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

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# Capella -- The Beacon Chain
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## Table of contents
<!-- TOC -->
<!-- START doctoc generated TOC please keep comment here to allow auto update -->
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- [Introduction](#introduction)
- [Custom types](#custom-types)
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- [Constants](#constants)
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- [Domain types](#domain-types)
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- [Preset](#preset)
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- [Max operations per block](#max-operations-per-block)
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- [Execution](#execution)
- [Containers](#containers)
- [New containers](#new-containers)
- [`Withdrawal`](#withdrawal)
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- [`BLSToExecutionChange`](#blstoexecutionchange)
- [`SignedBLSToExecutionChange`](#signedblstoexecutionchange)
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- [Extended Containers](#extended-containers)
- [`ExecutionPayload`](#executionpayload)
- [`ExecutionPayloadHeader`](#executionpayloadheader)
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- [`BeaconBlockBody`](#beaconblockbody)
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- [`BeaconState`](#beaconstate)
- [Helpers](#helpers)
- [Predicates](#predicates)
- [`has_eth1_withdrawal_credential`](#has_eth1_withdrawal_credential)
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- [`is_fully_withdrawable_validator`](#is_fully_withdrawable_validator)
- [`is_partially_withdrawable_validator`](#is_partially_withdrawable_validator)
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- [Beacon chain state transition function](#beacon-chain-state-transition-function)
- [Block processing](#block-processing)
- [New `get_expected_withdrawals`](#new-get_expected_withdrawals)
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- [New `process_withdrawals`](#new-process_withdrawals)
- [Modified `process_execution_payload`](#modified-process_execution_payload)
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- [Modified `process_operations`](#modified-process_operations)
- [New `process_bls_to_execution_change`](#new-process_bls_to_execution_change)
- [Testing](#testing)
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<!-- END doctoc generated TOC please keep comment here to allow auto update -->
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## Introduction
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Capella is a consensus-layer upgrade containing a number of features related
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to validator withdrawals. Including:
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* Automatic withdrawals of `withdrawable` validators.
* Partial withdrawals sweep for validators with 0x01 withdrawal
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credentials and balances in excess of `MAX_EFFECTIVE_BALANCE`.
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* Operation to change from `BLS_WITHDRAWAL_PREFIX` to
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`ETH1_ADDRESS_WITHDRAWAL_PREFIX` versioned withdrawal credentials to enable withdrawals for a validator.
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## 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` |
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## Constants
### Domain types
| Name | Value |
| - | - |
| `DOMAIN_BLS_TO_EXECUTION_CHANGE` | `DomainType('0x0A000000')` |
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## Preset
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### Max operations per block
| Name | Value |
| - | - |
| `MAX_BLS_TO_EXECUTION_CHANGES` | `2**4` (= 16) |
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### Execution
| Name | Value | Description |
| - | - | - |
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| `MAX_WITHDRAWALS_PER_PAYLOAD` | `uint64(2**4)` (= 16) | Maximum amount of withdrawals allowed in each payload |
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## Containers
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### New containers
#### `Withdrawal`
```python
class Withdrawal(Container):
index: WithdrawalIndex
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validator_index: ValidatorIndex
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address: ExecutionAddress
amount: Gwei
```
#### `BLSToExecutionChange`
```python
class BLSToExecutionChange(Container):
validator_index: ValidatorIndex
from_bls_pubkey: BLSPubkey
to_execution_address: ExecutionAddress
```
#### `SignedBLSToExecutionChange`
```python
class SignedBLSToExecutionChange(Container):
message: BLSToExecutionChange
signature: BLSSignature
```
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### Extended Containers
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#### `ExecutionPayload`
```python
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`
```python
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]
```
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#### `BeaconBlockBody`
```python
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]
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```
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#### `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[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
# Withdrawals
next_withdrawal_index: WithdrawalIndex # [New in Capella]
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last_withdrawal_validator_index: ValidatorIndex # [New in Capella]
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```
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## Helpers
### Predicates
#### `has_eth1_withdrawal_credential`
```python
def has_eth1_withdrawal_credential(validator: Validator) -> bool:
"""
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Check if ``validator`` has an 0x01 prefixed "eth1" withdrawal credential.
"""
return validator.withdrawal_credentials[:1] == ETH1_ADDRESS_WITHDRAWAL_PREFIX
```
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#### `is_fully_withdrawable_validator`
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```python
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def is_fully_withdrawable_validator(validator: Validator, balance: Gwei, epoch: Epoch) -> bool:
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"""
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Check if ``validator`` is fully withdrawable.
