# Ethereum 2.0 Phase 1 -- The Beacon Chain for Shards

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

## Table of contents

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TODO

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## Introduction

This document describes the extensions made to the Phase 0 design of The Beacon Chain
 to facilitate the new shards as part of Phase 1 of Eth2.

## Configuration

Configuration is not namespaced. Instead it is strictly an extension;
 no constants of phase 0 change, but new constants are adopted for changing behaviors.

### Misc

| Name | Value | Unit | Duration |
| - | - | - | - | 
| `MAX_SHARDS` | `2**10` (= 1024) |
| `ACTIVE_SHARDS` | `2**6` (= 64) |
| `ONLINE_PERIOD` | `2**3` (= 8) | epochs | ~51 min |
| `LIGHT_CLIENT_COMMITTEE_SIZE` | `2**7` (= 128) |
| `LIGHT_CLIENT_COMMITTEE_PERIOD` | `2**8` (= 256) | epochs | ~27 hours |
| `SHARD_COMMITTEE_PERIOD` | `2**8` (= 256) | epochs | ~27 hours |
| `SHARD_BLOCK_CHUNK_SIZE` | `2**18` (= 262,144) | |
| `MAX_SHARD_BLOCK_CHUNKS` | `2**2` (= 4) | |
| `BLOCK_SIZE_TARGET` | `3 * 2**16` (= 196,608) | |
| `SHARD_BLOCK_OFFSETS` | `[1, 2, 3, 5, 8, 13, 21, 34, 55, 89, 144, 233]` | |
| `MAX_SHARD_BLOCKS_PER_ATTESTATION` | `len(SHARD_BLOCK_OFFSETS)` | |
| `EMPTY_CHUNK_ROOT` | `hash_tree_root(BytesN[SHARD_BLOCK_CHUNK_SIZE]())` | |
| `MAX_GASPRICE` | `2**14` (= 16,384) | Gwei | |
| `MIN_GASPRICE` | `2**5` (= 32) | Gwei | |
| `GASPRICE_ADJUSTMENT_COEFFICIENT` | `2**3` (= 8) | |
| `DOMAIN_SHARD_LIGHT_CLIENT` | `192` | |
| `DOMAIN_SHARD_COMMITTEE` | `192` | |
| `DOMAIN_SHARD_PROPOSAL` | `193` | |

## Containers

### `ShardBlockWrapper`

_Wrapper for being broadcasted over the network._

```python
class ShardBlockWrapper(Container):
    shard_parent_root: Hash
    beacon_parent_root: Hash
    slot: Slot
    body: BytesN[SHARD_BLOCK_CHUNK_SIZE]
    signature: BLSSignature
```

### `ShardSignableHeader`

```python
class ShardSignableHeader(Container):
    shard_parent_root: Hash
    beacon_parent_root: Hash
    slot: Slot
    body_root: Hash
```

### `ShardState`

```python
class ShardState(Container):
    slot: Slot
    gasprice: Gwei
    data: Hash
    latest_block_root: Hash
```

### New `AttestationData`

```python
class AttestationData(Container):
    slot: Slot
    index: CommitteeIndex
    # LMD GHOST vote
    beacon_block_root: Hash
    # FFG vote
    source: Checkpoint
    target: Checkpoint
    # Current-slot shard block root
    head_shard_root: Hash
    # Shard transition root
    shard_transition_root: Hash
```

### `ShardTransition`

```python
class ShardTransition(Container):
    # Starting from slot
    start_slot: Slot
    # Shard block lengths
    shard_block_lengths: List[uint64, MAX_SHARD_BLOCKS_PER_ATTESTATION]
    # Shard data roots
    shard_data_roots: List[List[Hash, MAX_SHARD_BLOCK_CHUNKS], MAX_SHARD_BLOCKS_PER_ATTESTATION]
    # Intermediate shard states
    shard_states: List[ShardState, MAX_SHARD_BLOCKS_PER_ATTESTATION]
    # Proposer signature aggregate
    proposer_signature_aggregate: BLSSignature
```

