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Sharding -- The Beacon Chain

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

Table of contents

Introduction

This document describes the extensions made to the Phase 0 design of The Beacon Chain to support data sharding, based on the ideas here and more broadly here, using KZG10 commitments to commit to data to remove any need for fraud proofs (and hence, safety-critical synchrony assumptions) in the design.

Glossary

  • Data: A list of KZG points, to translate a byte string into
  • Blob: Data with commitments and meta-data, like a flattened bundle of L2 transactions.

Constants

The following values are (non-configurable) constants used throughout the specification.

Misc

Name Value Notes
FIELD_ELEMENTS_PER_SAMPLE uint64(2**4) (= 16) 31 * 16 = 496 bytes

Domain types

Name Value
DOMAIN_SHARD_SAMPLE DomainType('0x10000000')

Preset

Misc

Name Value Notes
MAX_SHARDS uint64(2**12) (= 4,096) Theoretical max shard count (used to determine data structure sizes)
ACTIVE_SHARDS uint64(2**8) (= 256) Initial shard count
MAX_PROPOSER_BLOCKS_BETWEEN_BUILDER_BLOCKS uint64(2**4) (= 16) TODO: Need to define what happens if there were more blocks without builder blocks

Time parameters

With the introduction of builder blocks the number of slots per epoch is doubled (it counts beacon blocks and builder blocks).

Name Value Unit Duration
SLOTS_PER_EPOCH uint64(2**6) (= 64) slots 8:32 minutes

Shard blob samples

Name Value Notes
SAMPLES_PER_BLOB uint64(2**9) (= 512) 248 * 512 = 126,976 bytes

Configuration

Note: Some preset variables may become run-time configurable for testnets, but default to a preset while the spec is unstable.
E.g. ACTIVE_SHARDS and SAMPLES_PER_BLOB.

Time parameters

Name Value Unit Duration
SECONDS_PER_SLOT uint64(8) seconds 8 seconds

Containers

New Containers

BuilderBlockBid

class BuilderBlockBid(Container):
    slot: Slot
    parent_block_root: Root

    execution_payload_root: Root

    sharded_data_commitment_root: Root # Root of the sharded data (only data, not beacon/builder block commitments)

    sharded_data_commitment_count: uint64 # Count of sharded data commitments

    bid: Gwei # Block builder bid paid to proposer

    validator_index: ValidatorIndex # Validator index for this bid
    
    # Block builders use an Eth1 address -- need signature as
    # block bid and data gas base fees will be charged to this address
    signature_y_parity: bool
    signature_r: uint256
    signature_s: uint256

BuilderBlockBidWithRecipientAddress

class BuilderBlockBidWithRecipientAddress(Container):
    builder_block_bid: Union[None, BuilderBlockBid]
    recipient_address: ExecutionAddress # Address to receive the block builder bid

ShardedCommitmentsContainer

class ShardedCommitmentsContainer(Container):
    sharded_commitments: List[KZGCommitment, 2 * MAX_SHARDS]

    # Aggregate degree proof for all sharded_commitments
    degree_proof: KZGCommitment

    # The sizes of the blocks encoded in the commitments (last builder and all beacon blocks since)
    included_block_sizes: List[uint64, MAX_PROPOSER_BLOCKS_BETWEEN_BUILDER_BLOCKS + 1]
    
    # Number of commitments that are for sharded data (no blocks)
    included_sharded_data_commitments: uint64

    # Random evaluation of beacon blocks + execution payload (this helps with quick verification)
    block_verification_kzg_proof: KZGCommitment

ShardSample

class ShardSample(Container):
    slot: Slot
    row: uint64
    column: uint64
    data: Vector[BLSFieldElement, FIELD_ELEMENTS_PER_SAMPLE]
    proof: KZGCommitment
    builder: ValidatorIndex
    signature: BLSSignature

Extended Containers

BeaconState

class BeaconState(bellatrix.BeaconState):
    blocks_since_builder_block: List[BeaconBlock, MAX_PROPOSER_BLOCKS_BETWEEN_BUILDER_BLOCKS]

BuilderBlockData

class BuilderBlockData(Container):
    execution_payload: ExecutionPayload
    sharded_commitments_container: ShardedCommitmentsContainer

