eth2.0-specs/specs/phase1/validator.md

Ethereum 2.0 Phase 1 -- Honest Validator

Notice: This document is a work-in-progress for researchers and implementers. This is an accompanying document to Ethereum 2.0 Phase 1, which describes the expected actions of a "validator" participating in the Ethereum 2.0 Phase 1 protocol.

Table of contents

• Introduction
• Prerequisites
• Constants
  • Misc
• Becoming a validator
• Beacon chain validator assignments
  • Lookahead
• Beacon chain responsibilities
  • Block proposal
    • Preparing for a BeaconBlock
    • Constructing the BeaconBlockBody
      • Custody slashings
      • Custody key reveals
      • Early derived secret reveals
      • Shard transitions
      • Light client fields
    • Packaging into a SignedBeaconBlock
  • Attesting
    • FullAttestationData
    • FullAttestation
    • Timing
    • Attestation data
      • Head shard root
      • Shard transition
    • Construct attestation
  • Attestation Aggregation
    • Broadcast aggregate
      • AggregateAndProof
      • SignedAggregateAndProof
  • Light client committee
    • Preparation
    • Light clent vote
      • Light client vote data
        • LightClientVoteData
      • Construct vote
        • LightClientVote
      • Broadcast
    • Light client vote aggregation
    • Aggregation selection
    • Construct aggregate
    • Broadcast aggregate
      • LightAggregateAndProof
      • SignedLightAggregateAndProof
• How to avoid slashing
  • Custody slashing

Introduction

This document represents the expected behavior of an "honest validator" with respect to Phase 1 of the Ethereum 2.0 protocol. This document does not distinguish between a "node" (i.e. the functionality of following and reading the beacon chain) and a "validator client" (i.e. the functionality of actively participating in consensus). The separation of concerns between these (potentially) two pieces of software is left as a design decision that is out of scope.

A validator is an entity that participates in the consensus of the Ethereum 2.0 protocol. This is an optional role for users in which they can post ETH as collateral and verify and attest to the validity of blocks to seek financial returns in exchange for building and securing the protocol. This is similar to proof-of-work networks in which miners provide collateral in the form of hardware/hash-power to seek returns in exchange for building and securing the protocol.

Prerequisites

This document is an extension of the Phase 0 -- Validator. All behaviors and definitions defined in the Phase 0 doc carry over unless explicitly noted or overridden.

All terminology, constants, functions, and protocol mechanics defined in the Phase 1 -- The Beacon Chain and Phase 1 -- Custody Game docs are requisite for this document and used throughout. Please see the Phase 1 docs before continuing and use as a reference throughout.

Constants

See constants from Phase 0 validator guide.

Misc

Name	Value	Unit	Duration
TARGET_LIGHT_CLIENT_AGGREGATORS_PER_SLOT	2**3 (= 8)	validators
LIGHT_CLIENT_PREPARATION_EPOCHS	2**2 (= 4)	epochs

Becoming a validator

Becoming a validator in Phase 1 is unchanged from Phase 0. See the Phase 0 validator guide for details.

Beacon chain validator assignments

Beacon chain validator assignments to beacon committees and beacon block proposal are unchanged from Phase 0. See the Phase 0 validator guide for details.

Lookahead

Lookahead for beacon committee assignments operates in the same manner as Phase 0, but committee members must join a shard block pubsub topic in addition to the committee attestation topic.

Specifically after finding stable peers of attestation subnets (see Phase 0) a validator should:

• Let shard = compute_shard_from_committee_index(state, committee_index, slot)
• Subscribe to the pubsub topic shard_{shard}_block (attestation subnet peers should have this topic available).

TODO: For now, the state we pass to compute_shard_from_committee_index is the current state without considering len(state.shard_states), i.e., the result from get_active_shard_count(state) changes. We should fix it when we have shard count update logic.

Beacon chain responsibilities

A validator has two primary responsibilities to the beacon chain: proposing blocks and creating attestations. Proposals happen infrequently, whereas attestations should be created once per epoch.