### New `Attestation`

```python
class Attestation(Container):
    aggregation_bits: Bitlist[MAX_VALIDATORS_PER_COMMITTEE]
    data: AttestationData
    custody_bits: List[Bitlist[MAX_VALIDATORS_PER_COMMITTEE], MAX_SHARD_BLOCKS_PER_ATTESTATION]
    signature: BLSSignature
```

### `AttestationAndCommittee`

```python
class AttestationAndCommittee(Container):
    committee: List[ValidatorIndex, MAX_VALIDATORS_PER_COMMITTEE]
    attestation: Attestation
```

### `CompactCommittee`

```python
class CompactCommittee(Container):
    pubkeys: List[BLSPubkey, MAX_VALIDATORS_PER_COMMITTEE]
    compact_validators: List[uint64, MAX_VALIDATORS_PER_COMMITTEE]
```

### `AttestationCustodyBitWrapper`

```python
class AttestationCustodyBitWrapper(Container):
    attestation_root: Hash
    block_index: uint64
    bit: bool
```

### New extended `PendingAttestation`

```python
class PendingAttestation(phase0.PendingAttestation):
    aggregation_bits: Bitlist[MAX_VALIDATORS_PER_COMMITTEE]
    data: AttestationData
    inclusion_delay: Slot
    proposer_index: ValidatorIndex
    crosslink_success: bool
```

### New extended `Validator`

```python
class Validator(phase0.Validator):
    pubkey: BLSPubkey
    withdrawal_credentials: Hash  # Commitment to pubkey for withdrawals
    effective_balance: Gwei  # Balance at stake
    slashed: boolean
    # Status epochs
    activation_eligibility_epoch: Epoch  # When criteria for activation were met
    activation_epoch: Epoch
    exit_epoch: Epoch
    withdrawable_epoch: Epoch  # When validator can withdraw funds
    # Custody game
    # next_custody_secret_to_reveal is initialised to the custody period
    # (of the particular validator) in which the validator is activated
    # = get_custody_period_for_validator(...)
    next_custody_secret_to_reveal: uint64
    max_reveal_lateness: Epoch
```

### New extended `BeaconBlockBody`

```python
class BeaconBlockBody(phase0.BeaconBlockBody):
    randao_reveal: BLSSignature
    eth1_data: Eth1Data  # Eth1 data vote
    graffiti: Bytes32  # Arbitrary data
    # Slashings
    proposer_slashings: List[ProposerSlashing, MAX_PROPOSER_SLASHINGS]
    attester_slashings: List[AttesterSlashing, MAX_ATTESTER_SLASHINGS]
    # Attesting
    attestations: List[Attestation, MAX_ATTESTATIONS]
    # Entry & exit
    deposits: List[Deposit, MAX_DEPOSITS]
    voluntary_exits: List[VoluntaryExit, MAX_VOLUNTARY_EXITS]
    # Custody game
    custody_chunk_challenges: List[CustodyChunkChallenge, MAX_CUSTODY_CHUNK_CHALLENGES]
    custody_bit_challenges: List[CustodyBitChallenge, MAX_CUSTODY_BIT_CHALLENGES]
    custody_key_reveals: List[CustodyKeyReveal, MAX_CUSTODY_KEY_REVEALS]
    early_derived_secret_reveals: List[EarlyDerivedSecretReveal, MAX_EARLY_DERIVED_SECRET_REVEALS]
    # Shards
    shard_transitions: Vector[ShardTransition, MAX_SHARDS]
    # Light clients
    light_client_signature_bitfield: Bitlist[LIGHT_CLIENT_COMMITTEE_SIZE]
    light_client_signature: BLSSignature
```

### New extended `BeaconBlock`

Note that the `body` has a new `BeaconBlockBody` definition.

```python
class BeaconBlock(phase0.BeaconBlock):
    slot: Slot
    parent_root: Hash
    state_root: Hash
    body: BeaconBlockBody
    signature: BLSSignature
```