BeaconBlockBody

class BeaconBlockBody(altair.BeaconBlockBody):
    payload_data: Union[BuilderBlockBid, BuilderBlockData]

Helper functions

Block processing

is_builder_block_slot

def is_builder_block_slot(slot: Slot) -> bool:
    return slot % 2 == 1

Beacon state accessors

get_active_shard_count

def get_active_shard_count(state: BeaconState, epoch: Epoch) -> uint64:
    """
    Return the number of active shards.
    Note that this puts an upper bound on the number of committees per slot.
    """
    return ACTIVE_SHARDS

Beacon chain state transition function

Block processing

process_block

def process_block(state: BeaconState, block: BeaconBlock) -> None:
    process_block_header(state, block)
    verify_builder_block_bid(state, block)
    process_sharded_data(state, block)
    if is_execution_enabled(state, block.body):
        process_execution_payload(state, block, EXECUTION_ENGINE)

    if not is_builder_block_slot(block.slot):
        process_randao(state, block.body)

    process_eth1_data(state, block.body)
    process_operations(state, block.body)
    process_sync_aggregate(state, block.body.sync_aggregate)

    if is_builder_block_slot(block.slot):
        state.blocks_since_builder_block = []
    state.blocks_since_builder_block.append(block)

Block header

def process_block_header(state: BeaconState, block: BeaconBlock) -> None:
    # Verify that the slots match
    assert block.slot == state.slot
    # Verify that the block is newer than latest block header
    assert block.slot > state.latest_block_header.slot
    # Verify that proposer index is the correct index
    if not is_builder_block_slot(block.slot):
        assert block.proposer_index == get_beacon_proposer_index(state)
    # Verify that the parent matches
    assert block.parent_root == hash_tree_root(state.latest_block_header)
    # Cache current block as the new latest block
    state.latest_block_header = BeaconBlockHeader(
        slot=block.slot,
        proposer_index=block.proposer_index,
        parent_root=block.parent_root,
        state_root=Bytes32(),  # Overwritten in the next process_slot call
        body_root=hash_tree_root(block.body),
    )

    # Verify proposer is not slashed
    proposer = state.validators[block.proposer_index]
    assert not proposer.slashed

Builder Block Bid

def verify_builder_block_bid(state: BeaconState, block: BeaconBlock) -> None:
    if is_builder_block_slot(block.slot):
        # Get last builder block bid
        assert state.blocks_since_builder_block[-1].body.payload_data.selector == 0
        builder_block_bid = state.blocks_since_builder_block[-1].body.payload_data.value.builder_block_bid
        assert builder_block_bid.slot + 1 == block.slot

        assert block.body.payload_data.selector == 1 # Verify that builder block does not contain bid

        builder_block_data = block.body.payload_data.value

        assert builder_block_bid.execution_payload_root == hash_tree_root(builder_block_data.execution_payload)

        assert builder_block_bid.sharded_data_commitment_count == builder_block_data.included_sharded_data_commitments

        assert builder_block_bid.sharded_data_commitment_root == hash_tree_root(builder_block_data.sharded_commitments[-builder_block_bid.included_sharded_data_commitments:])

        assert builder_block_bid.validator_index == block.proposer_index

    else:
        assert block.body.payload_data.selector == 0

        builder_block_bid = block.body.payload_data.value.builder_block_bid
        assert builder_block_bid.slot == block.slot
        assert builder_block_bid.parent_block_root == block.parent_root
        # We do not check that the builder address exists or has sufficient balance here.
        # If it does not have sufficient balance, the block proposer loses out, so it is their
        # responsibility to check.

        # Check that the builder is a slashable validator. We can probably reduce this requirement and only
        # ensure that they have 1 ETH in their account as a DOS protection.
        builder = state.validators[builder_block_bid.validator_index]
        assert is_slashable_validator(builder, get_current_epoch(state))

Sharded data

def process_sharded_data(state: BeaconState, block: BeaconBlock) -> None:
    if is_builder_block_slot(block.slot):
        assert block.body.payload_data.selector == 1
        sharded_commitments_container = block.body.payload_data.value.sharded_commitments_container

        # Verify not too many commitments
        assert len(sharded_commitments_container.sharded_commitments) // 2 <= get_active_shard_count(state, get_current_epoch(state))