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"""
return (
has_eth1_withdrawal_credential(validator)
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and validator.withdrawable_epoch <= epoch
and balance > 0
)
```
#### `is_partially_withdrawable_validator`
```python
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
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```
## Beacon chain state transition function
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### Block processing
```python
def process_block(state: BeaconState, block: BeaconBlock) -> None:
process_block_header(state, block)
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if is_execution_enabled(state, block.body):
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process_withdrawals(state, block.body.execution_payload) # [New in Capella]
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process_execution_payload(state, block.body.execution_payload, EXECUTION_ENGINE) # [Modified in Capella]
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process_randao(state, block.body)
process_eth1_data(state, block.body)
process_operations(state, block.body)
process_sync_aggregate(state, block.body.sync_aggregate)
```
#### New `get_expected_withdrawals`
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```python
def get_expected_withdrawals(state: BeaconState) -> Sequence[Withdrawal]:
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epoch = get_current_epoch(state)
withdrawal_index = state.next_withdrawal_index
index = ValidatorIndex((state.last_withdrawal_validator_index + 1) % len(state.validators))
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ret: List[Withdrawal] = []
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for probed in range(len(state.validators)):
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val = state.validators[index]
balance = state.balances[index]
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if is_fully_withdrawable_validator(val, balance, epoch):
withdrawal = Withdrawal(
index=withdrawal_index,
validator_index=index,
address=ExecutionAddress(val.withdrawal_credentials[12:]),
amount=balance,
)
ret.append(withdrawal)
withdrawal_index = WithdrawalIndex(withdrawal_index + 1)
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elif is_partially_withdrawable_validator(val, balance):
withdrawal = Withdrawal(
index=withdrawal_index,
validator_index=index,
address=ExecutionAddress(val.withdrawal_credentials[12:]),
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amount=balance - MAX_EFFECTIVE_BALANCE,
)
ret.append(withdrawal)
withdrawal_index = WithdrawalIndex(withdrawal_index + 1)
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if len(ret) == MAX_WITHDRAWALS_PER_PAYLOAD:
break
probed += 1
index = ValidatorIndex((index + probed) % len(state.validators))
return ret
```
#### New `process_withdrawals`
```python
def process_withdrawals(state: BeaconState, payload: ExecutionPayload) -> None:
expected_withdrawals = get_expected_withdrawals(state)
assert len(payload.withdrawals) == len(expected_withdrawals)
for expected_withdrawal, withdrawal in zip(get_expected_withdrawals(state), payload.withdrawals):
assert withdrawal == expected_withdrawal
decrease_balance(state, withdrawal.validator_index, withdrawal.amount)
state.next_withdrawal_index = WithdrawalIndex(withdrawal.index + 1)
state.last_withdrawal_validator_index = withdrawal.validator_index
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```
#### Modified `process_execution_payload`
*Note*: The function `process_execution_payload` is modified to use the new `ExecutionPayloadHeader` type.
```python
def process_execution_payload(state: BeaconState, payload: ExecutionPayload, execution_engine: ExecutionEngine) -> None:
# Verify consistency of the parent hash with respect to the previous execution payload header
if is_merge_transition_complete(state):
assert payload.parent_hash == state.latest_execution_payload_header.block_hash
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# Verify prev_randao
assert payload.prev_randao == get_randao_mix(state, get_current_epoch(state))
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# Verify timestamp
assert payload.timestamp == compute_timestamp_at_slot(state, state.slot)
# Verify the execution payload is valid
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assert execution_engine.notify_new_payload(payload)
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# 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,
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receipts_root=payload.receipts_root,
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logs_bloom=payload.logs_bloom,
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prev_randao=payload.prev_randao,
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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),
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withdrawals_root=hash_tree_root(payload.withdrawals), # [New in Capella]
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)
```
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#### Modified `process_operations`
*Note*: The function `process_operations` is modified to process `BLSToExecutionChange` operations included in the block.
```python
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]
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```
#### New `process_bls_to_execution_change`
```python
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:]
domain = get_domain(state, DOMAIN_BLS_TO_EXECUTION_CHANGE)
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
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+ address_change.to_execution_address
)
```
## Testing
*Note*: The function `initialize_beacon_state_from_eth1` is modified for pure Capella testing only.
Modifications include:
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1. Use `CAPELLA_FORK_VERSION` as the previous and current fork version.
2. Utilize the Capella `BeaconBlockBody` when constructing the initial `latest_block_header`.
```python
def initialize_beacon_state_from_eth1(eth1_block_hash: Hash32,
eth1_timestamp: uint64,
deposits: Sequence[Deposit],
execution_payload_header: ExecutionPayloadHeader=ExecutionPayloadHeader()
) -> BeaconState:
fork = Fork(
previous_version=CAPELLA_FORK_VERSION, # [Modified in Capella] for testing only
current_version=CAPELLA_FORK_VERSION, # [Modified in Capella]
epoch=GENESIS_EPOCH,
)
state = BeaconState(
genesis_time=eth1_timestamp + GENESIS_DELAY,
fork=fork,
eth1_data=Eth1Data(block_hash=eth1_block_hash, deposit_count=uint64(len(deposits))),
latest_block_header=BeaconBlockHeader(body_root=hash_tree_root(BeaconBlockBody())),
randao_mixes=[eth1_block_hash] * EPOCHS_PER_HISTORICAL_VECTOR, # Seed RANDAO with Eth1 entropy
)
# Process deposits
leaves = list(map(lambda deposit: deposit.data, deposits))
for index, deposit in enumerate(deposits):
deposit_data_list = List[DepositData, 2**DEPOSIT_CONTRACT_TREE_DEPTH](*leaves[:index + 1])
state.eth1_data.deposit_root = hash_tree_root(deposit_data_list)
process_deposit(state, deposit)
# Process activations
for index, validator in enumerate(state.validators):
balance = state.balances[index]
validator.effective_balance = min(balance - balance % EFFECTIVE_BALANCE_INCREMENT, MAX_EFFECTIVE_BALANCE)
if validator.effective_balance == MAX_EFFECTIVE_BALANCE:
validator.activation_eligibility_epoch = GENESIS_EPOCH
validator.activation_epoch = GENESIS_EPOCH
# Set genesis validators root for domain separation and chain versioning
state.genesis_validators_root = hash_tree_root(state.validators)
# Fill in sync committees
# Note: A duplicate committee is assigned for the current and next committee at genesis
state.current_sync_committee = get_next_sync_committee(state)
state.next_sync_committee = get_next_sync_committee(state)
# Initialize the execution payload header
state.latest_execution_payload_header = execution_payload_header
return state
```