These responsibilities are largely unchanged from Phase 0, but utilize the updated SignedBeaconBlock, BeaconBlock, BeaconBlockBody, Attestation, and AttestationData definitions found in Phase 1. Below notes only the additional and modified behavior with respect to Phase 0.

Phase 1 adds light client committees and associated responsibilities, discussed below.

Block proposal

Preparing for a BeaconBlock

slot, proposer_index, parent_root, state_root fields are unchanged.

Constructing the BeaconBlockBody

randao_reveal, eth1_data, and graffiti are unchanged.

proposer_slashings, deposits, and voluntary_exits are unchanged.

attester_slashings and attestations operate exactly as in Phase 0, but with new definitations of AttesterSlashing and Attestation, along with modified validation conditions found in process_attester_slashing and process_attestation.

Custody slashings

Up to MAX_CUSTODY_SLASHINGS, CustodySlashing objects can be included in the block. The custody slashings must satisfy the verification conditions found in custody slashings processing. The validator receives a small "whistleblower" reward for each custody slashing included (THIS IS NOT CURRENTLY THE CASE BUT PROBABLY SHOULD BE).

Custody key reveals

Up to MAX_CUSTODY_KEY_REVEALS, CustodyKeyReveal objects can be included in the block. The custody key reveals must satisfy the verification conditions found in custody key reveal processing. The validator receives a small reward for each custody key reveal included.

Early derived secret reveals

Up to MAX_EARLY_DERIVED_SECRET_REVEALS, EarlyDerivedSecretReveal objects can be included in the block. The early derived secret reveals must satisfy the verification conditions found in early derived secret reveal processing. The validator receives a small "whistleblower" reward for each early derived secrete reveal included.

Shard transitions

Exactly MAX_SHARDS ShardTransition objects are included in the block. Default each to an empty ShardTransition(). Then for each committee assigned to the slot with an associated committee_index and shard, set shard_transitions[shard] = full_transitions[winning_root] if the committee had enough weight to form a crosslink this slot.

Specifically:

• Call shards, winning_roots = get_shard_winning_roots(state, block.body.attestations)
• Let full_transitions be a dictionary mapping from the shard_transition_roots found in attestations to the corresponding full ShardTransition
• Then for each shard and winning_root in zip(shards, winning_roots) set shard_transitions[shard] = full_transitions[winning_root]

Note: The state passed into get_shard_winning_roots must be transitioned the slot of block.slot to run accurately due to the internal use of get_online_validator_indices and is_on_time_attestation.

def get_shard_winning_roots(state: BeaconState,
                            attestations: Sequence[Attestation]) -> Tuple[Sequence[Shard], Sequence[Root]]:
    shards = []
    winning_roots = []
    online_indices = get_online_validator_indices(state)
    on_time_attestation_slot = compute_previous_slot(state.slot)
    committee_count = get_committee_count_per_slot(state, compute_epoch_at_slot(on_time_attestation_slot))
    for committee_index in map(CommitteeIndex, range(committee_count)):
        shard = compute_shard_from_committee_index(state, committee_index, on_time_attestation_slot)
        # All attestations in the block for this committee/shard and are "on time"
        shard_attestations = [
            attestation for attestation in attestations
            if is_on_time_attestation(state, attestation) and attestation.data.index == committee_index
        ]
        committee = get_beacon_committee(state, on_time_attestation_slot, committee_index)

        # Loop over all shard transition roots, looking for a winning root
        shard_transition_roots = set([a.data.shard_transition_root for a in shard_attestations])
        for shard_transition_root in sorted(shard_transition_roots):
            transition_attestations = [
                a for a in shard_attestations
                if a.data.shard_transition_root == shard_transition_root
            ]
            transition_participants: Set[ValidatorIndex] = set()
            for attestation in transition_attestations:
                participants = get_attesting_indices(state, attestation.data, attestation.aggregation_bits)
                transition_participants = transition_participants.union(participants)

            enough_online_stake = (
                get_total_balance(state, online_indices.intersection(transition_participants)) * 3 >=
                get_total_balance(state, online_indices.intersection(committee)) * 2
            )
            if enough_online_stake:
                shards.append(shard)
                winning_roots.append(shard_transition_root)
                break

    return shards, winning_roots

Light client fields

First retrieve best_aggregate from get_best_light_client_aggregate(block, aggregates) where aggregates is a list of valid aggregated LightClientVotes for the previous slot.

