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eth2.0-specs/specs/sharding/beacon-chain.md

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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.
  • Builder: Independent actor that builds blobs and bids for proposal slots via fee-paying blob-headers, responsible for availability.
  • Shard proposer: Validator taking bids from blob builders for shard data opportunity, co-signs with builder to propose the blob.

Custom types

We define the following Python custom types for type hinting and readability:

Name SSZ equivalent Description
Shard uint64 A shard number
BLSCommitment Bytes48 A G1 curve point
BLSPoint uint256 A number x in the range 0 <= x < MODULUS
BuilderIndex uint64 Builder registry index

Constants

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

Misc

Name Value Notes
PRIMITIVE_ROOT_OF_UNITY 5 Primitive root of unity of the BLS12_381 (inner) modulus
DATA_AVAILABILITY_INVERSE_CODING_RATE 2**1 (= 2) Factor by which samples are extended for data availability encoding
POINTS_PER_SAMPLE uint64(2**3) (= 8) 31 * 8 = 248 bytes
MODULUS 0x73eda753299d7d483339d80809a1d80553bda402fffe5bfeffffffff00000001 (curve order of BLS12_381)

Domain types

Name Value
DOMAIN_SHARD_BLOB DomainType('0x80000000')

Shard Work Status

Name Value Notes
SHARD_WORK_UNCONFIRMED 0 Unconfirmed, nullified after confirmation time elapses
SHARD_WORK_CONFIRMED 1 Confirmed, reduced to just the commitment
SHARD_WORK_PENDING 2 Pending, a list of competing headers

Misc

TODO: PARTICIPATION_FLAG_WEIGHTS backwards-compatibility is difficult, depends on usage.

Name Value
PARTICIPATION_FLAG_WEIGHTS [TIMELY_SOURCE_WEIGHT, TIMELY_TARGET_WEIGHT, TIMELY_HEAD_WEIGHT, TIMELY_SHARD_WEIGHT]

Participation flag indices

Name Value
TIMELY_SHARD_FLAG_INDEX 3

Incentivization weights

TODO: determine weight for shard attestations

Name Value
TIMELY_SHARD_WEIGHT uint64(8)

TODO: WEIGHT_DENOMINATOR needs to be adjusted, but this breaks a lot of Altair code.

Preset

Misc

Name Value Notes
MAX_SHARDS uint64(2**10) (= 1,024) Theoretical max shard count (used to determine data structure sizes)
INITIAL_ACTIVE_SHARDS uint64(2**6) (= 64) Initial shard count
SAMPLE_PRICE_ADJUSTMENT_COEFFICIENT uint64(2**3) (= 8) Sample price may decrease/increase by at most exp(1 / this value) per epoch
MAX_SHARD_PROPOSER_SLASHINGS 2**4 (= 16) Maximum amount of shard proposer slashing operations per block
MAX_SHARD_HEADERS_PER_SHARD 4
SHARD_STATE_MEMORY_SLOTS uint64(2**8) (= 256) Number of slots for which shard commitments and confirmation status is directly available in the state
BLOB_BUILDER_REGISTRY_LIMIT uint64(2**40) (= 1,099,511,627,776) shard blob builders

Shard blob samples

Name Value Notes
MAX_SAMPLES_PER_BLOB uint64(2**11) (= 2,048) 248 * 2,048 = 507,904 bytes
TARGET_SAMPLES_PER_BLOB uint64(2**10) (= 1,024) 248 * 1,024 = 253,952 bytes

Precomputed size verification points

Name Value
G1_SETUP Type List[G1]. The G1-side trusted setup [G, G*s, G*s**2....]; note that the first point is the generator.
G2_SETUP Type List[G2]. The G2-side trusted setup [G, G*s, G*s**2....]
ROOT_OF_UNITY pow(PRIMITIVE_ROOT_OF_UNITY, (MODULUS - 1) // int(MAX_SAMPLES_PER_BLOB * POINTS_PER_SAMPLE), MODULUS)

Gwei values

Name Value Unit Description
MAX_SAMPLE_PRICE Gwei(2**33) (= 8,589,934,592) Gwei Max sample charged for a TARGET-sized shard blob
MIN_SAMPLE_PRICE Gwei(2**3) (= 8) Gwei Min sample price charged for a TARGET-sized shard blob

Configuration

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

Updated containers

The following containers have updated definitions to support Sharding.

