# EIP-4844 -- Honest Validator

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

## Table of contents

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- [Introduction](#introduction)
- [Prerequisites](#prerequisites)
- [Helpers](#helpers)
  - [`get_blobs_and_kzg_commitments`](#get_blobs_and_kzg_commitments)
- [Beacon chain responsibilities](#beacon-chain-responsibilities)
  - [Block and sidecar proposal](#block-and-sidecar-proposal)
    - [Constructing the `BeaconBlockBody`](#constructing-the-beaconblockbody)
      - [Blob KZG commitments](#blob-kzg-commitments)
    - [Constructing the `SignedBeaconBlockAndBlobsSidecar`](#constructing-the-signedbeaconblockandblobssidecar)
      - [Block](#block)
      - [Sidecar](#sidecar)

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## 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 [Capella -- Honest Validator](../capella/validator.md) 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](./beacon-chain.md) are requisite for this document and used throughout.
Please see related Beacon Chain doc before continuing and use them as a reference throughout.

## Helpers

### `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.

```python
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

1. After retrieving the execution payload from the execution engine as specified in Capella,
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`:

```python
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)]
```

3. 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:
```python
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_aggregate_kzg_proof(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.