### New extended `BeaconState`

Note that aside from the new additions, `Validator` and `PendingAttestation` have new definitions.

```python
class BeaconState(phase0.BeaconState):
    # Versioning
    genesis_time: uint64
    slot: Slot
    fork: Fork
    # History
    latest_block_header: BeaconBlockHeader
    block_roots: Vector[Hash, SLOTS_PER_HISTORICAL_ROOT]
    state_roots: Vector[Hash, SLOTS_PER_HISTORICAL_ROOT]
    historical_roots: List[Hash, HISTORICAL_ROOTS_LIMIT]
    # Eth1
    eth1_data: Eth1Data
    eth1_data_votes: List[Eth1Data, SLOTS_PER_ETH1_VOTING_PERIOD]
    eth1_deposit_index: uint64
    # Registry
    validators: List[Validator, VALIDATOR_REGISTRY_LIMIT]
    balances: List[Gwei, VALIDATOR_REGISTRY_LIMIT]
    # Randomness
    randao_mixes: Vector[Hash, EPOCHS_PER_HISTORICAL_VECTOR]
    # Slashings
    slashings: Vector[Gwei, EPOCHS_PER_SLASHINGS_VECTOR]  # Per-epoch sums of slashed effective balances
    # Attestations
    previous_epoch_attestations: List[PendingAttestation, MAX_ATTESTATIONS * SLOTS_PER_EPOCH]
    current_epoch_attestations: List[PendingAttestation, MAX_ATTESTATIONS * SLOTS_PER_EPOCH]
    # Finality
    justification_bits: Bitvector[JUSTIFICATION_BITS_LENGTH]  # Bit set for every recent justified epoch
    previous_justified_checkpoint: Checkpoint  # Previous epoch snapshot
    current_justified_checkpoint: Checkpoint
    finalized_checkpoint: Checkpoint
    # Phase 1
    shard_states: List[ShardState, MAX_SHARDS]
    online_countdown: Bytes[VALIDATOR_REGISTRY_LIMIT]
    current_light_committee: CompactCommittee
    next_light_committee: CompactCommittee
    # Custody game
    # TODO: custody game refactor, no challenge-records, immediate processing.
    custody_challenge_index: uint64
    # Future derived secrets already exposed; contains the indices of the exposed validator
    # at RANDAO reveal period % EARLY_DERIVED_SECRET_PENALTY_MAX_FUTURE_EPOCHS
    exposed_derived_secrets: Vector[List[ValidatorIndex, PLACEHOLDER],
                                    EARLY_DERIVED_SECRET_PENALTY_MAX_FUTURE_EPOCHS]
```

## Helper functions

### Crypto

#### `bls_verify_multiple`

`bls_verify_multiple` is a function for verifying a BLS signature constructed from multiple messages, as defined in the [BLS Signature spec](../bls_signature.md#bls_verify_multiple).


### Misc

#### `pack_compact_validator`

```python
def pack_compact_validator(index: int, slashed: bool, balance_in_increments: int) -> int:
    """
    Creates a compact validator object representing index, slashed status, and compressed balance.
    Takes as input balance-in-increments (// EFFECTIVE_BALANCE_INCREMENT) to preserve symmetry with
    the unpacking function.
    """
    return (index << 16) + (slashed << 15) + balance_in_increments
```

#### `committee_to_compact_committee`

```python
def committee_to_compact_committee(state: BeaconState, committee: Sequence[ValidatorIndex]) -> CompactCommittee:
    """
    Given a state and a list of validator indices, outputs the CompactCommittee representing them.
    """
    validators = [state.validators[i] for i in committee]
    compact_validators = [
        pack_compact_validator(i, v.slashed, v.effective_balance // EFFECTIVE_BALANCE_INCREMENT)
        for i, v in zip(committee, validators)
    ]
    pubkeys = [v.pubkey for v in validators]
    return CompactCommittee(pubkeys=pubkeys, compact_validators=compact_validators)
```

