eth2.0-specs/specs/_features/eip7549/beacon-chain.md

# EIP-7549 -- The Beacon Chain

## Table of contents

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- [Introduction](#introduction)
- [Preset](#preset)
- [Containers](#containers)
  - [Modified containers](#modified-containers)
    - [`Attestation`](#attestation)
    - [`IndexedAttestation`](#indexedattestation)
    - [`BeaconBlockBody`](#beaconblockbody)
- [Helper functions](#helper-functions)
  - [Misc](#misc)
    - [`get_committee_indices`](#get_committee_indices)
  - [Beacon state accessors](#beacon-state-accessors)
    - [Modified `get_attesting_indices`](#modified-get_attesting_indices)
  - [Block processing](#block-processing)
    - [Modified `process_attestation`](#modified-process_attestation)

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## Introduction

This is the beacon chain specification to move the attestation committee index outside of the signed message. For motivation, refer to [EIP-7549](https://eips.ethereum.org/EIPS/eip-7549).

*Note:* This specification is built upon [Deneb](../../deneb/beacon_chain.md) and is under active development.

## Preset

| Name | Value | Description |
| - | - | - |
| `MAX_ATTESTER_SLASHINGS_EIP7549`   | `2**0` (= 1) |
| `MAX_ATTESTATIONS_EIP7549` | `2**3` (= 8) |

## Containers

### Modified containers

#### `Attestation`

```python
class Attestation(Container):
    aggregation_bits: Bitlist[MAX_VALIDATORS_PER_COMMITTEE * MAX_COMMITTEES_PER_SLOT]  # [Modified in EIP7549]
    data: AttestationData
    committee_bits: Bitvector[MAX_COMMITTEES_PER_SLOT]  # [New in EIP7549]
    signature: BLSSignature
```

#### `IndexedAttestation`

```python
class IndexedAttestation(Container):
    # [Modified in EIP7549]
    attesting_indices: List[ValidatorIndex, MAX_VALIDATORS_PER_COMMITTEE * MAX_COMMITTEES_PER_SLOT]
    data: AttestationData
    signature: BLSSignature
```

#### `BeaconBlockBody`

```python
class BeaconBlockBody(Container):
    randao_reveal: BLSSignature
    eth1_data: Eth1Data  # Eth1 data vote
    graffiti: Bytes32  # Arbitrary data
    # Operations
    proposer_slashings: List[ProposerSlashing, MAX_PROPOSER_SLASHINGS]
    attester_slashings: List[AttesterSlashing, MAX_ATTESTER_SLASHINGS_EIP7549]  # [Modified in EIP7549]
    attestations: List[Attestation, MAX_ATTESTATIONS_EIP7549]  # [Modified in EIP7549]
    deposits: List[Deposit, MAX_DEPOSITS]
    voluntary_exits: List[SignedVoluntaryExit, MAX_VOLUNTARY_EXITS]
    sync_aggregate: SyncAggregate
    # Execution
    execution_payload: ExecutionPayload
    bls_to_execution_changes: List[SignedBLSToExecutionChange, MAX_BLS_TO_EXECUTION_CHANGES]
    blob_kzg_commitments: List[KZGCommitment, MAX_BLOB_COMMITMENTS_PER_BLOCK]
```

## Helper functions

### Misc

#### `get_committee_indices`

```python
def get_committee_indices(commitee_bits: Bitvector) -> Sequence[CommitteeIndex]:
    return [CommitteeIndex(index) for index, bit in enumerate(commitee_bits) if bit]
```

### Beacon state accessors

#### Modified `get_attesting_indices`

```python
def get_attesting_indices(state: BeaconState, attestation: Attestation) -> Set[ValidatorIndex]:
    """
    Return the set of attesting indices corresponding to ``aggregation_bits`` and ``committee_bits``.
    """
    output: Set[ValidatorIndex] = set()
    committee_indices = get_committee_indices(attestation.committee_bits)
    committee_offset = 0
    for index in committee_indices:
        committee = get_beacon_committee(state, attestation.data.slot, index)
        committee_attesters = set(
            index for i, index in enumerate(committee) if attestation.aggregation_bits[committee_offset + i])
        output = output.union(committee_attesters)

        committee_offset += len(committee)

    return output
```

### Block processing

#### Modified `process_attestation`

```python
def process_attestation(state: BeaconState, attestation: Attestation) -> None:
    data = attestation.data
    assert data.target.epoch in (get_previous_epoch(state), get_current_epoch(state))
    assert data.target.epoch == compute_epoch_at_slot(data.slot)
    assert data.slot + MIN_ATTESTATION_INCLUSION_DELAY <= state.slot

    # [Modified in EIP7549]
    assert data.index == 0
    committee_indices = get_committee_indices(attestation.committee_bits)
    participants_count = 0
    for index in committee_indices:
        assert index < get_committee_count_per_slot(state, data.target.epoch)
        committee = get_beacon_committee(state, data.slot, index)
        participants_count += len(committee)

    assert len(attestation.aggregation_bits) == participants_count

    # Participation flag indices
    participation_flag_indices = get_attestation_participation_flag_indices(state, data, state.slot - data.slot)

    # Verify signature
    assert is_valid_indexed_attestation(state, get_indexed_attestation(state, attestation))

    # Update epoch participation flags
    if data.target.epoch == get_current_epoch(state):
        epoch_participation = state.current_epoch_participation
    else:
        epoch_participation = state.previous_epoch_participation

    proposer_reward_numerator = 0
    for index in get_attesting_indices(state, attestation):
        for flag_index, weight in enumerate(PARTICIPATION_FLAG_WEIGHTS):
            if flag_index in participation_flag_indices and not has_flag(epoch_participation[index], flag_index):
                epoch_participation[index] = add_flag(epoch_participation[index], flag_index)
                proposer_reward_numerator += get_base_reward(state, index) * weight

    # Reward proposer
    proposer_reward_denominator = (WEIGHT_DENOMINATOR - PROPOSER_WEIGHT) * WEIGHT_DENOMINATOR // PROPOSER_WEIGHT
    proposer_reward = Gwei(proposer_reward_numerator // proposer_reward_denominator)
    increase_balance(state, get_beacon_proposer_index(state), proposer_reward)
```