# Altair -- Minimal Light Client

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

## Table of contents

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- [Introduction](#introduction)
- [Constants](#constants)
- [Preset](#preset)
  - [Misc](#misc)
- [Containers](#containers)
  - [`LightClientSnapshot`](#lightclientsnapshot)
  - [`LightClientUpdate`](#lightclientupdate)
  - [`LightClientStore`](#lightclientstore)
- [Helper functions](#helper-functions)
  - [`get_subtree_index`](#get_subtree_index)
- [Light client state updates](#light-client-state-updates)
    - [`validate_light_client_update`](#validate_light_client_update)
    - [`apply_light_client_update`](#apply_light_client_update)
    - [`process_light_client_update`](#process_light_client_update)

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## Introduction

The beacon chain is designed to be light client friendly for constrained environments to
access Ethereum with reasonable safety and liveness.
Such environments include resource-constrained devices (e.g. phones for trust-minimised wallets)
and metered VMs (e.g. blockchain VMs for cross-chain bridges).

This document suggests a minimal light client design for the beacon chain that
uses sync committees introduced in [this beacon chain extension](./beacon-chain.md).

## Constants

| Name | Value |
| - | - |
| `FINALIZED_ROOT_INDEX` | `get_generalized_index(BeaconState, 'finalized_checkpoint', 'root')` |
| `NEXT_SYNC_COMMITTEE_INDEX` | `get_generalized_index(BeaconState, 'next_sync_committee')` |

## Preset

### Misc

| Name | Value |
| - | - |
| `MIN_SYNC_COMMITTEE_PARTICIPANTS` | `1` |

## Containers

### `LightClientSnapshot`

```python
class LightClientSnapshot(Container):
    # Beacon block header
    header: BeaconBlockHeader
    # Sync committees corresponding to the header
    current_sync_committee: SyncCommittee
    next_sync_committee: SyncCommittee
```

### `LightClientUpdate`

```python
class LightClientUpdate(Container):
    # Update beacon block header
    header: BeaconBlockHeader
    # Next sync committee corresponding to the header
    next_sync_committee: SyncCommittee
    next_sync_committee_branch: Vector[Bytes32, floorlog2(NEXT_SYNC_COMMITTEE_INDEX)]
    # Finality proof for the update header
    finality_header: BeaconBlockHeader
    finality_branch: Vector[Bytes32, floorlog2(FINALIZED_ROOT_INDEX)]
    # Sync committee aggregate signature
    sync_committee_bits: Bitvector[SYNC_COMMITTEE_SIZE]
    sync_committee_signature: BLSSignature
    # Fork version for the aggregate signature
    fork_version: Version
```

### `LightClientStore`

```python
@dataclass
class LightClientStore(object):
    snapshot: LightClientSnapshot
    valid_updates: Set[LightClientUpdate]
```

## Helper functions

### `get_subtree_index`

```python
def get_subtree_index(generalized_index: GeneralizedIndex) -> uint64:
    return uint64(generalized_index % 2**(floorlog2(generalized_index)))
```

## Light client state updates

A light client maintains its state in a `store` object of type `LightClientStore` and receives `update` objects of type `LightClientUpdate`. Every `update` triggers `process_light_client_update(store, update, current_slot)` where `current_slot` is the current slot based on some local clock.

#### `validate_light_client_update`

```python
def validate_light_client_update(snapshot: LightClientSnapshot,
                                 update: LightClientUpdate,
                                 genesis_validators_root: Root) -> None:
    # Verify update slot is larger than snapshot slot
    assert update.header.slot > snapshot.header.slot

    # Verify update does not skip a sync committee period
    snapshot_period = compute_epoch_at_slot(snapshot.header.slot) // EPOCHS_PER_SYNC_COMMITTEE_PERIOD
    update_period = compute_epoch_at_slot(update.header.slot) // EPOCHS_PER_SYNC_COMMITTEE_PERIOD
    assert update_period in (snapshot_period, snapshot_period + 1)

    # Verify update header root is the finalized root of the finality header, if specified
    if update.finality_header == BeaconBlockHeader():
        signed_header = update.header
        assert update.finality_branch == [Bytes32() for _ in range(floorlog2(FINALIZED_ROOT_INDEX))]
    else:
        signed_header = update.finality_header
        assert is_valid_merkle_branch(
            leaf=hash_tree_root(update.header),
            branch=update.finality_branch,
            depth=floorlog2(FINALIZED_ROOT_INDEX),
            index=get_subtree_index(FINALIZED_ROOT_INDEX),
            root=update.finality_header.state_root,
        )

    # Verify update next sync committee if the update period incremented
    if update_period == snapshot_period:
        sync_committee = snapshot.current_sync_committee
        assert update.next_sync_committee_branch == [Bytes32() for _ in range(floorlog2(NEXT_SYNC_COMMITTEE_INDEX))]
    else:
        sync_committee = snapshot.next_sync_committee
        assert is_valid_merkle_branch(
            leaf=hash_tree_root(update.next_sync_committee),
            branch=update.next_sync_committee_branch,
            depth=floorlog2(NEXT_SYNC_COMMITTEE_INDEX),
            index=get_subtree_index(NEXT_SYNC_COMMITTEE_INDEX),
            root=update.header.state_root,
        )

    # Verify sync committee has sufficient participants
    assert sum(update.sync_committee_bits) >= MIN_SYNC_COMMITTEE_PARTICIPANTS

    # Verify sync committee aggregate signature
    participant_pubkeys = [pubkey for (bit, pubkey) in zip(update.sync_committee_bits, sync_committee.pubkeys) if bit]
    domain = compute_domain(DOMAIN_SYNC_COMMITTEE, update.fork_version, genesis_validators_root)
    signing_root = compute_signing_root(signed_header, domain)
    assert bls.FastAggregateVerify(participant_pubkeys, signing_root, update.sync_committee_signature)
```

#### `apply_light_client_update`

```python
def apply_light_client_update(snapshot: LightClientSnapshot, update: LightClientUpdate) -> None:
    snapshot_period = compute_epoch_at_slot(snapshot.header.slot) // EPOCHS_PER_SYNC_COMMITTEE_PERIOD
    update_period = compute_epoch_at_slot(update.header.slot) // EPOCHS_PER_SYNC_COMMITTEE_PERIOD
    if update_period == snapshot_period + 1:
        snapshot.current_sync_committee = snapshot.next_sync_committee
        snapshot.next_sync_committee = update.next_sync_committee
    snapshot.header = update.header
```

#### `process_light_client_update`

```python
def process_light_client_update(store: LightClientStore, update: LightClientUpdate, current_slot: Slot,
                                genesis_validators_root: Root) -> None:
    validate_light_client_update(store.snapshot, update, genesis_validators_root)
    store.valid_updates.add(update)

    update_timeout = SLOTS_PER_EPOCH * EPOCHS_PER_SYNC_COMMITTEE_PERIOD
    if (
        sum(update.sync_committee_bits) * 3 >= len(update.sync_committee_bits) * 2
        and update.finality_header != BeaconBlockHeader()
    ):
        # Apply update if (1) 2/3 quorum is reached and (2) we have a finality proof.
        # Note that (2) means that the current light client design needs finality.
        # It may be changed to re-organizable light client design. See the on-going issue consensus-specs#2182.
        apply_light_client_update(store.snapshot, update)
        store.valid_updates = set()
    elif current_slot > store.snapshot.header.slot + update_timeout:
        # Forced best update when the update timeout has elapsed
        apply_light_client_update(store.snapshot,
                                  max(store.valid_updates, key=lambda update: sum(update.sync_committee_bits)))
        store.valid_updates = set()
```