#### `chunks_to_body_root`

```python
def chunks_to_body_root(chunks):
    return hash_tree_root(chunks + [EMPTY_CHUNK_ROOT] * (MAX_SHARD_BLOCK_CHUNKS - len(chunks)))
```

### Beacon state accessors

#### `get_online_validator_indices`

```python
def get_online_validator_indices(state: BeaconState) -> Set[ValidatorIndex]:
    active_validators = get_active_validator_indices(state, get_current_epoch(state))
    return set([i for i in active_validators if state.online_countdown[i] != 0])
```

#### `get_shard_committee`

```python
def get_shard_committee(beacon_state: BeaconState, epoch: Epoch, shard: Shard) -> Sequence[ValidatorIndex]:
    source_epoch = epoch - epoch % SHARD_COMMITTEE_PERIOD 
    if source_epoch > 0:
        source_epoch -= SHARD_COMMITTEE_PERIOD
    active_validator_indices = get_active_validator_indices(beacon_state, source_epoch)
    seed = get_seed(beacon_state, source_epoch, DOMAIN_SHARD_COMMITTEE)
    return compute_committee(active_validator_indices, seed, 0, ACTIVE_SHARDS)
```

#### `get_shard_proposer_index`

```python
def get_shard_proposer_index(beacon_state: BeaconState, slot: Slot, shard: Shard) -> ValidatorIndex:
    committee = get_shard_committee(beacon_state, slot_to_epoch(slot), shard)
    r = bytes_to_int(get_seed(beacon_state, get_current_epoch(state), DOMAIN_SHARD_COMMITTEE)[:8])
    return committee[r % len(committee)]
```

#### `get_light_client_committee`

```python
def get_light_client_committee(beacon_state: BeaconState, epoch: Epoch) -> Sequence[ValidatorIndex]:
    source_epoch = epoch - epoch % LIGHT_CLIENT_COMMITTEE_PERIOD 
    if source_epoch > 0:
        source_epoch -= LIGHT_CLIENT_COMMITTEE_PERIOD
    active_validator_indices = get_active_validator_indices(beacon_state, source_epoch)
    seed = get_seed(beacon_state, source_epoch, DOMAIN_SHARD_LIGHT_CLIENT)
    return compute_committee(active_validator_indices, seed, 0, ACTIVE_SHARDS)[:TARGET_COMMITTEE_SIZE]
```

#### `get_indexed_attestation`

```python
def get_indexed_attestation(beacon_state: BeaconState, attestation: Attestation) -> AttestationAndCommittee:
    committee = get_beacon_committee(beacon_state, attestation.data.slot, attestation.data.index)
    return AttestationAndCommittee(committee, attestation)
```

#### `get_updated_gasprice`

```python
def get_updated_gasprice(prev_gasprice: Gwei, length: uint8) -> Gwei:
    if length > BLOCK_SIZE_TARGET:
        delta = prev_gasprice * (length - BLOCK_SIZE_TARGET) // BLOCK_SIZE_TARGET // GASPRICE_ADJUSTMENT_COEFFICIENT
        return min(prev_gasprice + delta, MAX_GASPRICE)
    else:
        delta = prev_gasprice * (BLOCK_SIZE_TARGET - length) // BLOCK_SIZE_TARGET // GASPRICE_ADJUSTMENT_COEFFICIENT
        return max(prev_gasprice, MIN_GASPRICE + delta) - delta
```

#### `get_shard`

```python
def get_shard(state: BeaconState, attestation: Attestation) -> Shard:
    return Shard((attestation.data.index + get_start_shard(state, attestation.data.slot)) % ACTIVE_SHARDS)
```

#### `get_offset_slots`

```python
def get_offset_slots(state: BeaconState, start_slot: Slot) -> Sequence[Slot]:
    return [start_slot + x for x in SHARD_BLOCK_OFFSETS if start_slot + x < state.slot]
```