        # Verify the degree proof
        r = hash_to_bls_field(sharded_commitments_container.sharded_commitments, 0)
        r_powers = compute_powers(r, len(sharded_commitments_container.sharded_commitments))
        combined_commitment = elliptic_curve_lincomb(sharded_commitments_container.sharded_commitments, r_powers)

        payload_field_elements_per_blob = SAMPLES_PER_BLOB * FIELD_ELEMENTS_PER_SAMPLE // 2

        verify_degree_proof(combined_commitment, payload_field_elements_per_blob, sharded_commitments_container.degree_proof)

        # Verify that the 2*N commitments lie on a degree < N polynomial
        low_degree_check(sharded_commitments_container.sharded_commitments)

        # Verify that blocks since the last builder block have been included
        blocks_chunked = [bytes_to_field_elements(ssz_serialize(block)) for block in state.blocks_since_builder_block]
        block_vectors = []

        for block_chunked in blocks_chunked:
            for i in range(0, len(block_chunked), payload_field_elements_per_blob):
                block_vectors.append(block_chunked[i:i + payload_field_elements_per_blob])

        number_of_blobs = len(block_vectors)
        r = hash_to_bls_field(sharded_commitments_container.sharded_commitments[:number_of_blobs], 0)
        x = hash_to_bls_field(sharded_commitments_container.sharded_commitments[:number_of_blobs], 1)

        r_powers = compute_powers(r, number_of_blobs)
        combined_vector = vector_lincomb(block_vectors, r_powers)
        combined_commitment = elliptic_curve_lincomb(sharded_commitments_container.sharded_commitments[:number_of_blobs], r_powers)
        y = evaluate_polynomial_in_evaluation_form(combined_vector, x)

        verify_kzg_proof(combined_commitment, x, y, sharded_commitments_container.block_verification_kzg_proof)

        # Verify that number of sharded data commitments is correctly indicated
        assert 2 * (number_of_blobs + included_sharded_data_commitments) == len(sharded_commitments_container.sharded_commitments)

Execution payload

def process_execution_payload(state: BeaconState, block: BeaconBlock, execution_engine: ExecutionEngine) -> None:
    if is_builder_block_slot(block.slot):
        assert block.body.payload_data.selector == 1
        payload = block.body.payload_data.value.execution_payload

        # Verify consistency of the parent hash with respect to the previous execution payload header
        if is_merge_transition_complete(state):
            assert payload.parent_hash == state.latest_execution_payload_header.block_hash
        # Verify random
        assert payload.random == get_randao_mix(state, get_current_epoch(state))
        # Verify timestamp
        assert payload.timestamp == compute_timestamp_at_slot(state, state.slot)

        # Get sharded data commitments
        sharded_commitments_container = block.body.sharded_commitments_container
        sharded_data_commitments = sharded_commitments_container.sharded_commitments[-sharded_commitments_container.included_sharded_data_commitments:]

        # Get all unprocessed builder block bids
        unprocessed_builder_block_bid_with_recipient_addresses = []
        for block in state.blocks_since_builder_block[1:]:
            unprocessed_builder_block_bid_with_recipient_addresses.append(block.body.builder_block_bid_with_recipient_address.value)

        # Verify the execution payload is valid
        # The execution engine gets two extra payloads: One for the sharded data commitments (these are needed to verify type 3 transactions)
        # and one for all so far unprocessed builder block bids:
        # * The execution engine needs to transfer the balance from the bidder to the proposer.
        # * The execution engine needs to deduct data gas fees from the bidder balances
        assert execution_engine.execute_payload(payload,
                                                sharded_data_commitments,
                                                unprocessed_builder_block_bid_with_recipient_addresses)

        # Cache execution payload header
        state.latest_execution_payload_header = ExecutionPayloadHeader(
            parent_hash=payload.parent_hash,
            fee_recipient=payload.fee_recipient,
            state_root=payload.state_root,
            receipt_root=payload.receipt_root,
            logs_bloom=payload.logs_bloom,
            random=payload.random,
            block_number=payload.block_number,
            gas_limit=payload.gas_limit,
            gas_used=payload.gas_used,
            timestamp=payload.timestamp,
            extra_data=payload.extra_data,
            base_fee_per_gas=payload.base_fee_per_gas,
            block_hash=payload.block_hash,
            transactions_root=hash_tree_root(payload.transactions),
        )