Then:

• Set light_client_bits = best_aggregate.aggregation_bits
• Set light_client_signature = best_aggregate.signature

def get_best_light_client_aggregate(block: BeaconBlock,
                                    aggregates: Sequence[LightClientVote]) -> LightClientVote:
    viable_aggregates = [
        aggregate for aggregate in aggregates
        if (
            aggregate.data.slot == compute_previous_slot(block.slot)
            and aggregate.data.beacon_block_root == block.parent_root
        )
    ]

    return max(
        viable_aggregates,
        # Ties broken by lexicographically by hash_tree_root
        key=lambda a: (len([i for i in a.aggregation_bits if i == 1]), hash_tree_root(a)),
        default=LightClientVote(),
    )

Packaging into a SignedBeaconBlock

Packaging into a SignedBeaconBlock is unchanged from Phase 0.

Attesting

A validator is expected to create, sign, and broadcast an attestation during each epoch.

Assignments and the core of this duty are unchanged from Phase 0. There are a few additional fields related to the assigned shard chain.

The Attestation and AttestationData defined in the Phase 1 Beacon Chain spec utilizes shard_transition_root: Root rather than a full ShardTransition. For the purposes of the validator and p2p layer, a modified FullAttestationData and containing FullAttestation are used to send the accompanying ShardTransition in its entirety. Note that due to the properties of SSZ hash_tree_root, the root and signatures of AttestationData and FullAttestationData are equivalent.

FullAttestationData

class FullAttestationData(Container):
    slot: Slot
    index: CommitteeIndex
    # LMD GHOST vote
    beacon_block_root: Root
    # FFG vote
    source: Checkpoint
    target: Checkpoint
    # Current-slot shard block root
    shard_head_root: Root
    # Full shard transition
    shard_transition: ShardTransition

FullAttestation

class FullAttestation(Container):
    aggregation_bits: Bitlist[MAX_VALIDATORS_PER_COMMITTEE]
    data: FullAttestationData
    signature: BLSSignature

Timing

Note the timing of when to create/broadcast is altered from Phase 1.

A validator should create and broadcast the attestation to the associated attestation subnet when either (a) the validator has received a valid BeaconBlock from the expected beacon block proposer and a valid ShardBlock for the expected shard block proposer for the assigned slot or (b) one-half of the slot has transpired (SECONDS_PER_SLOT / 2 seconds after the start of slot) -- whichever comes first.

Attestation data

attestation_data is constructed in the same manner as Phase 0 but uses FullAttestationData with the addition of two fields -- shard_head_root and shard_transition.

• Let head_block be the result of running the fork choice during the assigned slot.
• Let head_state be the state of head_block processed through any empty slots up to the assigned slot using process_slots(state, slot).
• Let shard_head_block be the result of running the fork choice on the assigned shard chain during the assigned slot.
• Let shard_blocks be the shard blocks in the chain starting immediately after the most recent crosslink (head_state.shard_transitions[shard].latest_block_root) up to the shard_head_block (i.e. the value of the shard fork choice store of get_pending_shard_blocks(store, shard_store)).

Note: We assume that the fork choice only follows branches with valid offset_slots with respect to the most recent beacon state shard transition for the queried shard.

Head shard root

Set attestation_data.shard_head_root = hash_tree_root(shard_head_block).