### Predicates

#### Updated `is_valid_indexed_attestation`

Note that this replaces the Phase 0 `is_valid_indexed_attestation`.

```python
def is_valid_indexed_attestation(state: BeaconState, indexed_attestation: AttestationAndCommittee) -> bool:
    """
    Check if ``indexed_attestation`` has valid indices and signature.
    """

    # Verify aggregate signature
    all_pubkeys = []
    all_message_hashes = []
    aggregation_bits = indexed_attestation.attestation.aggregation_bits
    assert len(aggregation_bits) == len(indexed_attestation.committee)
    for i, custody_bits in enumerate(indexed_attestation.attestation.custody_bits):
        assert len(custody_bits) == len(indexed_attestation.committee)
        for participant, abit, cbit in zip(indexed_attestation.committee, aggregation_bits, custody_bits):
            if abit:
                all_pubkeys.append(state.validators[participant].pubkey)
                # Note: only 2N distinct message hashes
                all_message_hashes.append(hash_tree_root(
                    AttestationCustodyBitWrapper(hash_tree_root(indexed_attestation.data), i, cbit)
                ))
            else:
                assert cbit == False
        
    return bls_verify_multiple(
        pubkeys=all_pubkeys,
        message_hashes=all_message_hashes,
        signature=indexed_attestation.signature,
        domain=get_domain(state, DOMAIN_BEACON_ATTESTER, indexed_attestation.data.target.epoch),
    )
```


### Block processing

```python
def process_block(state: BeaconState, block: BeaconBlock) -> None:
    process_block_header(state, block)
    process_randao(state, block.body)
    process_eth1_data(state, block.body)
    verify_shard_transition_false_positives(state, block)
    process_light_client_signatures(state, block)
    process_operations(state, block.body)
```


#### Operations

```python
def process_operations(state: BeaconState, body: BeaconBlockBody) -> None:
    # Verify that outstanding deposits are processed up to the maximum number of deposits
    assert len(body.deposits) == min(MAX_DEPOSITS, state.eth1_data.deposit_count - state.eth1_deposit_index)
    
    def for_ops(operations, fn):
        for operation in operations:
            fn(state, operation)
    
    for_ops(body.proposer_slashings, process_proposer_slashing)
    for_ops(body.attester_slashings, process_attester_slashing)

    # New attestation processing
    process_attestations(state, body, body.attestations)

    for_ops(body.deposits, process_deposit)
    for_ops(body.voluntary_exits, process_voluntary_exit)

    # See custody game spec.
    process_custody_game_operations(state, body)

    # TODO process_operations(body.shard_receipt_proofs, process_shard_receipt_proofs)
```

##### New Attestation processing

###### `validate_attestation`

```python
def validate_attestation(state: BeaconState, attestation: Attestation) -> None:
    data = attestation.data
    assert data.index < ACTIVE_SHARDS
    shard = get_shard(state, attestation)
    proposer_index = get_beacon_proposer_index(state)

    # Signature check
    assert is_valid_indexed_attestation(state, get_indexed_attestation(state, attestation))
    # Type 1: on-time attestations
    if attestation.custody_bits != []:
        # Correct slot
        assert data.slot == state.slot
        # Correct data root count
        assert len(attestation.custody_bits) == len(get_offset_slots(state, state.shard_next_slots[shard]))
        # Correct parent block root
        assert data.beacon_block_root == get_block_root_at_slot(state, state.slot - 1)
    # Type 2: delayed attestations
    else:
        assert state.slot - compute_start_slot_at_epoch(slot_to_epoch(data.slot)) < EPOCH_LENGTH
        assert data.shard_transition_root == Hash()
```

