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Gloas -- Honest Builder

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

Introduction

This is an accompanying document which describes the expected actions of a "builder" participating in the Ethereum proof-of-stake protocol.

With the Gloas upgrade, the protocol introduces a new type of staked actor (not a validator) called a builder. Since builders are not validators, they do not perform validator duties (e.g., attesting and proposing) and therefore do not earn yield on their stake. Builders have the option to produce execution payloads by submitting bids. This document is a collection of guidelines for builders.

Becoming a builder

Builder withdrawal credentials

When submitting a deposit to the deposit contract, the withdrawal_credentials field determines whether the staked actor will be a validator or a builder. To be recognized as a builder, the withdrawal_credentials must use the BUILDER_WITHDRAWAL_PREFIX.

The withdrawal_credentials field must be:

  • withdrawal_credentials[:1] == BUILDER_WITHDRAWAL_PREFIX (0x03)
  • withdrawal_credentials[1:12] == b'\x00' * 11
  • withdrawal_credentials[12:] == builder_execution_address

Where builder_execution_address is an execution-layer address that will receive withdrawals.

Submit deposit

Builders follow the same deposit process as validators, but with the builder-specific withdrawal credentials. The deposit must include:

  • pubkey: The builder's BLS public key.
  • withdrawal_credentials: With the BUILDER_WITHDRAWAL_PREFIX (0x03) prefix.
  • amount: At least MIN_DEPOSIT_AMOUNT gwei.
  • signature: BLS signature over the deposit data.

Process deposit

The beacon chain processes builder deposits identically to validator deposits, with the withdrawal credentials using BUILDER_WITHDRAWAL_PREFIX.

Builder index

Once the deposit is processed, the builder is assigned a unique builder_index within the builder registry. This index is used to identify the builder in execution payload bids and envelopes.

Activation

Builders are active as soon as the deposit is processed on the consensus layer.

Builder activities

Builders have two optional activities: submitting bids and submitting payloads. Builders can submit bids to produce execution payloads. They can broadcast these bids in the form of SignedExecutionPayloadBid objects. These objects encode a commitment to reveal an execution payload in exchange for a payment. When their bids are chosen by the corresponding proposer, builders are expected to broadcast an accompanying SignedExecutionPayloadEnvelope object honoring the commitment. If a proposer accepts a builder's bid, the builder will pay the proposer what it promised whether it submits the payload or not.

Constructing the SignedExecutionPayloadBid

Builders can broadcast a payload bid for the current or the next slot's proposer to include. They produce a SignedExecutionPayloadBid as follows.

  1. Set bid.parent_block_hash to the current head of the execution chain (this can be obtained from the beacon state as state.latest_block_hash).
  2. Set bid.parent_block_root to be the head of the consensus chain; this can be obtained from the beacon state as hash_tree_root(state.latest_block_header). The parent_block_root and parent_block_hash must be compatible, in the sense that they both should come from the same state by the method described in this and the previous point.
  3. Construct an execution payload. This can be performed with an external execution engine via a call to engine_getPayloadV5.
  4. Set bid.block_hash to be the block hash of the constructed payload, that is payload.block_hash.
  5. Set bid.prev_randao to be the previous RANDAO of the constructed payload, that is payload.prev_randao.
  6. Set bid.fee_recipient to be an execution address to receive the payment. The proposer's preferred fee recipient can be obtained from the SignedProposerPreferences associated with bid.slot.
  7. Set bid.gas_limit to be the gas limit of the constructed payload. The proposer's preferred gas limit can be obtained from the SignedProposerPreferences associated with bid.slot.
  8. Set bid.builder_index to be the index of the builder performing these actions.
  9. Set bid.slot to be the slot for which this bid is aimed. This slot MUST be either the current slot or the next slot.
  10. Set bid.value to be the value (in gwei) that the builder will pay the proposer if the bid is accepted. The builder MUST have enough excess balance to fulfill this bid and all pending payments.
  11. Set bid.execution_payment to zero. A non-zero value indicates a trusted execution-layer payment. Bids with non-zero execution_payment MUST NOT be broadcast to the execution_payload_bid gossip topic.
  12. Set bid.blob_kzg_commitments_root to be the hash_tree_root of the blobsbundle.commitments field returned by engine_getPayloadV5.

After building the bid, the builder obtains a signature of the bid by using:

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def get_execution_payload_bid_signature(
    state: BeaconState, bid: ExecutionPayloadBid, privkey: int
) -> BLSSignature:
    domain = get_domain(state, DOMAIN_BEACON_BUILDER, compute_epoch_at_slot(bid.slot))
    signing_root = compute_signing_root(bid, domain)
    return bls.Sign(privkey, signing_root)

Then the builder assembles signed_execution_payload_bid = SignedExecutionPayloadBid(message=bid, signature=signature) and broadcasts it on the execution_payload_bid global gossip topic.