AttestationData

class AttestationData(Container):
    slot: Slot
    index: CommitteeIndex
    # LMD GHOST vote
    beacon_block_root: Root
    # FFG vote
    source: Checkpoint
    target: Checkpoint
    # Hash-tree-root of ShardBlob
    shard_blob_root: Root  # [New in Sharding]

BeaconBlockBody

class BeaconBlockBody(merge.BeaconBlockBody):  # [extends The Merge block body]
    shard_proposer_slashings: List[ShardProposerSlashing, MAX_SHARD_PROPOSER_SLASHINGS]
    shard_headers: List[SignedShardBlobHeader, MAX_SHARDS * MAX_SHARD_HEADERS_PER_SHARD]

BeaconState

class BeaconState(merge.BeaconState):
    # Blob builder registry.
    blob_builders: List[Builder, BLOB_BUILDER_REGISTRY_LIMIT]
    blob_builder_balances: List[Gwei, BLOB_BUILDER_REGISTRY_LIMIT]
    # A ring buffer of the latest slots, with information per active shard.
    shard_buffer: Vector[List[ShardWork, MAX_SHARDS], SHARD_STATE_MEMORY_SLOTS]
    shard_sample_price: uint64

New containers

Builder

class Builder(Container):
    pubkey: BLSPubkey
    # TODO: fields for either an expiry mechanism (refunding execution account with remaining balance) 
    #  and/or a builder-transaction mechanism.

DataCommitment

class DataCommitment(Container):
    # KZG10 commitment to the data
    point: BLSCommitment
    # Length of the data in samples
    samples_count: uint64

AttestedDataCommitment

class AttestedDataCommitment(Container):
    # KZG10 commitment to the data, and length
    commitment: DataCommitment
    # hash_tree_root of the ShardBlobHeader (stored so that attestations can be checked against it)
    root: Root
    # The proposer who included the shard-header
    includer_index: ValidatorIndex

ShardBlobBody

Unsigned shard data, bundled by a shard-builder. Unique, signing different bodies as shard proposer for the same (slot, shard) is slashable.

class ShardBlobBody(Container):
    # The actual data commitment
    commitment: DataCommitment
    # Proof that the degree < commitment.samples_count * POINTS_PER_SAMPLE
    degree_proof: BLSCommitment
    # The actual data. Should match the commitment and degree proof.
    data: List[BLSPoint, POINTS_PER_SAMPLE * MAX_SAMPLES_PER_BLOB]
    # fee payment fields (EIP 1559 like)
    # TODO: express in MWei instead?
    max_priority_fee_per_sample: Gwei
    max_fee_per_sample: Gwei

ShardBlobBodySummary

Summary version of the ShardBlobBody, omitting the data payload, while preserving the data-commitments.

The commitments are not further collapsed to a single hash, to avoid an extra network roundtrip between proposer and builder, to include the header on-chain more quickly.

class ShardBlobBodySummary(Container):
    # The actual data commitment
    commitment: DataCommitment
    # Proof that the degree < commitment.samples_count * POINTS_PER_SAMPLE
    degree_proof: BLSCommitment
    # Hash-tree-root as summary of the data field
    data_root: Root
    # fee payment fields (EIP 1559 like)
    # TODO: express in MWei instead?
    max_priority_fee_per_sample: Gwei
    max_fee_per_sample: Gwei

ShardBlob

ShardBlobBody wrapped with the header data that is unique to the shard blob proposal.

class ShardBlob(Container):
    slot: Slot
    shard: Shard
    # Builder of the data, pays data-fee to proposer
    builder_index: BuilderIndex
    # Proposer of the shard-blob
    proposer_index: ValidatorIndex
    # Blob contents
    body: ShardBlobBody

