237 lines
8.7 KiB
Markdown
237 lines
8.7 KiB
Markdown
# EIP-4844 -- Honest Validator
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**Notice**: This document is a work-in-progress for researchers and implementers.
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## Table of contents
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<!-- TOC -->
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<!-- START doctoc generated TOC please keep comment here to allow auto update -->
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<!-- DON'T EDIT THIS SECTION, INSTEAD RE-RUN doctoc TO UPDATE -->
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- [Introduction](#introduction)
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- [Prerequisites](#prerequisites)
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- [Helpers](#helpers)
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- [`is_data_available`](#is_data_available)
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- [`hash_to_bls_field`](#hash_to_bls_field)
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- [`compute_powers`](#compute_powers)
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- [`vector_lincomb`](#vector_lincomb)
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- [`verify_blobs_sidecar`](#verify_blobs_sidecar)
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- [Beacon chain responsibilities](#beacon-chain-responsibilities)
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- [Block proposal](#block-proposal)
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- [Constructing the `BeaconBlockBody`](#constructing-the-beaconblockbody)
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- [Blob commitments](#blob-commitments)
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- [Beacon Block publishing time](#beacon-block-publishing-time)
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<!-- END doctoc generated TOC please keep comment here to allow auto update -->
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<!-- /TOC -->
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## Introduction
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This document represents the changes to be made in the code of an "honest validator" to implement EIP-4844.
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## Prerequisites
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This document is an extension of the [Bellatrix -- Honest Validator](../bellatrix/validator.md) guide.
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All behaviors and definitions defined in this document, and documents it extends, carry over unless explicitly noted or overridden.
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All terminology, constants, functions, and protocol mechanics defined in the updated [Beacon Chain doc of EIP4844](./beacon-chain.md) are requisite for this document and used throughout.
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Please see related Beacon Chain doc before continuing and use them as a reference throughout.
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## Custom types
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| Name | SSZ equivalent | Description |
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| - | - | - |
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| `Polynomial` | `List[BLSFieldElement, MAX_BLOBS_PER_BLOCK]` | a polynomial in evaluation form |
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## Containers
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### `BlobsAndCommmitments`
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```python
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class BlobsAndCommmitments(Container):
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blobs: List[Blob, MAX_BLOBS_PER_BLOCK]
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kzg_commitments: List[KZGCommitment, MAX_BLOBS_PER_BLOCK]
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```
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### `PolynomialAndCommitment`
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```python
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class PolynomialAndCommitment(Container):
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polynomial: Polynomial
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kzg_commitment: KZGCommitment
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```
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## Helpers
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### `is_data_available`
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The implementation of `is_data_available` is meant to change with later sharding upgrades.
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Initially, it requires every verifying actor to retrieve the matching `BlobsSidecar`,
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and verify the sidecar with `verify_blobs_sidecar`.
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Without the sidecar the block may be processed further optimistically,
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but MUST NOT be considered valid until a valid `BlobsSidecar` has been downloaded.
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```python
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def is_data_available(slot: Slot, beacon_block_root: Root, kzgs: Sequence[KZGCommitment]):
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sidecar = retrieve_blobs_sidecar(slot, beacon_block_root) # implementation dependent, raises an exception if not available
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verify_blobs_sidecar(slot, beacon_block_root, kzgs, sidecar)
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```
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### `hash_to_bls_field`
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```python
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def hash_to_bls_field(x: Container) -> BLSFieldElement:
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"""
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This function is used to generate Fiat-Shamir challenges. The output is not uniform over the BLS field.
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"""
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return int.from_bytes(hash_tree_root(x), "little") % BLS_MODULUS
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```
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### `compute_powers`
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```python
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def compute_powers(x: BLSFieldElement, n: uint64) -> Sequence[BLSFieldElement]:
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"""
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Return ``x`` to power of [0, n-1].
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"""
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current_power = 1
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powers = []
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for _ in range(n):
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powers.append(BLSFieldElement(current_power))
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current_power = current_power * int(x) % BLS_MODULUS
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return powers
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```
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### `compute_aggregated_poly_and_commitment`
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```python
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def compute_aggregated_poly_and_commitment(
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blobs: Sequence[BLSFieldElement],
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kzg_commitments: Sequence[KZGCommitment]) -> Tuple[Polynomial, KZGCommitment]:
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"""
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Return the aggregated polynomial and aggregated KZG commitment.