###### `apply_shard_transition`

```python
def apply_shard_transition(state: BeaconState, shard: Shard, transition: ShardTransition) -> None:
    # Slot the attestation starts counting from
    start_slot = state.shard_next_slots[shard]

    # Correct data root count
    offset_slots = get_offset_slots(state, start_slot)
    assert len(transition.shard_data_roots) == len(transition.shard_states) == len(transition.shard_block_lengths) == len(offset_slots)
    assert transition.start_slot == start_slot

    # Reonstruct shard headers
    headers = []
    proposers = []
    shard_parent_root = state.shard_states[shard].latest_block_root
    for i in range(len(offset_slots)):
        if any(transition.shard_data_roots):
            headers.append(ShardSignableHeader(
                shard_parent_root=shard_parent_root,
                parent_hash=get_block_root_at_slot(state, state.slot-1),
                slot=offset_slots[i],
                body_root=chunks_to_body_root(transition.shard_data_roots[i])
            ))
            proposers.append(get_shard_proposer_index(state, shard, offset_slots[i]))
            shard_parent_root = hash_tree_root(headers[-1])

    # Verify correct calculation of gas prices and slots and chunk roots
    prev_gasprice = state.shard_states[shard].gasprice
    for i in range(len(offset_slots)):
        shard_state, block_length, chunks = transition.shard_states[i], transition.shard_block_lengths[i], transition.shard_data_roots[i]
        assert shard_state.gasprice == get_updated_gasprice(prev_gasprice, block_length)
        assert shard_state.slot == offset_slots[i]
        assert len(chunks) == block_length // SHARD_BLOCK_CHUNK_SIZE
        prev_gasprice = shard_state.gasprice

    # Verify combined proposer signature
    assert bls_verify_multiple(
        pubkeys=[state.validators[proposer].pubkey for proposer in proposers],
        message_hashes=[hash_tree_root(header) for header in headers],
        signature=transition.proposer_signature_aggregate,
        domain=DOMAIN_SHARD_PROPOSAL
    )

    # Save updated state
    state.shard_states[shard] = transition.shard_states[-1]
    state.shard_states[shard].slot = state.slot - 1
```

###### `process_attestations`

```python
def process_attestations(state: BeaconState, block_body: BeaconBlockBody, attestations: Sequence[Attestation]) -> None:
    # Basic validation
    for attestation in attestations:
       validate_attestation(state, attestation)
    # Process crosslinks
    online_indices = get_online_validator_indices(state)
    winners = set()
    for shard in range(ACTIVE_SHARDS):
        success = False
        # All attestations in the block for this shard
        this_shard_attestations = [attestation for attestation in attestations if get_shard(state, attestation) == shard and attestation.data.slot == state.slot]
        # The committee for this shard
        this_shard_committee = get_beacon_committee(state, get_current_epoch(state), shard)
        # Loop over all shard transition roots
        for shard_transition_root in sorted(set([attestation.data.shard_transition_root for attestation in this_shard_attestations])):
            all_participants = set()
            participating_attestations = []
            for attestation in this_shard_attestations:
                participating_attestations.append(attestation)
                if attestation.data.shard_transition_root == shard_transition_root:
                    all_participants = all_participants.union(get_attesting_indices(state, attestation.data, attestation.aggregation_bits))
                if (
                    get_total_balance(state, online_indices.intersection(all_participants)) * 3 >=
                    get_total_balance(state, online_indices.intersection(this_shard_committee)) * 2
                    and success is False
                ):
                    # Attestation <-> shard transition consistency
                    assert shard_transition_root == hash_tree_root(block_body.shard_transition)
                    assert attestation.data.head_shard_root == chunks_to_body_root(block_body.shard_transition.shard_data_roots[-1])
                    # Apply transition
                    apply_shard_transition(state, shard, block_body.shard_transition)
                    # Apply proposer reward and cost
                    estimated_attester_reward = sum([get_base_reward(state, attester) for attester in all_participants])
                    increase_balance(state, proposer, estimated_attester_reward // PROPOSER_REWARD_COEFFICIENT)
                    for shard_state, slot, length in zip(block_body.shard_transition.shard_states, offset_slots, block_body.shard_transition.shard_block_lengths):
                        decrease_balance(state, get_shard_proposer_index(state, shard, slot), shard_state.gasprice * length)
                    winners.add((shard, shard_transition_root))
                    success = True
        if not success:
            assert block_body.shard_transitions[shard] == ShardTransition()
    for attestation in attestations:
        pending_attestation = PendingAttestation(
            aggregation_bits=attestation.aggregation_bits,
            data=attestation.data,
            inclusion_delay=state.slot - data.slot,
            crosslink_success=(get_shard(state, attestation), attestation.shard_transition_root) in winners and attestation.data.slot == state.slot,
            proposer_index=proposer_index
        )
        if attestation.data.target.epoch == get_current_epoch(state):
            assert attestation.data.source == state.current_justified_checkpoint
            state.current_epoch_attestations.append(pending_attestation)
        else:
            assert attestation.data.source == state.previous_justified_checkpoint
            state.previous_epoch_attestations.append(pending_attestation)
```