Constructing the DataColumnSidecars

Modified get_data_column_sidecars

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def get_data_column_sidecars(
    # [Modified in Gloas:EIP7732]
    # Removed `signed_block_header`
    # [New in Gloas:EIP7732]
    beacon_block_root: Root,
    # [New in Gloas:EIP7732]
    slot: Slot,
    kzg_commitments: List[KZGCommitment, MAX_BLOB_COMMITMENTS_PER_BLOCK],
    # [Modified in Gloas:EIP7732]
    # Removed `kzg_commitments_inclusion_proof`
    cells_and_kzg_proofs: Sequence[
        Tuple[Vector[Cell, CELLS_PER_EXT_BLOB], Vector[KZGProof, CELLS_PER_EXT_BLOB]]
    ],
) -> Sequence[DataColumnSidecar]:
    """
    Given a beacon block root and the commitments, cells/proofs associated with
    each blob in the block, assemble the sidecars which can be distributed to peers.
    """
    assert len(cells_and_kzg_proofs) == len(kzg_commitments)

    sidecars = []
    for column_index in range(NUMBER_OF_COLUMNS):
        column_cells, column_proofs = [], []
        for cells, proofs in cells_and_kzg_proofs:
            column_cells.append(cells[column_index])
            column_proofs.append(proofs[column_index])
        sidecars.append(
            DataColumnSidecar(
                index=column_index,
                column=column_cells,
                kzg_commitments=kzg_commitments,
                kzg_proofs=column_proofs,
                slot=slot,
                beacon_block_root=beacon_block_root,
            )
        )
    return sidecars

Modified get_data_column_sidecars_from_block

Note: The function get_data_column_sidecars_from_block is modified to include the list of blob KZG commitments and to use beacon_block_root instead of header and inclusion proof computations.

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def get_data_column_sidecars_from_block(
    signed_block: SignedBeaconBlock,
    # [New in Gloas:EIP7732]
    blob_kzg_commitments: List[KZGCommitment, MAX_BLOB_COMMITMENTS_PER_BLOCK],
    cells_and_kzg_proofs: Sequence[
        Tuple[Vector[Cell, CELLS_PER_EXT_BLOB], Vector[KZGProof, CELLS_PER_EXT_BLOB]]
    ],
) -> Sequence[DataColumnSidecar]:
    """
    Given a signed block and the cells/proofs associated with each blob in the
    block, assemble the sidecars which can be distributed to peers.
    """
    beacon_block_root = hash_tree_root(signed_block.message)
    return get_data_column_sidecars(
        beacon_block_root,
        signed_block.message.slot,
        blob_kzg_commitments,
        cells_and_kzg_proofs,
    )

Constructing the SignedExecutionPayloadEnvelope

When the proposer publishes a valid SignedBeaconBlock containing a signed commitment by the builder, the builder is later expected to broadcast the corresponding SignedExecutionPayloadEnvelope that fulfills this commitment. See below for a special case of an honestly withheld payload.

To construct the ExecutionPayloadEnvelope the builder must perform the following steps. We alias block to be the corresponding BeaconBlock and alias bid to be the committed ExecutionPayloadBid in block.body.signed_execution_payload_bid.message.

  1. Set envelope.payload to be the ExecutionPayload constructed when creating the corresponding bid. This payload MUST have the same block hash as bid.block_hash.
  2. Set envelope.execution_requests to be the ExecutionRequests associated with payload.
  3. Set envelope.builder_index to be the index of the builder performing these steps. This field MUST be bid.builder_index.
  4. Set envelope.beacon_block_root to be hash_tree_root(block).
  5. Set envelope.slot to be block.slot.
  6. Set envelope.blob_kzg_commitments to be the commitments field of the blobs bundle constructed when constructing the bid. This field MUST have a hash_tree_root equal to bid.blob_kzg_commitments_root.

After setting these parameters, the builder assembles signed_execution_payload_envelope = SignedExecutionPayloadEnvelope(message=envelope, signature=BLSSignature()), then verify that the envelope is valid with process_execution_payload(state, signed_execution_payload_envelope, execution_engine, verify=False). This function should not trigger an exception.

  1. Set envelope.state_root to hash_tree_root(state).

After preparing the envelope the builder should sign the envelope using:

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def get_execution_payload_envelope_signature(
    state: BeaconState, envelope: ExecutionPayloadEnvelope, privkey: int
) -> BLSSignature:
    domain = get_domain(state, DOMAIN_BEACON_BUILDER, compute_epoch_at_slot(state.slot))
    signing_root = compute_signing_root(envelope, domain)
    return bls.Sign(privkey, signing_root)

Then the builder assembles signed_execution_payload_envelope = SignedExecutionPayloadEnvelope(message=envelope, signature=signature) and broadcasts it on the execution_payload global gossip topic.

Honest payload withheld messages

An honest builder that has seen a SignedBeaconBlock referencing his signed bid, but that block was not timely and thus it is not the head of the builder's chain, may choose to withhold their execution payload. For this, the builder should act as if no block was produced and not broadcast the payload.