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ePBS -- Honest Validator

This document represents the changes and additions to the Honest validator guide included in the ePBS fork.

Table of Contents

Validator assignment

A validator may be a member of the new Payload Timeliness Committee (PTC) for a given slot. To check for PTC assignments the validator uses the helper get_ptc_assignment(state, epoch, validator_index) where epoch <= next_epoch.

PTC committee selection is only stable within the context of the current and next epoch.

def get_ptc_assignment(
        state: BeaconState,
        epoch: Epoch,
        validator_index: ValidatorIndex) -> Optional[Slot]:
    """
    Returns the slot during the requested epoch in which the validator with index `validator_index`
    is a member of the PTC. Returns None if no assignment is found. 
    """
    next_epoch = Epoch(get_current_epoch(state) + 1)
    assert epoch <= next_epoch

    start_slot = compute_start_slot_at_epoch(epoch)
    for slot in range(start_slot, start_slot + SLOTS_PER_EPOCH):
        if validator_index in get_ptc(state, Slot(slot)):
            return Slot(slot)
    return None

Lookahead

[New in EIP-7732]

get_ptc_assignment should be called at the start of each epoch to get the assignment for the next epoch (current_epoch + 1). A validator should plan for future assignments by noting their assigned PTC slot.

Beacon chain responsibilities

All validator responsibilities remain unchanged other than the following:

  • Proposers are no longer required to broadcast BlobSidecar objects, as this becomes a builder's duty.
  • Some validators are selected per slot to become PTC members, these validators must broadcast PayloadAttestationMessage objects during the assigned slot before the deadline of 3 * SECONDS_PER_SLOT // INTERVALS_PER_SLOT seconds into the slot.

Attestation

Attestation duties are not changed for validators, however the attestation deadline is implicitly changed by the change in INTERVALS_PER_SLOT.

Sync Committee participations

Sync committee duties are not changed for validators, however the submission deadline is implicitly changed by the change in INTERVALS_PER_SLOT.

Block proposal

Validators are still expected to propose SignedBeaconBlock at the beginning of any slot during which is_proposer(state, validator_index) returns true. The mechanism to prepare this beacon block and related sidecars differs from previous forks as follows

Constructing the new signed_execution_payload_header_envelope field in BeaconBlockBody

To obtain signed_execution_payload_header, a block proposer building a block on top of a state must take the following actions:

  • Listen to the execution_payload_header gossip global topic and save an accepted signed_execution_payload_header from a builder. Proposer MAY obtain these signed messages by other off-protocol means.
  • The signed_execution_payload_header must satisfy the verification conditions found in process_execution_payload_header, that is
    • The header signature must be valid
    • The builder balance can cover the header value
    • The header slot is for the proposal block slot
    • The header parent block hash equals the state's latest_block_hash.
    • The header parent block root equals the current block's parent_root.
  • Select one bid and set body.signed_execution_payload_header = signed_execution_payload_header

Constructing the new payload_attestations field in BeaconBlockBody

Up to MAX_PAYLOAD_ATTESTATIONS, aggregate payload attestations can be included in the block. The validator will have to

  • Listen to the payload_attestation_message gossip global topic
  • The payload attestations added must satisfy the verification conditions found in payload attestation gossip validation and payload attestation processing. This means
    • The data.beacon_block_root corresponds to block.parent_root.
    • The slot of the parent block is exactly one slot before the proposing slot.
    • The signature of the payload attestation data message verifies correctly.
  • The proposer needs to aggregate all payload attestations with the same data into a given PayloadAttestation object. For this it needs to fill the aggregation_bits field by using the relative position of the validator indices with respect to the PTC that is obtained from get_ptc(state, block_slot - 1).
  • The proposer should only include payload attestations that are consistent with the current block they are proposing. That is, if the previous block had a payload, they should only include attestations with payload_status = PAYLOAD_PRESENT. Proposers are penalized for attestations that are not-consistent with their view.

Blob sidecars

The blob sidecars are no longer broadcast by the validator, and thus their construction is not necessary. This deprecates the corresponding sections from the honest validator guide in the Electra fork, moving them, albeit with some modifications, to the honest Builder guide

Payload timeliness attestation

Some validators are selected to submit payload timeliness attestations. Validators should call get_ptc_assignment at the beginning of an epoch to be prepared to submit their PTC attestations during the next epoch.

A validator should create and broadcast the payload_attestation_message to the global execution attestation subnet not after SECONDS_PER_SLOT * 3 / INTERVALS_PER_SLOT seconds since the start of slot

Constructing a payload attestation

If a validator is in the payload attestation committee for the current slot (as obtained from get_ptc_assignment above) then the validator should prepare a PayloadAttestationMessage for the current slot, according to the logic in get_payload_attestation_message below and broadcast it not after SECONDS_PER_SLOT * 3 / INTERVALS_PER_SLOT seconds since the start of the slot, to the global payload_attestation_message pubsub topic.

The validator creates payload_attestation_message as follows:

  • If the validator has not seen any beacon block for the assigned slot, do not submit a payload attestation. It will be ignored anyway.
  • Set data.beacon_block_root be the HTR of the beacon block seen for the assigned slot
  • Set data.slot to be the assigned slot.
  • Set data.payload_status as follows
    • If a SignedExecutionPayloadEnvelope has been seen referencing the block data.beacon_block_root and the envelope has payload_withheld = False, set to PAYLOAD_PRESENT.
    • If a SignedExecutionPayloadEnvelope has been seen referencing the block data.beacon_block_root and the envelope has payload_withheld = True, set to PAYLOAD_WITHHELD.
    • If no SignedExecutionPayloadEnvelope has been seen referencing the block data.beacon_block_root set to PAYLOAD_ABSENT.
  • Set payload_attestation_message.validator_index = validator_index where validator_index is the validator chosen to submit. The private key mapping to state.validators[validator_index].pubkey is used to sign the payload timeliness attestation.
  • Sign the payload_attestation_message.data using the helper get_payload_attestation_message_signature.

Notice that the attester only signs the PayloadAttestationData and not the validator_index field in the message. Proposers need to aggregate these attestations as described above.

def get_payload_attestation_message_signature(
        state: BeaconState, attestation: PayloadAttestationMessage, privkey: int) -> BLSSignature:
    domain = get_domain(state, DOMAIN_PTC_ATTESTER, compute_epoch_at_slot(attestation.data.slot))
    signing_root = compute_signing_root(attestation.data, domain)
    return bls.Sign(privkey, signing_root)

Remark Validators do not need to check the full validity of the ExecutionPayload contained in within the envelope, but the checks in the P2P guide should pass for the SignedExecutionPayloadEnvelope.