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"""
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# Generate random linear combination challenges
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r = hash_to_bls_field(BlobsAndCommmitments(blobs=blobs, kzg_commitments=kzg_commitments))
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r_powers = compute_powers(r, len(kzg_commitments))
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# Create aggregated polynomial in evaluation form
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aggregated_poly = Polynomial(matrix_lincomb(blobs, r_powers))
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# Compute commitment to aggregated polynomial
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aggregated_poly_commitment = KZGCommitment(lincomb(kzg_commitments, r_powers))
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return aggregated_poly, aggregated_poly_commitment
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```
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### `verify_blobs_sidecar`
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```python
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def verify_blobs_sidecar(slot: Slot, beacon_block_root: Root,
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expected_kzg_commitments: Sequence[KZGCommitment], blobs_sidecar: BlobsSidecar) -> bool:
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assert slot == blobs_sidecar.beacon_block_slot
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assert beacon_block_root == blobs_sidecar.beacon_block_root
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blobs = blobs_sidecar.blobs
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kzg_aggregated_proof = blobs_sidecar.kzg_aggregated_proof
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assert len(expected_kzg_commitments) == len(blobs)
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aggregated_poly, aggregated_poly_commitment = compute_aggregated_poly_and_commitment(
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blobs,
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expected_kzg_commitments,
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)
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# Generate challenge `x` and evaluate the aggregated polynomial at `x`
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x = hash_to_bls_field(
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PolynomialAndCommitment(polynomial=aggregated_poly, kzg_commitment=aggregated_poly_commitment)
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)
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# Evaluate aggregated polynomial at `x` (evaluation function checks for div-by-zero)
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y = evaluate_polynomial_in_evaluation_form(aggregated_poly, x)
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# Verify aggregated proof
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return verify_kzg_proof(aggregated_poly_commitment, x, y, kzg_aggregated_proof)
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```
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### `compute_proof_from_blobs`
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```python
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def compute_proof_from_blobs(blobs: Sequence[BLSFieldElement]) -> KZGProof:
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commitments = [blob_to_kzg_commitment(blob) for blob in blobs]
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aggregated_poly, aggregated_poly_commitment = compute_aggregated_poly_and_commitment(blobs, commitments)
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x = hash_to_bls_field(PolynomialAndCommitment(
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polynomial=aggregated_poly,
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kzg_commitment=aggregated_poly_commitment,
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))
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return compute_kzg_proof(aggregated_poly, x)
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```
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## Beacon chain responsibilities
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All validator responsibilities remain unchanged other than those noted below.
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Namely, the blob handling and the addition of `BlobsSidecar`.
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### Block proposal
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#### Constructing the `BeaconBlockBody`
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##### Blob KZG commitments
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After retrieving the execution payload from the execution engine as specified in Bellatrix,
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the blobs are retrieved and processed:
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```
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# execution_payload = execution_engine.get_payload(payload_id)
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# block.body.execution_payload = execution_payload
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# ...
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blobs, blob_kzg_commitments = get_blobs(payload_id)
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# Optionally sanity-check that the KZG commitments match the versioned hashes in the transactions
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assert verify_kzg_commitments_against_transactions(execution_payload.transactions, blob_kzg_commitments)
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# Optionally sanity-check that the KZG commitments match the blobs (as produced by the execution engine)
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assert len(blob_kzg_commitments) == len(blobs)
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assert [blob_to_kzg_commitment(blob) == commitment for blob, commitment in zip(blobs, blob_kzg_commitments)]
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# Update the block body
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block.body.blob_kzg_commitments = blob_kzg_commitments
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```
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The `blobs` should be held with the block in preparation of publishing.
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Without the `blobs`, the published block will effectively be ignored by honest validators.
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Note: This API is *unstable*. `get_blobs` and `get_payload` may be unified.
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Implementers may also retrieve blobs individually per transaction.
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### Beacon Block publishing time
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Before publishing a prepared beacon block proposal, the corresponding blobs are packaged into a sidecar object for distribution to the network:
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```python
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def get_blobs_sidecar(block: BeaconBlock, blobs: Sequence[Blob]) -> BlobsSidecar:
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return BlobsSidecar(
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beacon_block_root=hash_tree_root(block),
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beacon_block_slot=block.slot,
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blobs=blobs,
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kzg_aggregated_proof=compute_proof_from_blobs(blobs),
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)
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```
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And then signed:
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```python
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def get_signed_blobs_sidecar(state: BeaconState, blobs_sidecar: BlobsSidecar, privkey: int) -> SignedBlobsSidecar:
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domain = get_domain(state, DOMAIN_BLOBS_SIDECAR, blobs_sidecar.beacon_block_slot // SLOTS_PER_EPOCH)
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signing_root = compute_signing_root(blobs_sidecar, domain)
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signature = bls.Sign(privkey, signing_root)
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return SignedBlobsSidecar(message=blobs_sidecar, signature=signature)
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```
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This `signed_blobs_sidecar` is then published to the global `blobs_sidecar` topic as soon as the `beacon_block` is published.
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After publishing the sidecar peers on the network may request the sidecar through sync-requests, or a local user may be interested.
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The validator MUST hold on to blobs for `MIN_EPOCHS_FOR_BLOBS_SIDECARS_REQUESTS` epochs and serve when capable,
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to ensure the data-availability of these blobs throughout the network.
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After `MIN_EPOCHS_FOR_BLOBS_SIDECARS_REQUESTS` nodes MAY prune the blobs and/or stop serving them.
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