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EIP-4844 -- 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 EIP-4844.

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 EIP4844 are requisite for this document and used throughout. Please see related Beacon Chain doc before continuing and use them as a reference throughout.

Custom types

Name SSZ equivalent Description
Polynomial List[BLSFieldElement, FIELD_ELEMENTS_PER_BLOB] a polynomial in evaluation form

Containers

BlobsAndCommitments

class BlobsAndCommitments(Container):
    blobs: List[Blob, MAX_BLOBS_PER_BLOCK]
    kzg_commitments: List[KZGCommitment, MAX_BLOBS_PER_BLOCK]

PolynomialAndCommitment

class PolynomialAndCommitment(Container):
    polynomial: Polynomial
    kzg_commitment: KZGCommitment

Helpers

is_data_available

The implementation of is_data_available is meant to change with later sharding upgrades. Initially, it requires every verifying actor to retrieve the matching BlobsSidecar, and validate the sidecar with validate_blobs_sidecar.

Without the sidecar the block may be processed further optimistically, but MUST NOT be considered valid until a valid BlobsSidecar has been downloaded.

def is_data_available(slot: Slot, beacon_block_root: Root, blob_kzg_commitments: Sequence[KZGCommitment]) -> bool:
    # `retrieve_blobs_sidecar` is implementation dependent, raises an exception if not available.
    sidecar = retrieve_blobs_sidecar(slot, beacon_block_root)
    validate_blobs_sidecar(slot, beacon_block_root, blob_kzg_commitments, sidecar)

    return True

hash_to_bls_field

def hash_to_bls_field(x: Container) -> BLSFieldElement:
    """
    Compute 32-byte hash of serialized container and convert it to BLS field.
    The output is not uniform over the BLS field.
    """
    return bytes_to_bls_field(hash(ssz_serialize(x)))

compute_powers

def compute_powers(x: BLSFieldElement, n: uint64) -> Sequence[BLSFieldElement]:
    """
    Return ``x`` to power of [0, n-1].
    """
    current_power = 1
    powers = []
    for _ in range(n):
        powers.append(BLSFieldElement(current_power))
        current_power = current_power * int(x) % BLS_MODULUS
    return powers

compute_aggregated_poly_and_commitment

def compute_aggregated_poly_and_commitment(
        blobs: Sequence[Blob],
        kzg_commitments: Sequence[KZGCommitment]) -> Tuple[Polynomial, KZGCommitment]:
    """
    Return the aggregated polynomial and aggregated KZG commitment.
    """
    # Generate random linear combination challenges
    r = hash_to_bls_field(BlobsAndCommitments(blobs=blobs, kzg_commitments=kzg_commitments))
    r_powers = compute_powers(r, len(kzg_commitments))

    # Create aggregated polynomial in evaluation form
    aggregated_poly = Polynomial(vector_lincomb(blobs, r_powers))

    # Compute commitment to aggregated polynomial
    aggregated_poly_commitment = KZGCommitment(g1_lincomb(kzg_commitments, r_powers))

    return aggregated_poly, aggregated_poly_commitment

validate_blobs_sidecar

def validate_blobs_sidecar(slot: Slot,
                           beacon_block_root: Root,
                           expected_kzg_commitments: Sequence[KZGCommitment],
                           blobs_sidecar: BlobsSidecar) -> None:
    assert slot == blobs_sidecar.beacon_block_slot
    assert beacon_block_root == blobs_sidecar.beacon_block_root
    blobs = blobs_sidecar.blobs
    kzg_aggregated_proof = blobs_sidecar.kzg_aggregated_proof
    assert len(expected_kzg_commitments) == len(blobs)

    aggregated_poly, aggregated_poly_commitment = compute_aggregated_poly_and_commitment(
        blobs,
        expected_kzg_commitments,
    )

    # Generate challenge `x` and evaluate the aggregated polynomial at `x`
    x = hash_to_bls_field(
        PolynomialAndCommitment(polynomial=aggregated_poly, kzg_commitment=aggregated_poly_commitment)
    )
    # Evaluate aggregated polynomial at `x` (evaluation function checks for div-by-zero)
    y = evaluate_polynomial_in_evaluation_form(aggregated_poly, x)

