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Capella -- Honest Validator

Notice: This document is a work-in-progress for researchers and implementers.

Table of contents

Introduction

This document represents the changes to be made in the code of an "honest validator" to implement the Capella upgrade.

Prerequisites

This document is an extension of the Bellatrix -- Honest Validator guide. All behaviors and definitions defined in this document, and documents it extends, carry over unless explicitly noted or overridden.

All terminology, constants, functions, and protocol mechanics defined in the updated Beacon Chain doc of Capella are requisite for this document and used throughout. Please see related Beacon Chain doc before continuing and use them as a reference throughout.

Helpers

Modified GetPayloadResponse

@dataclass
class GetPayloadResponse(object):
    execution_payload: ExecutionPayload
    block_value: uint256

Protocols

ExecutionEngine

Modified get_payload

get_payload returns the upgraded Capella ExecutionPayload type.

Beacon chain responsibilities

All validator responsibilities remain unchanged other than those noted below.

Block proposal

Constructing the BeaconBlockBody

ExecutionPayload

ExecutionPayloads are constructed as they were in Bellatrix, except that the expected withdrawals for the slot must be gathered from the state (utilizing the helper get_expected_withdrawals) and passed into the ExecutionEngine within prepare_execution_payload.

Note: In this section, state is the state of the slot for the block proposal without the block yet applied. That is, state is the previous_state processed through any empty slots up to the assigned slot using process_slots(previous_state, slot).

Note: The only change made to prepare_execution_payload is to call get_expected_withdrawals() to set the new withdrawals field of PayloadAttributes.

def prepare_execution_payload(state: BeaconState,
                              safe_block_hash: Hash32,
                              finalized_block_hash: Hash32,
                              suggested_fee_recipient: ExecutionAddress,
                              execution_engine: ExecutionEngine) -> Optional[PayloadId]:
    # [Modified in Capella] Removed `is_merge_transition_complete` check in Capella
    parent_hash = state.latest_execution_payload_header.block_hash

    # Set the forkchoice head and initiate the payload build process
    payload_attributes = PayloadAttributes(
        timestamp=compute_timestamp_at_slot(state, state.slot),
        prev_randao=get_randao_mix(state, get_current_epoch(state)),
        suggested_fee_recipient=suggested_fee_recipient,
        withdrawals=get_expected_withdrawals(state),  # [New in Capella]
    )
    return execution_engine.notify_forkchoice_updated(
        head_block_hash=parent_hash,
        safe_block_hash=safe_block_hash,
        finalized_block_hash=finalized_block_hash,
        payload_attributes=payload_attributes,
    )
BLS to execution changes

Up to MAX_BLS_TO_EXECUTION_CHANGES, BLSToExecutionChange objects can be included in the block. The BLS to execution changes must satisfy the verification conditions found in BLS to execution change processing.

Enabling validator withdrawals

Validator balances are withdrawn periodically via an automatic process. For exited validators, the full balance is withdrawn. For active validators, the balance in excess of MAX_EFFECTIVE_BALANCE is withdrawn.

There is one prerequisite for this automated process: the validator's withdrawal credentials pointing to an execution layer address, i.e. having an ETH1_ADDRESS_WITHDRAWAL_PREFIX.

If a validator has a BLS_WITHDRAWAL_PREFIX withdrawal credential prefix, to participate in withdrawals the validator must create a one-time message to change their withdrawal credential from the version authenticated with a BLS key to the version compatible with the execution layer. This message -- a BLSToExecutionChange -- is available starting in Capella

Validators who wish to enable withdrawals MUST assemble, sign, and broadcast this message so that it is accepted on the beacon chain. Validators who do not want to enable withdrawals and have the BLS_WITHDRAWAL_PREFIX version of withdrawal credentials can delay creating this message until they are ready to enable withdrawals.

Changing from BLS to execution withdrawal credentials

First, the validator must construct a valid BLSToExecutionChange message. This message contains the validator_index for the validator who wishes to change their credentials, the from_bls_pubkey -- the BLS public key corresponding to the withdrawal BLS secret key used to form the BLS_WITHDRAWAL_PREFIX withdrawal credential, and the to_execution_address specifying the execution layer address to which the validator's balances will be withdrawn.

Note: The withdrawal key pair used to construct the BLS_WITHDRAWAL_PREFIX withdrawal credential should be distinct from the signing key pair used to operate the validator under typical circumstances. Consult your validator deposit tooling documentation for further details if you are not aware of the difference.

Warning: This message can only be included on-chain once and is irreversible so ensure the correctness and accessibility to to_execution_address.

Next, the validator signs the assembled message: BLSToExecutionChange with the withdrawal BLS secret key and this signature is placed into a SignedBLSToExecutionChange message along with the inner BLSToExecutionChange message. Note that the SignedBLSToExecutionChange message should pass all of the validations in process_bls_to_execution_change.

The SignedBLSToExecutionChange message should then be submitted to the consensus layer network. Once included on-chain, the withdrawal credential change takes effect. No further action is required for a validator to enter into the automated withdrawal process.

Note: A node should prioritize locally received BLSToExecutionChange operations to ensure these changes make it on-chain through self published blocks even if the rest of the network censors.