9.6 KiB
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 SignedBeaconBlockAndBlobsSidecar.
Block and sidecar proposal
Constructing the BeaconBlockBody
Blob KZG commitments
- After retrieving the execution payload from the execution engine as specified in Bellatrix,
use the
payload_idto retrieveblobsandblob_kzg_commitmentsviaget_blobs_and_kzg_commitments(payload_id). - Validate
blobsandblob_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)]
- If valid, set
block.body.blob_kzg_commitments = blob_kzg_commitments.
Constructing the SignedBeaconBlockAndBlobsSidecar
To construct a SignedBeaconBlockAndBlobsSidecar, a signed_beacon_block_and_blobs_sidecar is defined with the necessary context for block and sidecar proposal.
Block
Set signed_beacon_block_and_blobs_sidecar.beacon_block = block where block is obtained above.
Sidecar
Coupled with block, the corresponding blobs are packaged into a sidecar object for distribution to the network.
Set signed_beacon_block_and_blobs_sidecar.blobs_sidecar = sidecar where sidecar is obtained from:
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),
)
This signed_beacon_block_and_blobs_sidecar is then published to the global beacon_block_and_blobs_sidecar topic.
After publishing the peers on the network may request the sidecar through sync-requests, or a local user may be interested.
The validator MUST hold on to sidecars 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 sidecars and/or stop serving them.