    # Verify aggregated proof
    assert verify_kzg_proof(aggregated_poly_commitment, x, y, kzg_aggregated_proof)

compute_proof_from_blobs

def compute_proof_from_blobs(blobs: Sequence[Blob]) -> KZGProof:
    commitments = [blob_to_kzg_commitment(blob) for blob in blobs]
    aggregated_poly, aggregated_poly_commitment = compute_aggregated_poly_and_commitment(blobs, commitments)
    x = hash_to_bls_field(PolynomialAndCommitment(
        polynomial=aggregated_poly,
        kzg_commitment=aggregated_poly_commitment,
    ))
    return compute_kzg_proof(aggregated_poly, x)

get_blobs_and_kzg_commitments

The interface to retrieve blobs and corresponding kzg commitments.

Note: This API is unstable. get_blobs_and_kzg_commitments and get_payload may be unified. Implementers may also retrieve blobs individually per transaction.

def get_blobs_and_kzg_commitments(payload_id: PayloadId) -> Tuple[Sequence[BLSFieldElement], Sequence[KZGCommitment]]:
    ...

Beacon chain responsibilities

All validator responsibilities remain unchanged other than those noted below. Namely, the blob handling and the addition of BlobsSidecar.

Block proposal

Constructing the BeaconBlockBody

Blob KZG commitments
  1. After retrieving the execution payload from the execution engine as specified in Bellatrix, use the payload_id to retrieve blobs and blob_kzg_commitments via get_blobs_and_kzg_commitments(payload_id).
  2. Validate blobs and blob_kzg_commitments:
def validate_blobs_and_kzg_commitments(execution_payload: ExecutionPayload,
                                       blobs: Sequence[Blob],
                                       blob_kzg_commitments: Sequence[KZGCommitment]) -> None:
    # Optionally sanity-check that the KZG commitments match the versioned hashes in the transactions
    assert verify_kzg_commitments_against_transactions(execution_payload.transactions, blob_kzg_commitments)

    # Optionally sanity-check that the KZG commitments match the blobs (as produced by the execution engine)
    assert len(blob_kzg_commitments) == len(blobs)
    assert [blob_to_kzg_commitment(blob) == commitment for blob, commitment in zip(blobs, blob_kzg_commitments)]
  1. If valid, set block.body.blob_kzg_commitments = blob_kzg_commitments.

Note that the blobs should be held with the block in preparation of publishing. Without the blobs, the published block will effectively be ignored by honest validators.

Beacon Block publishing time

Before publishing a prepared beacon block proposal, the corresponding blobs are packaged into a sidecar object for distribution to the network:

def get_blobs_sidecar(block: BeaconBlock, blobs: Sequence[Blob]) -> BlobsSidecar:
    return BlobsSidecar(
        beacon_block_root=hash_tree_root(block),
        beacon_block_slot=block.slot,
        blobs=blobs,
        kzg_aggregated_proof=compute_proof_from_blobs(blobs),
    )

And then signed:

def get_signed_blobs_sidecar(state: BeaconState, blobs_sidecar: BlobsSidecar, privkey: int) -> SignedBlobsSidecar:
    domain = get_domain(state, DOMAIN_BLOBS_SIDECAR, blobs_sidecar.beacon_block_slot // SLOTS_PER_EPOCH)
    signing_root = compute_signing_root(blobs_sidecar, domain)
    signature = bls.Sign(privkey, signing_root)
    return SignedBlobsSidecar(message=blobs_sidecar, signature=signature)

This signed_blobs_sidecar is then published to the global blobs_sidecar topic as soon as the signed_beacon_block is published.

After publishing the sidecar peers on the network may request the sidecar through sync-requests, or a local user may be interested. The validator MUST hold on to blobs for MIN_EPOCHS_FOR_BLOBS_SIDECARS_REQUESTS epochs and serve when capable, to ensure the data-availability of these blobs throughout the network.

After MIN_EPOCHS_FOR_BLOBS_SIDECARS_REQUESTS nodes MAY prune the blobs and/or stop serving them.