# Ethereum 2.0 HF1 ## Table of contents - [Introduction](#introduction) - [Constants](#constants) - [Participation flags](#participation-flags) - [Participation rewards](#participation-rewards) - [Misc](#misc) - [Configuration](#configuration) - [Constants](#constants-1) - [Misc](#misc-1) - [Time parameters](#time-parameters) - [Domain types](#domain-types) - [Containers](#containers) - [Extended containers](#extended-containers) - [`BeaconBlockBody`](#beaconblockbody) - [`BeaconState`](#beaconstate) - [New containers](#new-containers) - [`SyncCommittee`](#synccommittee) - [Helper functions](#helper-functions) - [`Predicates`](#predicates) - [`get_base_reward`](#get_base_reward) - [`get_base_reward_per_eth`](#get_base_reward_per_eth) - [`eth2_fast_aggregate_verify`](#eth2_fast_aggregate_verify) - [Beacon state accessors](#beacon-state-accessors) - [`get_sync_committee_indices`](#get_sync_committee_indices) - [`get_sync_committee`](#get_sync_committee) - [`get_unslashed_participating_indices`](#get_unslashed_participating_indices) - [`get_flag_deltas`](#get_flag_deltas) - [`get_inactivity_penalty_deltas`](#get_inactivity_penalty_deltas) - [Block processing](#block-processing) - [New `process_attestation`](#new-process_attestation) - [Sync committee processing](#sync-committee-processing) - [Epoch processing](#epoch-processing) - [New `process_justification_and_finalization`](#new-process_justification_and_finalization) - [New `process_rewards_and_penalties`](#new-process_rewards_and_penalties) - [Final updates](#final-updates) ## Introduction This is a patch implementing the first hard fork to the beacon chain, tentatively named HF1 pending a permanent name. It has three main features: * Light client support via sync committees * Incentive accounting reforms, reducing spec complexity and reducing the cost of processing chains that have very little or zero participation for a long span of epochs * Fork choice rule changes to address weaknesses recently discovered in the existing fork choice ## Constants ### Participation flags | Name | Value | | - | - | | `TIMELY_HEAD_FLAG` | `0` | | `TIMELY_SOURCE_FLAG` | `1` | | `TIMELY_TARGET_FLAG` | `2` | ### Participation rewards | Name | Value | | - | - | | `TIMELY_HEAD_NUMERATOR` | `12` | | `TIMELY_SOURCE_NUMERATOR` | `12` | | `TIMELY_TARGET_NUMERATOR` | `32` | | `REWARD_DENOMINATOR` | `64` | The reward fractions add up to 7/8, leaving the remaining 1/8 for proposer rewards and other future micro-rewards. ### Misc | Name | Value | | - | - | | `PARTICIPATION_FLAGS_LENGTH` | `8` | | `FLAGS_AND_NUMERATORS` | `((TIMELY_HEAD_FLAG, TIMELY_HEAD_NUMERATOR), (TIMELY_SOURCE_FLAG, TIMELY_SOURCE_NUMERATOR), (TIMELY_TARGET_FLAG, TIMELY_TARGET_NUMERATOR))` | ## Configuration ### Constants | Name | Value | | - | - | | `G2_POINT_AT_INFINITY` | `BLSSignature(b'\xc0' + b'\x00' * 95)` | | `GWEI_PER_ETH` | `10**9` | ### 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` ```python class BeaconBlockBody(phase0.BeaconBlockBody): # Sync committee aggregate signature sync_committee_bits: Bitvector[SYNC_COMMITTEE_SIZE] sync_committee_signature: BLSSignature ``` #### `BeaconState` ```python class BeaconState(Container): # Versioning genesis_time: uint64 genesis_validators_root: Root slot: Slot fork: Fork # History latest_block_header: BeaconBlockHeader block_roots: Vector[Root, SLOTS_PER_HISTORICAL_ROOT] state_roots: Vector[Root, SLOTS_PER_HISTORICAL_ROOT] historical_roots: List[Root, HISTORICAL_ROOTS_LIMIT] # Eth1 eth1_data: Eth1Data eth1_data_votes: List[Eth1Data, EPOCHS_PER_ETH1_VOTING_PERIOD * SLOTS_PER_EPOCH] eth1_deposit_index: uint64 # Registry validators: List[Validator, VALIDATOR_REGISTRY_LIMIT] balances: List[Gwei, VALIDATOR_REGISTRY_LIMIT] # Randomness randao_mixes: Vector[Bytes32, EPOCHS_PER_HISTORICAL_VECTOR] # Slashings slashings: Vector[Gwei, EPOCHS_PER_SLASHINGS_VECTOR] # Per-epoch sums of slashed effective balances # Participation previous_epoch_participation: List[Bitvector[PARTICIPATION_FLAGS_LENGTH], VALIDATOR_REGISTRY_LIMIT] current_epoch_participation: List[Bitvector[PARTICIPATION_FLAGS_LENGTH], VALIDATOR_REGISTRY_LIMIT] # Finality justification_bits: Bitvector[JUSTIFICATION_BITS_LENGTH] # Bit set for every recent justified epoch previous_justified_checkpoint: Checkpoint current_justified_checkpoint: Checkpoint finalized_checkpoint: Checkpoint # Light client sync committees current_sync_committee: SyncCommittee next_sync_committee: SyncCommittee # Denominator to real-time-updated balances (NOT effective balances!) balance_denominator: uint64 ``` ### New containers #### `SyncCommittee` ```python class SyncCommittee(Container): pubkeys: Vector[BLSPubkey, SYNC_COMMITTEE_SIZE] pubkey_aggregates: Vector[BLSPubkey, SYNC_COMMITTEE_SIZE // SYNC_COMMITTEE_PUBKEY_AGGREGATES_SIZE] ``` ## Helper functions ### `Predicates` #### `get_base_reward` *Note*: The function `get_base_reward` is modified with the removal of `BASE_REWARDS_PER_EPOCH`. Additionally, it is split into `get_base_reward_per_eth` to allow penalties to be computed to the denominator. ```python def get_base_reward(state: BeaconState, index: ValidatorIndex) -> Gwei: return get_base_reward_per_eth(state) * state.validators[index].effective_balance // GWEI_PER_ETH ``` #### `get_base_reward_per_eth` ```python def get_base_reward_per_eth(state: BeaconState) -> Gwei: total_balance = get_total_active_balance(state) return Gwei(GWEI_PER_ETH * BASE_REWARD_FACTOR // integer_squareroot(total_balance)) ``` #### `eth2_fast_aggregate_verify` ```python 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` ```python 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` ```python 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) ``` #### `get_unslashed_participating_indices` ```python def get_unslashed_participating_indices(state: BeaconState, flag: uint8, epoch: Epoch) -> Set[ValidatorIndex]: assert epoch in (get_previous_epoch(state), get_current_epoch(state)) if epoch == get_current_epoch(state): epoch_participation = state.current_epoch_participation else: epoch_participation = state.previous_epoch_participation participating_indices = [index for index in get_active_validator_indices(state, epoch) if epoch_participation[index][flag]] return set(filter(lambda index: not state.validators[index].slashed, participating_indices)) ``` #### `get_flag_deltas` ```python def get_flag_deltas(state: BeaconState, flag: uint8, numerator: uint64) -> Tuple[Sequence[Gwei], Gwei]: """ Computes the rewards and penalties associated with a particular duty, by scanning through the participation flags to determine who participated and who did not and assigning them the appropriate rewards and penalties. """ rewards = [Gwei(0)] * len(state.validators) unslashed_participating_indices = get_unslashed_participating_indices(state, flag, get_previous_epoch(state)) increment = EFFECTIVE_BALANCE_INCREMENT # Factored out from balances to avoid uint64 overflow unslashed_participating_increments = get_total_balance(state, unslashed_participating_indices) // increment active_increments = get_total_active_balance(state) // increment for index in get_eligible_validator_indices(state): base_reward = get_base_reward(state, index) if index in unslashed_participating_indices: if is_in_inactivity_leak(state): # Optimal participatition is fully rewarded to cancel the inactivity penalty rewards[index] = base_reward * numerator // REWARD_DENOMINATOR else: rewards[index] = ( (base_reward * unslashed_participating_increments // active_increments + base_reward) * numerator // REWARD_DENOMINATOR ) return rewards, get_base_reward_per_eth(state) * numerator // REWARD_DENOMINATOR ``` ##### `get_inactivity_penalty_deltas` *Note*: The function `get_inactivity_penalty_deltas` is modified in the selection of matching target indices and the removal of `BASE_REWARDS_PER_EPOCH`. ```python def get_inactivity_penalty_deltas(state: BeaconState) -> Tuple[Sequence[Gwei], Sequence[Gwei]]: """ Compute the penalties associated with the inactivity leak, by scanning through the participation flags to determine who participated and who did not, applying the leak penalty globally and applying compensatory rewards to participants. """ rewards = [Gwei(0) for _ in range(len(state.validators))] if is_in_inactivity_leak(state): reward_numerator_sum = sum(numerator for (_, numerator) in FLAGS_AND_NUMERATORS) matching_target_attesting_indices = get_unslashed_participating_indices( state, TIMELY_TARGET_FLAG, get_previous_epoch(state) ) for index in get_eligible_validator_indices(state): if index in matching_target_attesting_indices: effective_balance = state.validators[index].effective_balance rewards[index] += Gwei(effective_balance * get_finality_delay(state) // INACTIVITY_PENALTY_QUOTIENT) return rewards, Gwei(GWEI_PER_ETH * get_finality_delay(state) // INACTIVITY_PENALTY_QUOTIENT) ``` ### Block processing ```python 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) ``` #### New `process_attestation` *Note*: The function `process_attestation` is modified to do incentive accounting with epoch participation flags. ```python def process_attestation(state: BeaconState, attestation: Attestation) -> None: data = attestation.data assert data.target.epoch in (get_previous_epoch(state), get_current_epoch(state)) assert data.target.epoch == compute_epoch_at_slot(data.slot) assert data.slot + MIN_ATTESTATION_INCLUSION_DELAY <= state.slot <= data.slot + SLOTS_PER_EPOCH assert data.index < get_committee_count_per_slot(state, data.target.epoch) committee = get_beacon_committee(state, data.slot, data.index) assert len(attestation.aggregation_bits) == len(committee) if data.target.epoch == get_current_epoch(state): epoch_participation = state.current_epoch_participation justified_checkpoint = state.current_justified_checkpoint else: epoch_participation = state.previous_epoch_participation justified_checkpoint = state.previous_justified_checkpoint # Matching roots is_matching_head = data.beacon_block_root == get_block_root_at_slot(state, data.slot) is_matching_source = data.source == justified_checkpoint is_matching_target = data.target.root == get_block_root(state, data.target.epoch) assert is_matching_source # Participation flags participation_flags = [] if is_matching_head and state.slot <= data.slot + MIN_ATTESTATION_INCLUSION_DELAY: participation_flags.append(TIMELY_HEAD_FLAG) if is_matching_source and state.slot <= data.slot + integer_squareroot(SLOTS_PER_EPOCH): participation_flags.append(TIMELY_SOURCE_FLAG) if is_matching_target and state.slot <= data.slot + SLOTS_PER_EPOCH: participation_flags.append(TIMELY_TARGET_FLAG) # Update epoch participation flags proposer_reward_numerator = 0 for index in get_attesting_indices(state, data, attestation.aggregation_bits): for flag, numerator in FLAGS_AND_NUMERATORS: if flag in participation_flags and not epoch_participation[index][flag]: epoch_participation[index][flag] = True proposer_reward_numerator += get_base_reward(state, index) * numerator # Reward proposer proposer_reward = Gwei(proposer_reward_numerator // (REWARD_DENOMINATOR * PROPOSER_REWARD_QUOTIENT)) increase_balance(state, get_beacon_proposer_index(state), proposer_reward) # Verify signature assert is_valid_indexed_attestation(state, get_indexed_attestation(state, attestation)) ``` #### Sync committee processing ```python 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) max_participant_reward = base_reward - base_reward // PROPOSER_REWARD_QUOTIENT reward = Gwei(max_participant_reward * active_validator_count // len(committee_indices) // SLOTS_PER_EPOCH) increase_balance(state, participant_index, reward) proposer_reward += base_reward // PROPOSER_REWARD_QUOTIENT # Reward beacon proposer increase_balance(state, get_beacon_proposer_index(state), proposer_reward) ``` ### Epoch processing #### New `process_justification_and_finalization` *Note*: The function `process_justification_and_finalization` is modified with `matching_target_attestations` replaced by `matching_target_indices`. ```python def process_justification_and_finalization(state: BeaconState) -> None: # Initial FFG checkpoint values have a `0x00` stub for `root`. # Skip FFG updates in the first two epochs to avoid corner cases that might result in modifying this stub. if get_current_epoch(state) <= GENESIS_EPOCH + 1: return previous_epoch = get_previous_epoch(state) current_epoch = get_current_epoch(state) old_previous_justified_checkpoint = state.previous_justified_checkpoint old_current_justified_checkpoint = state.current_justified_checkpoint # Process justifications state.previous_justified_checkpoint = state.current_justified_checkpoint state.justification_bits[1:] = state.justification_bits[:JUSTIFICATION_BITS_LENGTH - 1] state.justification_bits[0] = 0b0 matching_target_indices = get_unslashed_participating_indices(state, TIMELY_TARGET_FLAG, previous_epoch) if get_total_balance(state, matching_target_indices) * 3 >= get_total_active_balance(state) * 2: state.current_justified_checkpoint = Checkpoint(epoch=previous_epoch, root=get_block_root(state, previous_epoch)) state.justification_bits[1] = 0b1 matching_target_indices = get_unslashed_participating_indices(state, TIMELY_TARGET_FLAG, current_epoch) if get_total_balance(state, matching_target_indices) * 3 >= get_total_active_balance(state) * 2: state.current_justified_checkpoint = Checkpoint(epoch=current_epoch, root=get_block_root(state, current_epoch)) state.justification_bits[0] = 0b1 # Process finalizations bits = state.justification_bits # The 2nd/3rd/4th most recent epochs are justified, the 2nd using the 4th as source if all(bits[1:4]) and old_previous_justified_checkpoint.epoch + 3 == current_epoch: state.finalized_checkpoint = old_previous_justified_checkpoint # The 2nd/3rd most recent epochs are justified, the 2nd using the 3rd as source if all(bits[1:3]) and old_previous_justified_checkpoint.epoch + 2 == current_epoch: state.finalized_checkpoint = old_previous_justified_checkpoint # The 1st/2nd/3rd most recent epochs are justified, the 1st using the 3rd as source if all(bits[0:3]) and old_current_justified_checkpoint.epoch + 2 == current_epoch: state.finalized_checkpoint = old_current_justified_checkpoint # The 1st/2nd most recent epochs are justified, the 1st using the 2nd as source if all(bits[0:2]) and old_current_justified_checkpoint.epoch + 1 == current_epoch: state.finalized_checkpoint = old_current_justified_checkpoint ``` #### New `process_rewards_and_penalties` *Note*: The function `process_rewards_and_penalties` is modified to use participation flag deltas. ```python def process_rewards_and_penalties(state: BeaconState) -> None: # No rewards are applied at the end of `GENESIS_EPOCH` because rewards are for work done in the previous epoch if get_current_epoch(state) == GENESIS_EPOCH: return flag_deltas = [get_flag_deltas(state, flag, numerator) for (flag, numerator) in FLAGS_AND_NUMERATORS] deltas = flag_deltas + [get_inactivity_penalty_deltas(state)] for (rewards, penalty) in deltas: for index in range(len(state.validators)): increase_balance(state, ValidatorIndex(index), rewards[index]) # Bounds-friendly expansion for `denom *= (1 + penalty // GWEI_PER_ETH)` state.balance_denominator = ( state.balance_denominator + penalty + (state.balance_denominator - GWEI_PER_ETH) * penalty // GWEI_PER_ETH ) ``` #### Final updates *Note*: The function `process_final_updates` is modified to handle sync committee updates, replacement of `PendingAttestation`s with participation flags, and a sliding-denominator-aware hysteresis mechanism. ```python def process_final_updates(state: BeaconState) -> None: current_epoch = get_current_epoch(state) next_epoch = Epoch(current_epoch + 1) # Reset eth1 data votes if next_epoch % EPOCHS_PER_ETH1_VOTING_PERIOD == 0: state.eth1_data_votes = [] # Update sync committees 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) # Update effective balances with hysteresis for index, validator in enumerate(state.validators): balance_increments = state.balances[index] * HYSTERESIS_QUOTIENT // state.balance_denominator effective_increments = validator.effective_balance // GWEI_PER_ETH * HYSTERESIS_QUOTIENT if ( balance_increments + HYSTERESIS_DOWNWARD_MULTIPLIER < effective_increments or effective_increments + HYSTERESIS_UPWARD_MULTIPLIER < balance_increments or get_current_epoch(state) == validator.withdrawable_epoch ): validator.effective_balance = (state.balances[index] // state.balance_denominator) * GWEI_PER_ETH if state.balance_denominator >= 2 * GWEI_PER_ETH: state.balances = [x // 2 for x in state.balances] state.balance_denominator //= 2 # Reset slashings state.slashings[next_epoch % EPOCHS_PER_SLASHINGS_VECTOR] = Gwei(0) # Set randao mix state.randao_mixes[next_epoch % EPOCHS_PER_HISTORICAL_VECTOR] = get_randao_mix(state, current_epoch) # Set historical root accumulator if next_epoch % (SLOTS_PER_HISTORICAL_ROOT // SLOTS_PER_EPOCH) == 0: historical_batch = HistoricalBatch(block_roots=state.block_roots, state_roots=state.state_roots) state.historical_roots.append(hash_tree_root(historical_batch)) # Rotate current/previous epoch participation flags state.previous_epoch_participation = state.current_epoch_participation state.current_epoch_participation = []