ShardBlobHeader

Header version of ShardBlob.

class ShardBlobHeader(Container):
    slot: Slot
    shard: Shard
    # Builder of the data, pays data-fee to proposer
    builder_index: BuilderIndex
    # Proposer of the shard-blob
    proposer_index: ValidatorIndex
    # Blob contents, without the full data
    body_summary: ShardBlobBodySummary

SignedShardBlob

Full blob data, signed by the shard builder (ensuring fee payment) and shard proposer (ensuring a single proposal).

class SignedShardBlob(Container):
    message: ShardBlob
    signature: BLSSignature

SignedShardBlobHeader

Header of the blob, the signature is equally applicable to SignedShardBlob. Shard proposers can accept SignedShardBlobHeader as a data-transaction by co-signing the header.

class SignedShardBlobHeader(Container):
    message: ShardBlobHeader
    # Signature by builder.
    # Once accepted by proposer, the signatures is the aggregate of both.
    signature: BLSSignature

PendingShardHeader

class PendingShardHeader(Container):
    # The commitment that is attested
    attested: AttestedDataCommitment
    # Who voted for the header
    votes: Bitlist[MAX_VALIDATORS_PER_COMMITTEE]
    # Sum of effective balances of votes
    weight: Gwei
    # When the header was last updated, as reference for weight accuracy
    update_slot: Slot

ShardBlobReference

Reference version of ShardBlobHeader, substituting the body for just a hash-tree-root.

class ShardBlobReference(Container):
    slot: Slot
    shard: Shard
    # Builder of the data
    builder_index: BuilderIndex
    # Proposer of the shard-blob
    proposer_index: ValidatorIndex
    # Blob hash-tree-root for slashing reference
    body_root: Root

ShardProposerSlashing

class ShardProposerSlashing(Container):
    slot: Slot
    shard: Shard
    proposer_index: ValidatorIndex
    builder_index_1: BuilderIndex
    builder_index_2: BuilderIndex
    body_root_1: Root
    body_root_2: Root
    signature_1: BLSSignature
    signature_2: BLSSignature

ShardWork

class ShardWork(Container):
    # Upon confirmation the data is reduced to just the commitment.
    status: Union[                                                   # See Shard Work Status enum
              None,                                                  # SHARD_WORK_UNCONFIRMED
              AttestedDataCommitment,                                # SHARD_WORK_CONFIRMED
              List[PendingShardHeader, MAX_SHARD_HEADERS_PER_SHARD]  # SHARD_WORK_PENDING
            ]

Helper functions

Misc

next_power_of_two

def next_power_of_two(x: int) -> int:
    return 2 ** ((x - 1).bit_length())

compute_previous_slot

def compute_previous_slot(slot: Slot) -> Slot:
    if slot > 0:
        return Slot(slot - 1)
    else:
        return Slot(0)

compute_updated_sample_price

def compute_updated_sample_price(prev_price: Gwei, samples_length: uint64, active_shards: uint64) -> Gwei:
    adjustment_quotient = active_shards * SLOTS_PER_EPOCH * SAMPLE_PRICE_ADJUSTMENT_COEFFICIENT
    if samples_length > TARGET_SAMPLES_PER_BLOB:
        delta = max(1, prev_price * (samples_length - TARGET_SAMPLES_PER_BLOB) // TARGET_SAMPLES_PER_BLOB // adjustment_quotient)
        return min(prev_price + delta, MAX_SAMPLE_PRICE)
    else:
        delta = max(1, prev_price * (TARGET_SAMPLES_PER_BLOB - samples_length) // TARGET_SAMPLES_PER_BLOB // adjustment_quotient)
        return max(prev_price, MIN_SAMPLE_PRICE + delta) - delta

compute_committee_source_epoch

def compute_committee_source_epoch(epoch: Epoch, period: uint64) -> Epoch:
    """
    Return the source epoch for computing the committee.
    """
    source_epoch = Epoch(epoch - epoch % period)
    if source_epoch >= period:
        source_epoch -= period  # `period` epochs lookahead
    return source_epoch