Shard transition

Set shard_transition to the value returned by get_shard_transition(head_state, shard, shard_blocks).

def get_shard_transition_fields(
    beacon_state: BeaconState,
    shard: Shard,
    shard_blocks: Sequence[SignedShardBlock],
) -> Tuple[Sequence[uint64], Sequence[Root], Sequence[ShardState]]:
    shard_states = []
    shard_data_roots = []
    shard_block_lengths = []

    shard_state = beacon_state.shard_states[shard]
    shard_block_slots = [shard_block.message.slot for shard_block in shard_blocks]
    offset_slots = compute_offset_slots(
        get_latest_slot_for_shard(beacon_state, shard),
        Slot(beacon_state.slot + 1),
    )
    for slot in offset_slots:
        if slot in shard_block_slots:
            shard_block = shard_blocks[shard_block_slots.index(slot)]
            shard_data_roots.append(hash_tree_root(shard_block.message.body))
        else:
            shard_block = SignedShardBlock(message=ShardBlock(slot=slot, shard=shard))
            shard_data_roots.append(Root())
        shard_state = shard_state.copy()
        shard_state_transition(shard_state, shard_block, validate=False)
        shard_states.append(shard_state)
        shard_block_lengths.append(len(shard_block.message.body))

    return shard_block_lengths, shard_data_roots, shard_states

def get_shard_transition(beacon_state: BeaconState,
                         shard: Shard,
                         shard_blocks: Sequence[SignedShardBlock]) -> ShardTransition:
    offset_slots = compute_offset_slots(
        get_latest_slot_for_shard(beacon_state, shard),
        Slot(beacon_state.slot + 1),
    )
    shard_block_lengths, shard_data_roots, shard_states = (
        get_shard_transition_fields(beacon_state, shard, shard_blocks)
    )

    if len(shard_blocks) > 0:
        proposer_signatures = [shard_block.signature for shard_block in shard_blocks]
        proposer_signature_aggregate = bls.Aggregate(proposer_signatures)
    else:
        proposer_signature_aggregate = NO_SIGNATURE

    return ShardTransition(
        start_slot=offset_slots[0],
        shard_block_lengths=shard_block_lengths,
        shard_data_roots=shard_data_roots,
        shard_states=shard_states,
        proposer_signature_aggregate=proposer_signature_aggregate,
    )

Construct attestation

Next, the validator creates attestation, a FullAttestation as defined above.

attestation.data, attestation.aggregation_bits, and attestation.signature are unchanged from Phase 0. But safety/validity in signing the message is premised upon calculation of the "custody bit" [TODO].

Attestation Aggregation

Some validators are selected to locally aggregate attestations with a similar attestation_data to their constructed attestation for the assigned slot.

Aggregation selection and the core of this duty are largely unchanged from Phase 0. Any additional components or changes are noted.

Broadcast aggregate

Note the timing of when to broadcast aggregates is altered in Phase 1+.

If the validator is selected to aggregate (is_aggregator), then they broadcast their best aggregate as a SignedAggregateAndProof to the global aggregate channel (beacon_aggregate_and_proof) three-fourths of the way through the slot-that is, SECONDS_PER_SLOT * 3 / 4 seconds after the start of slot.

AggregateAndProof

AggregateAndProof is unchanged other than the contained Attestation.

class AggregateAndProof(Container):
    aggregator_index: ValidatorIndex
    aggregate: Attestation
    selection_proof: BLSSignature

SignedAggregateAndProof

AggregateAndProof is unchanged other than the contained AggregateAndProof.

class SignedAggregateAndProof(Container):
    message: AggregateAndProof
    signature: BLSSignature

Light client committee

In addition to the core beacon chain responsibilities, Phase 1 adds an additional role -- the Light Client Committee -- to aid in light client functionality.

Validators serve on the light client committee for LIGHT_CLIENT_COMMITTEE_PERIOD epochs and the assignment to be on a committee is known LIGHT_CLIENT_COMMITTEE_PERIOD epochs in advance.

