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Capella -- The Beacon Chain

Table of contents

Introduction

Capella is a consensus-layer upgrade containing a number of features related to validator withdrawals. Including: * Automatic withdrawals of withdrawable validators. * Partial withdrawals sweep for validators with 0x01 withdrawal credentials and balances in excess of MAX_EFFECTIVE_BALANCE. * Operation to change from BLS_WITHDRAWAL_PREFIX to ETH1_ADDRESS_WITHDRAWAL_PREFIX versioned withdrawal credentials to enable withdrawals for a validator.

Another new feature is the new independent state and block historical accumulators that replace the original singular historical roots. With these accumulators, it becomes possible to validate the entire block history that led up to that particular state without any additional information beyond the state and the blocks.

Custom types

We define the following Python custom types for type hinting and readability:

Name SSZ equivalent Description
WithdrawalIndex uint64 an index of a Withdrawal

Domain types

Name Value
DOMAIN_BLS_TO_EXECUTION_CHANGE DomainType('0x0A000000')

Preset

Max operations per block

Name Value
MAX_BLS_TO_EXECUTION_CHANGES 2**4 (= 16)

Execution

Name Value Description
MAX_WITHDRAWALS_PER_PAYLOAD uint64(2**4) (= 16) Maximum amount of withdrawals allowed in each payload

Withdrawals processing

Name Value
MAX_VALIDATORS_PER_WITHDRAWALS_SWEEP 16384 (= 2**14 )

Containers

New containers

Withdrawal

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class Withdrawal(Container):
    index: WithdrawalIndex
    validator_index: ValidatorIndex
    address: ExecutionAddress
    amount: Gwei

BLSToExecutionChange

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class BLSToExecutionChange(Container):
    validator_index: ValidatorIndex
    from_bls_pubkey: BLSPubkey
    to_execution_address: ExecutionAddress

SignedBLSToExecutionChange

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class SignedBLSToExecutionChange(Container):
    message: BLSToExecutionChange
    signature: BLSSignature

HistoricalSummary

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class HistoricalSummary(Container):
    """
    `HistoricalSummary` matches the components of the phase0 `HistoricalBatch`
    making the two hash_tree_root-compatible.
    """
    block_summary_root: Root
    state_summary_root: Root

Extended Containers

ExecutionPayload

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class ExecutionPayload(Container):
    # Execution block header fields
    parent_hash: Hash32
    fee_recipient: ExecutionAddress  # 'beneficiary' in the yellow paper
    state_root: Bytes32
    receipts_root: Bytes32
    logs_bloom: ByteVector[BYTES_PER_LOGS_BLOOM]
    prev_randao: Bytes32  # 'difficulty' in the yellow paper
    block_number: uint64  # 'number' in the yellow paper
    gas_limit: uint64
    gas_used: uint64
    timestamp: uint64
    extra_data: ByteList[MAX_EXTRA_DATA_BYTES]
    base_fee_per_gas: uint256
    # Extra payload fields
    block_hash: Hash32  # Hash of execution block
    transactions: List[Transaction, MAX_TRANSACTIONS_PER_PAYLOAD]
    withdrawals: List[Withdrawal, MAX_WITHDRAWALS_PER_PAYLOAD]  # [New in Capella]

ExecutionPayloadHeader

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class ExecutionPayloadHeader(Container):
    # Execution block header fields
    parent_hash: Hash32
    fee_recipient: ExecutionAddress
    state_root: Bytes32
    receipts_root: Bytes32
    logs_bloom: ByteVector[BYTES_PER_LOGS_BLOOM]
    prev_randao: Bytes32
    block_number: uint64
    gas_limit: uint64
    gas_used: uint64
    timestamp: uint64
    extra_data: ByteList[MAX_EXTRA_DATA_BYTES]
    base_fee_per_gas: uint256
    # Extra payload fields
    block_hash: Hash32  # Hash of execution block
    transactions_root: Root
    withdrawals_root: Root  # [New in Capella]

BeaconBlockBody

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class BeaconBlockBody(Container):
    randao_reveal: BLSSignature
    eth1_data: Eth1Data  # Eth1 data vote
    graffiti: Bytes32  # Arbitrary data
    # Operations
    proposer_slashings: List[ProposerSlashing, MAX_PROPOSER_SLASHINGS]
    attester_slashings: List[AttesterSlashing, MAX_ATTESTER_SLASHINGS]
    attestations: List[Attestation, MAX_ATTESTATIONS]
    deposits: List[Deposit, MAX_DEPOSITS]
    voluntary_exits: List[SignedVoluntaryExit, MAX_VOLUNTARY_EXITS]
    sync_aggregate: SyncAggregate
    # Execution
    execution_payload: ExecutionPayload
    # Capella operations
    bls_to_execution_changes: List[SignedBLSToExecutionChange, MAX_BLS_TO_EXECUTION_CHANGES]  # [New in Capella]