batch_apply_participation_flag

def batch_apply_participation_flag(state: BeaconState, bits: Bitlist[MAX_VALIDATORS_PER_COMMITTEE],
                                   epoch: Epoch, full_committee: Sequence[ValidatorIndex], flag_index: int):
    if epoch == get_current_epoch(state):
        epoch_participation = state.current_epoch_participation
    else:
        epoch_participation = state.previous_epoch_participation
    for bit, index in zip(bits, full_committee):
        if bit:
            epoch_participation[index] = add_flag(epoch_participation[index], flag_index)

Beacon state accessors

Updated get_committee_count_per_slot

def get_committee_count_per_slot(state: BeaconState, epoch: Epoch) -> uint64:
    """
    Return the number of committees in each slot for the given ``epoch``.
    """
    return max(uint64(1), min(
        get_active_shard_count(state, epoch),
        uint64(len(get_active_validator_indices(state, epoch))) // SLOTS_PER_EPOCH // TARGET_COMMITTEE_SIZE,
    ))

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 INITIAL_ACTIVE_SHARDS

get_shard_proposer_index

def get_shard_proposer_index(state: BeaconState, slot: Slot, shard: Shard) -> ValidatorIndex:
    """
    Return the proposer's index of shard block at ``slot``.
    """
    epoch = compute_epoch_at_slot(slot)
    seed = hash(get_seed(state, epoch, DOMAIN_SHARD_BLOB) + uint_to_bytes(slot) + uint_to_bytes(shard))
    indices = get_active_validator_indices(state, epoch)
    return compute_proposer_index(state, indices, seed)

get_start_shard

def get_start_shard(state: BeaconState, slot: Slot) -> Shard:
    """
    Return the start shard at ``slot``.
    """
    epoch = compute_epoch_at_slot(Slot(slot))
    committee_count = get_committee_count_per_slot(state, epoch)
    active_shard_count = get_active_shard_count(state, epoch)
    return committee_count * slot % active_shard_count

compute_shard_from_committee_index

def compute_shard_from_committee_index(state: BeaconState, slot: Slot, index: CommitteeIndex) -> Shard:
    active_shards = get_active_shard_count(state, compute_epoch_at_slot(slot))
    assert index < active_shards
    return Shard((index + get_start_shard(state, slot)) % active_shards)

compute_committee_index_from_shard

def compute_committee_index_from_shard(state: BeaconState, slot: Slot, shard: Shard) -> CommitteeIndex:
    epoch = compute_epoch_at_slot(slot)
    active_shards = get_active_shard_count(state, epoch)
    index = CommitteeIndex((active_shards + shard - get_start_shard(state, slot)) % active_shards)
    assert index < get_committee_count_per_slot(state, epoch)
    return index

Block processing

def process_block(state: BeaconState, block: BeaconBlock) -> None:
    process_block_header(state, block)
    # is_execution_enabled is omitted, execution is enabled by default.
    process_execution_payload(state, block.body.execution_payload, EXECUTION_ENGINE)
    process_randao(state, block.body)
    process_eth1_data(state, block.body)
    process_operations(state, block.body)  # [Modified in Sharding]
    process_sync_aggregate(state, block.body.sync_aggregate)

Operations

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: Sequence[Any], fn: Callable[[BeaconState, Any], None]) -> None:
        for operation in operations:
            fn(state, operation)

    for_ops(body.proposer_slashings, process_proposer_slashing)
    for_ops(body.attester_slashings, process_attester_slashing)
    # New shard proposer slashing processing
    for_ops(body.shard_proposer_slashings, process_shard_proposer_slashing)

    # Limit is dynamic: based on active shard count
    assert len(body.shard_headers) <= MAX_SHARD_HEADERS_PER_SHARD * get_active_shard_count(state, get_current_epoch(state))
    for_ops(body.shard_headers, process_shard_header)

    # New attestation processing
    for_ops(body.attestations, process_attestation)
    for_ops(body.deposits, process_deposit)
    for_ops(body.voluntary_exits, process_voluntary_exit)