Preparation

When get_current_epoch(state) % LIGHT_CLIENT_COMMITTEE_PERIOD == LIGHT_CLIENT_COMMITTEE_PERIOD - LIGHT_CLIENT_PREPARATION_EPOCHS each validator must check if they are in the next period light client committee by calling is_in_next_light_client_committee().

If the validator is in the next light client committee, they must join the light_client_votes pubsub topic to begin duties at the start of the next period.

def is_in_next_light_client_committee(state: BeaconState, index: ValidatorIndex) -> bool:
    next_committee = get_light_client_committee(state, get_current_epoch(state) + LIGHT_CLIENT_COMMITTEE_PERIOD)
    return index in next_committee

Light clent vote

During a period of epochs that the validator is a part of the light client committee (validator_index in get_light_client_committee(state, epoch)), the validator creates and broadcasts a LightClientVote at each slot.

A validator should create and broadcast the light_client_vote to the light_client_votes pubsub topic when either (a) the validator has received a valid block from the expected block proposer for the current slot or (b) two-thirds of the slot have transpired (SECONDS_PER_SLOT / 3 seconds after the start of slot) -- whichever comes first.

• Let light_client_committee = get_light_client_committee(state, compute_epoch_at_slot(slot))

Light client vote data

First the validator constructs light_client_vote_data, a LightClientVoteData object.

• Let head_block be the result of running the fork choice during the assigned slot.
• Set light_client_vote.slot = slot.
• Set light_client_vote.beacon_block_root = hash_tree_root(head_block).

LightClientVoteData

class LightClientVoteData(Container):
    slot: Slot
    beacon_block_root: Root

Construct vote

Then the validator constructs light_client_vote, a LightClientVote object.

• Set light_client_vote.data = light_client_vote_data.
• Set light_client_vote.aggregation_bits to be a Bitvector[LIGHT_CLIENT_COMMITTEE_SIZE], where the bit of the index of the validator in the light_client_committee is set to 0b1 and all other bits are are set to 0b0.
• Set light_client_vote.signature = vote_signature where vote_signature is obtained from:

def get_light_client_vote_signature(state: BeaconState,
                                    light_client_vote_data: LightClientVoteData,
                                    privkey: int) -> BLSSignature:
    domain = get_domain(state, DOMAIN_LIGHT_CLIENT, compute_epoch_at_slot(light_client_vote_data.slot))
    signing_root = compute_signing_root(light_client_vote_data, domain)
    return bls.Sign(privkey, signing_root)

LightClientVote

class LightClientVote(Container):
    data: LightClientVoteData
    aggregation_bits: Bitvector[LIGHT_CLIENT_COMMITTEE_SIZE]
    signature: BLSSignature

Broadcast

Finally, the validator broadcasts light_client_vote to the light_client_votes pubsub topic.

Light client vote aggregation

Some validators in the light client committee are selected to locally aggregate light client votes with a similar light_client_vote_data to their constructed light_client_vote for the assigned slot.

Aggregation selection

A validator is selected to aggregate based upon the return value of is_light_client_aggregator().

def get_light_client_slot_signature(state: BeaconState, slot: Slot, privkey: int) -> BLSSignature:
    domain = get_domain(state, DOMAIN_LIGHT_SELECTION_PROOF, compute_epoch_at_slot(slot))
    signing_root = compute_signing_root(slot, domain)
    return bls.Sign(privkey, signing_root)

def is_light_client_aggregator(state: BeaconState, slot: Slot, slot_signature: BLSSignature) -> bool:
    committee = get_light_client_committee(state, compute_epoch_at_slot(slot))
    modulo = max(1, len(committee) // TARGET_LIGHT_CLIENT_AGGREGATORS_PER_SLOT)
    return bytes_to_int(hash(slot_signature)[0:8]) % modulo == 0

Construct aggregate

If the validator is selected to aggregate (is_light_client_aggregator()), they construct an aggregate light client vote via the following.

Collect light_client_votes seen via gossip during the slot that have an equivalent light_client_vote_data to that constructed by the validator, and create a aggregate_light_client_vote: LightClientVote with the following fields.