BeaconState

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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]  # Frozen in Capella, replaced by historical_summaries
    # 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[ParticipationFlags, VALIDATOR_REGISTRY_LIMIT]
    current_epoch_participation: List[ParticipationFlags, 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
    # Inactivity
    inactivity_scores: List[uint64, VALIDATOR_REGISTRY_LIMIT]
    # Sync
    current_sync_committee: SyncCommittee
    next_sync_committee: SyncCommittee
    # Execution
    latest_execution_payload_header: ExecutionPayloadHeader  # [Modified in Capella]
    # Withdrawals
    next_withdrawal_index: WithdrawalIndex  # [New in Capella]
    next_withdrawal_validator_index: ValidatorIndex  # [New in Capella]
    # Deep history valid from Capella onwards
    historical_summaries: List[HistoricalSummary, HISTORICAL_ROOTS_LIMIT]  # [New in Capella]

Helpers

Predicates

has_eth1_withdrawal_credential

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def has_eth1_withdrawal_credential(validator: Validator) -> bool:
    """
    Check if ``validator`` has an 0x01 prefixed "eth1" withdrawal credential.
    """
    return validator.withdrawal_credentials[:1] == ETH1_ADDRESS_WITHDRAWAL_PREFIX

is_fully_withdrawable_validator

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def is_fully_withdrawable_validator(validator: Validator, balance: Gwei, epoch: Epoch) -> bool:
    """
    Check if ``validator`` is fully withdrawable.
    """
    return (
        has_eth1_withdrawal_credential(validator)
        and validator.withdrawable_epoch <= epoch
        and balance > 0
    )

is_partially_withdrawable_validator

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def is_partially_withdrawable_validator(validator: Validator, balance: Gwei) -> bool:
    """
    Check if ``validator`` is partially withdrawable.
    """
    has_max_effective_balance = validator.effective_balance == MAX_EFFECTIVE_BALANCE
    has_excess_balance = balance > MAX_EFFECTIVE_BALANCE
    return has_eth1_withdrawal_credential(validator) and has_max_effective_balance and has_excess_balance

Beacon chain state transition function

Epoch processing

Note: The function process_historical_summaries_update replaces process_historical_roots_update in Capella.

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def process_epoch(state: BeaconState) -> None:
    process_justification_and_finalization(state)
    process_inactivity_updates(state)
    process_rewards_and_penalties(state)
    process_registry_updates(state)
    process_slashings(state)
    process_eth1_data_reset(state)
    process_effective_balance_updates(state)
    process_slashings_reset(state)
    process_randao_mixes_reset(state)
    process_historical_summaries_update(state)  # [Modified in Capella]
    process_participation_flag_updates(state)
    process_sync_committee_updates(state)

Historical summaries updates

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def process_historical_summaries_update(state: BeaconState) -> None:
    # Set historical block root accumulator.
    next_epoch = Epoch(get_current_epoch(state) + 1)
    if next_epoch % (SLOTS_PER_HISTORICAL_ROOT // SLOTS_PER_EPOCH) == 0:
        historical_summary = HistoricalSummary(
            block_summary_root=hash_tree_root(state.block_roots),
            state_summary_root=hash_tree_root(state.state_roots),
        )
        state.historical_summaries.append(historical_summary)

Block processing

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def process_block(state: BeaconState, block: BeaconBlock) -> None:
    process_block_header(state, block)
    # [Modified in Capella] Removed `is_execution_enabled` check in Capella
    process_withdrawals(state, block.body.execution_payload)  # [New in Capella]
    process_execution_payload(state, block.body, EXECUTION_ENGINE)  # [Modified in Capella]
    process_randao(state, block.body)
    process_eth1_data(state, block.body)
    process_operations(state, block.body)  # [Modified in Capella]
    process_sync_aggregate(state, block.body.sync_aggregate)

New get_expected_withdrawals

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def get_expected_withdrawals(state: BeaconState) -> Sequence[Withdrawal]:
    epoch = get_current_epoch(state)
    withdrawal_index = state.next_withdrawal_index
    validator_index = state.next_withdrawal_validator_index
    withdrawals: List[Withdrawal] = []
    bound = min(len(state.validators), MAX_VALIDATORS_PER_WITHDRAWALS_SWEEP)
    for _ in range(bound):
        validator = state.validators[validator_index]
        balance = state.balances[validator_index]
        if is_fully_withdrawable_validator(validator, balance, epoch):
            withdrawals.append(Withdrawal(
                index=withdrawal_index,
                validator_index=validator_index,
                address=ExecutionAddress(validator.withdrawal_credentials[12:]),
                amount=balance,
            ))
            withdrawal_index += WithdrawalIndex(1)
        elif is_partially_withdrawable_validator(validator, balance):
            withdrawals.append(Withdrawal(
                index=withdrawal_index,
                validator_index=validator_index,
                address=ExecutionAddress(validator.withdrawal_credentials[12:]),
                amount=balance - MAX_EFFECTIVE_BALANCE,
            ))
            withdrawal_index += WithdrawalIndex(1)
        if len(withdrawals) == MAX_WITHDRAWALS_PER_PAYLOAD:
            break
        validator_index = ValidatorIndex((validator_index + 1) % len(state.validators))
    return withdrawals

