[index] += Gwei((BASE_REWARDS_PER_EPOCH - 1) * base_reward - get_proposer_reward(state, index))
            if index not in matching_target_attesting_indices:
                effective_balance = state.validators[index].effective_balance
                penalties[index] += Gwei(effective_balance * get_finality_delay(state) // INACTIVITY_PENALTY_QUOTIENT)

    # No rewards associated with inactivity penalties
    rewards = [Gwei(0) for _ in range(len(state.validators))]
    return rewards, penalties

Final updates

Note: The function process_final_updates is modified to handle sync committee updates.

def process_final_updates(state: BeaconState) -> None:
    # FIXME: unfold the full `process_final_updates` to avoid side effects.
    phase0.process_final_updates(state)
    next_epoch = get_current_epoch(state) + Epoch(1)
    if next_epoch % EPOCHS_PER_SYNC_COMMITTEE_PERIOD == 0:
        state.current_sync_committee = state.next_sync_committee
        state.next_sync_committee = get_sync_committee(state, next_epoch + EPOCHS_PER_SYNC_COMMITTEE_PERIOD)