• Set aggregate_light_client_vote.data = light_client_vote_data where light_client_vote_data is the LightClientVoteData object that is the same for each individual light client vote being aggregated.
• Set aggregate_light_client_vote.aggregation_bits to be a Bitvector[LIGHT_CLIENT_COMMITTEE_SIZE], where each bit set from each individual light client vote is set to 0b1.
• Set aggregate_light_client_vote.signature = aggregate_light_client_signature where aggregate_light_client_signature is obtained from get_aggregate_light_client_signature.

def get_aggregate_light_client_signature(light_client_votes: Sequence[LightClientVote]) -> BLSSignature:
    signatures = [light_client_vote.signature for light_client_vote in light_client_votes]
    return bls.Aggregate(signatures)

Broadcast aggregate

If the validator is selected to aggregate (is_light_client_aggregator), then they broadcast their best aggregate light client vote as a SignedLightAggregateAndProof to the global aggregate light client vote channel (aggregate_light_client_votes) two-thirds of the way through the slot-that is, SECONDS_PER_SLOT * 2 / 3 seconds after the start of slot.

Selection proofs are provided in LightAggregateAndProof to prove to the gossip channel that the validator has been selected as an aggregator.

LightAggregateAndProof messages are signed by the aggregator and broadcast inside of SignedLightAggregateAndProof objects to prevent a class of DoS attacks and message forgeries.

First, light_aggregate_and_proof = get_light_aggregate_and_proof(state, validator_index, aggregate_light_client_vote, privkey) is constructed.

def get_light_aggregate_and_proof(state: BeaconState,
                                  aggregator_index: ValidatorIndex,
                                  aggregate: LightClientVote,
                                  privkey: int) -> LightAggregateAndProof:
    return LightAggregateAndProof(
        aggregator_index=aggregator_index,
        aggregate=aggregate,
        selection_proof=get_light_client_slot_signature(state, aggregate.data.slot, privkey),
    )

Then signed_light_aggregate_and_proof = SignedLightAggregateAndProof(message=light_aggregate_and_proof, signature=signature) is constructed and broadast. Where signature is obtained from:

def get_light_aggregate_and_proof_signature(state: BeaconState,
                                            aggregate_and_proof: LightAggregateAndProof,
                                            privkey: int) -> BLSSignature:
    aggregate = aggregate_and_proof.aggregate
    domain = get_domain(state, DOMAIN_LIGHT_AGGREGATE_AND_PROOF, compute_epoch_at_slot(aggregate.data.slot))
    signing_root = compute_signing_root(aggregate_and_proof, domain)
    return bls.Sign(privkey, signing_root)

LightAggregateAndProof

class LightAggregateAndProof(Container):
    aggregator_index: ValidatorIndex
    aggregate: LightClientVote
    selection_proof: BLSSignature

SignedLightAggregateAndProof

class SignedLightAggregateAndProof(Container):
    message: LightAggregateAndProof
    signature: BLSSignature

How to avoid slashing

Proposer and Attester slashings described in Phase 0 remain in place with the addition of the following.

Custody slashing

To avoid custody slashings, the attester must never sign any shard transition for which the custody bit is one. The custody bit is computed using the custody secret:

def get_custody_secret(state: BeaconState,
                       validator_index: ValidatorIndex,
                       privkey: int,
                       epoch: Epoch=None) -> BLSSignature:
    if epoch is None:
        epoch = get_current_epoch(state)
    period = get_custody_period_for_validator(validator_index, epoch)
    epoch_to_sign = get_randao_epoch_for_custody_period(period, validator_index)
    domain = get_domain(state, DOMAIN_RANDAO, epoch_to_sign)
    signing_root = compute_signing_root(Epoch(epoch_to_sign), domain)
    return bls.Sign(privkey, signing_root)

Note that the valid custody secret is always the one for the attestation target epoch, not to be confused with the epoch in which the shard block was generated. While they are the same most of the time, getting this wrong at custody epoch boundaries would result in a custody slashing.