#### Shard transition false positives

```python
def verify_shard_transition_false_positives(state: BeaconState, block_body: BeaconBlockBody) -> None:
    # Verify that a `shard_transition` in a block is empty if an attestation was not processed for it
    for shard in range(ACTIVE_SHARDS):
        if state.shard_states[shard].slot != state.slot - 1:
            assert block_body.shard_transition[shard] == ShardTransition()
```

#### Light client processing

```python
def process_light_client_signatures(state: BeaconState, block_body: BeaconBlockBody) -> None:
    committee = get_light_client_committee(state, get_current_epoch(state))
    assert len(block_body.light_client_signature_bitfield) == len(committee)
    total_reward = Gwei(0)
    signer_keys = []
    for i, bit in enumerate(block_body.light_client_signature_bitfield):
        if bit:
            signer_keys.append(state.validators[committee[i]].pubkey)
            increase_balance(state, committee[i], get_base_reward(state, committee[i]))
            total_reward += get_base_reward(state, committee[i])

    increase_balance(state, get_beacon_proposer_index(state), total_reward // PROPOSER_REWARD_COEFFICIENT)
    
    assert bls_verify(
        pubkey=bls_aggregate_pubkeys(signer_keys),
        message_hash=get_block_root_at_slot(state, state.slot - 1),
        signature=block_body.light_client_signature,
        domain=DOMAIN_LIGHT_CLIENT
    )
```


### Epoch transition

This epoch transition overrides the phase0 epoch transition:

```python
def process_epoch(state: BeaconState) -> None:
    process_justification_and_finalization(state)
    process_rewards_and_penalties(state)
    process_registry_updates(state)
    process_reveal_deadlines(state)
    process_slashings(state)
    process_final_updates(state)
    process_custody_final_updates(state)
    process_online_tracking(state)
    process_light_client_committee_updates(state)
```

#### Custody game updates

`process_reveal_deadlines` and `process_custody_final_updates` are defined in [the Custody Game spec](./1_custody-game.md), 

#### Online-tracking

```python
def process_online_tracking(state: BeaconState) -> None:
    # Slowly remove validators from the "online" set if they do not show up
    for index in range(len(state.validators)):
        if state.online_countdown[index] != 0:
            state.online_countdown[index] = state.online_countdown[index] - 1

    # Process pending attestations
    for pending_attestation in state.current_epoch_attestations + state.previous_epoch_attestations:
        for index in get_attesting_indices(state, pending_attestation.data, pending_attestation.aggregation_bits):
            state.online_countdown[index] = ONLINE_PERIOD
```

#### Light client committee updates

```python
def process_light_client_committee_updates(state: BeaconState) -> None:
    # Update light client committees
    if get_current_epoch(state) % LIGHT_CLIENT_COMMITTEE_PERIOD == 0:
        state.current_light_committee = state.next_light_committee
        new_committee = get_light_client_committee(state, get_current_epoch(state) + LIGHT_CLIENT_COMMITTEE_PERIOD)
        state.next_light_committee = committee_to_compact_committee(state, new_committee)
```