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

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# Capella -- The Beacon Chain
## Table of contents
<!-- TOC -->
<!-- START doctoc generated TOC please keep comment here to allow auto update -->
<!-- DON'T EDIT THIS SECTION, INSTEAD RE-RUN doctoc TO UPDATE -->
- [Introduction](#introduction)
- [Custom types](#custom-types)
- [Domain types](#domain-types)
- [Preset](#preset)
- [Max operations per block](#max-operations-per-block)
- [Execution](#execution)
- [Withdrawals processing](#withdrawals-processing)
- [Containers](#containers)
- [New containers](#new-containers)
- [`Withdrawal`](#withdrawal)
- [`BLSToExecutionChange`](#blstoexecutionchange)
- [`SignedBLSToExecutionChange`](#signedblstoexecutionchange)
- [`HistoricalSummary`](#historicalsummary)
- [Extended Containers](#extended-containers)
- [`ExecutionPayload`](#executionpayload)
- [`ExecutionPayloadHeader`](#executionpayloadheader)
- [`BeaconBlockBody`](#beaconblockbody)
- [`BeaconState`](#beaconstate)
- [Helpers](#helpers)
- [Predicates](#predicates)
- [`has_eth1_withdrawal_credential`](#has_eth1_withdrawal_credential)
- [`is_fully_withdrawable_validator`](#is_fully_withdrawable_validator)
- [`is_partially_withdrawable_validator`](#is_partially_withdrawable_validator)
- [Beacon chain state transition function](#beacon-chain-state-transition-function)
- [Epoch processing](#epoch-processing)
- [Historical summaries updates](#historical-summaries-updates)
- [Block processing](#block-processing)
- [New `get_expected_withdrawals`](#new-get_expected_withdrawals)
- [New `process_withdrawals`](#new-process_withdrawals)
- [Modified `process_execution_payload`](#modified-process_execution_payload)
- [Modified `process_operations`](#modified-process_operations)
- [New `process_bls_to_execution_change`](#new-process_bls_to_execution_change)
- [Testing](#testing)
<!-- END doctoc generated TOC please keep comment here to allow auto update -->
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## 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 state/block historical accumulators. It becomes possible to validate
the entire block history that led up to that particular state without executing the transitions
and checking them one by one in backward order using a parent chain.
## 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`
```python
class Withdrawal(Container):
index: WithdrawalIndex
validator_index: ValidatorIndex
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
```
#### `HistoricalSummary`
```python
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`
```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]
```
#### `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]
```
#### `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] # 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
# 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`
```python
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`
```python
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`
```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
```
## Beacon chain state transition function
### Epoch processing
*Note*: The function `process_historical_summaries_update` replaces `process_historical_roots_update` in Bellatrix.
```python
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
```python
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
```python
def process_block(state: BeaconState, block: BeaconBlock) -> None:
process_block_header(state, block)
if is_execution_enabled(state, block.body):
process_withdrawals(state, block.body.execution_payload) # [New in Capella]
process_execution_payload(state, block.body.execution_payload, 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`
```python
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`
```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(expected_withdrawals, payload.withdrawals):
assert withdrawal == expected_withdrawal
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.
```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
# 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.notify_new_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.
```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]
```
#### 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
+ address_change.to_execution_address
)
```
## Testing
*Note*: The function `initialize_beacon_state_from_eth1` is modified for pure Capella testing only.
Modifications include:
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
```
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