New process_withdrawals

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def process_withdrawals(state: BeaconState, payload: ExecutionPayload) -> None:
    expected_withdrawals = get_expected_withdrawals(state)
    assert payload.withdrawals == expected_withdrawals

    for withdrawal in expected_withdrawals:
        decrease_balance(state, withdrawal.validator_index, withdrawal.amount)

    # Update the next withdrawal index if this block contained withdrawals
    if len(expected_withdrawals) != 0:
        latest_withdrawal = expected_withdrawals[-1]
        state.next_withdrawal_index = WithdrawalIndex(latest_withdrawal.index + 1)

    # Update the next validator index to start the next withdrawal sweep
    if len(expected_withdrawals) == MAX_WITHDRAWALS_PER_PAYLOAD:
        # Next sweep starts after the latest withdrawal's validator index
        next_validator_index = ValidatorIndex((expected_withdrawals[-1].validator_index + 1) % len(state.validators))
        state.next_withdrawal_validator_index = next_validator_index
    else:
        # Advance sweep by the max length of the sweep if there was not a full set of withdrawals
        next_index = state.next_withdrawal_validator_index + MAX_VALIDATORS_PER_WITHDRAWALS_SWEEP
        next_validator_index = ValidatorIndex(next_index % len(state.validators))
        state.next_withdrawal_validator_index = next_validator_index

Modified process_execution_payload

Note: The function process_execution_payload is modified to use the new ExecutionPayloadHeader type and removed the is_merge_transition_complete check.

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def process_execution_payload(state: BeaconState, body: BeaconBlockBody, execution_engine: ExecutionEngine) -> None:
    payload = body.execution_payload
    # [Modified in Capella] Removed `is_merge_transition_complete` check in Capella
    # Verify consistency of the parent hash with respect to the previous execution payload header
    assert payload.parent_hash == state.latest_execution_payload_header.block_hash
    # Verify prev_randao
    assert payload.prev_randao == get_randao_mix(state, get_current_epoch(state))
    # Verify timestamp
    assert payload.timestamp == compute_timestamp_at_slot(state, state.slot)
    # Verify the execution payload is valid
    assert execution_engine.verify_and_notify_new_payload(NewPayloadRequest(execution_payload=payload))
    # Cache execution payload header
    state.latest_execution_payload_header = ExecutionPayloadHeader(
        parent_hash=payload.parent_hash,
        fee_recipient=payload.fee_recipient,
        state_root=payload.state_root,
        receipts_root=payload.receipts_root,
        logs_bloom=payload.logs_bloom,
        prev_randao=payload.prev_randao,
        block_number=payload.block_number,
        gas_limit=payload.gas_limit,
        gas_used=payload.gas_used,
        timestamp=payload.timestamp,
        extra_data=payload.extra_data,
        base_fee_per_gas=payload.base_fee_per_gas,
        block_hash=payload.block_hash,
        transactions_root=hash_tree_root(payload.transactions),
        withdrawals_root=hash_tree_root(payload.withdrawals),  # [New in Capella]
    )

Modified process_operations

Note: The function process_operations is modified to process BLSToExecutionChange operations included in the block.

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def process_operations(state: BeaconState, body: BeaconBlockBody) -> None:
    # Verify that outstanding deposits are processed up to the maximum number of deposits
    assert len(body.deposits) == min(MAX_DEPOSITS, state.eth1_data.deposit_count - state.eth1_deposit_index)

    def for_ops(operations: Sequence[Any], fn: Callable[[BeaconState, Any], None]) -> None:
        for operation in operations:
            fn(state, operation)

    for_ops(body.proposer_slashings, process_proposer_slashing)
    for_ops(body.attester_slashings, process_attester_slashing)
    for_ops(body.attestations, process_attestation)
    for_ops(body.deposits, process_deposit)
    for_ops(body.voluntary_exits, process_voluntary_exit)
    for_ops(body.bls_to_execution_changes, process_bls_to_execution_change)  # [New in Capella]

New process_bls_to_execution_change

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def process_bls_to_execution_change(state: BeaconState,
                                    signed_address_change: SignedBLSToExecutionChange) -> None:
    address_change = signed_address_change.message

    assert address_change.validator_index < len(state.validators)

    validator = state.validators[address_change.validator_index]

    assert validator.withdrawal_credentials[:1] == BLS_WITHDRAWAL_PREFIX
    assert validator.withdrawal_credentials[1:] == hash(address_change.from_bls_pubkey)[1:]

    # Fork-agnostic domain since address changes are valid across forks
    domain = compute_domain(DOMAIN_BLS_TO_EXECUTION_CHANGE, genesis_validators_root=state.genesis_validators_root)
    signing_root = compute_signing_root(address_change, domain)
    assert bls.Verify(address_change.from_bls_pubkey, signing_root, signed_address_change.signature)

    validator.withdrawal_credentials = (
        ETH1_ADDRESS_WITHDRAWAL_PREFIX
        + b'\x00' * 11
        + address_change.to_execution_address
    )