    # TODO: to avoid parallel shards racing, and avoid inclusion-order problems,
    #  update the fee price per slot, instead of per header.
    # state.shard_sample_price = compute_updated_sample_price(state.shard_sample_price, ?, shard_count)
Extended Attestation processing
def process_attestation(state: BeaconState, attestation: Attestation) -> None:
    altair.process_attestation(state, attestation)
    process_attested_shard_work(state, attestation)

def process_attested_shard_work(state: BeaconState, attestation: Attestation) -> None:
    attestation_shard = compute_shard_from_committee_index(
        state,
        attestation.data.slot,
        attestation.data.index,
    )
    full_committee = get_beacon_committee(state, attestation.data.slot, attestation.data.index)

    buffer_index = attestation.data.slot % SHARD_STATE_MEMORY_SLOTS
    committee_work = state.shard_buffer[buffer_index][attestation_shard]

    # Skip attestation vote accounting if the header is not pending
    if committee_work.status.selector != SHARD_WORK_PENDING:
        # If the data was already confirmed, check if this matches, to apply the flag to the attesters.
        if committee_work.status.selector == SHARD_WORK_CONFIRMED:
            attested: AttestedDataCommitment = committee_work.status.value
            if attested.root == attestation.data.shard_blob_root:
                batch_apply_participation_flag(state, attestation.aggregation_bits,
                                               attestation.data.target.epoch,
                                               full_committee, TIMELY_SHARD_FLAG_INDEX)
        return

    current_headers: Sequence[PendingShardHeader] = committee_work.status.value

    # Find the corresponding header, abort if it cannot be found
    header_index = len(current_headers)
    for i, header in enumerate(current_headers):
        if attestation.data.shard_blob_root == header.attested.root:
            header_index = i
            break

    # Attestations for an unknown header do not count towards shard confirmations, but can otherwise be valid.
    if header_index == len(current_headers):
        # Note: Attestations may be re-included if headers are included late.
        return

    pending_header: PendingShardHeader = current_headers[header_index]

    # The weight may be outdated if it is not the initial weight, and from a previous epoch
    if pending_header.weight != 0 and compute_epoch_at_slot(pending_header.update_slot) < get_current_epoch(state):
        pending_header.weight = sum(state.validators[index].effective_balance for index, bit
                                    in zip(full_committee, pending_header.votes) if bit)

    pending_header.update_slot = state.slot

    full_committee_balance = Gwei(0)
    # Update votes bitfield in the state, update weights
    for i, bit in enumerate(attestation.aggregation_bits):
        weight = state.validators[full_committee[i]].effective_balance
        full_committee_balance += weight
        if bit:
            if not pending_header.votes[i]:
                pending_header.weight += weight
                pending_header.votes[i] = True

    # Check if the PendingShardHeader is eligible for expedited confirmation, requiring 2/3 of balance attesting
    if pending_header.weight * 3 >= full_committee_balance * 2:
        # participants of the winning header are remembered with participation flags
        batch_apply_participation_flag(state, pending_header.votes, attestation.data.target.epoch,
                                       full_committee, TIMELY_SHARD_FLAG_INDEX)

        if pending_header.attested.commitment == DataCommitment():
            # The committee voted to not confirm anything
            state.shard_buffer[buffer_index][attestation_shard].status.change(
                selector=SHARD_WORK_UNCONFIRMED,
                value=None,
            )
        else:
            state.shard_buffer[buffer_index][attestation_shard].status.change(
                selector=SHARD_WORK_CONFIRMED,
                value=pending_header.attested,
            )
process_shard_header
def process_shard_header(state: BeaconState, signed_header: SignedShardBlobHeader) -> None:
    header: ShardBlobHeader = signed_header.message
    slot = header.slot
    shard = header.shard

    # Verify the header is not 0, and not from the future.
    assert Slot(0) < slot <= state.slot
    header_epoch = compute_epoch_at_slot(slot)
    # Verify that the header is within the processing time window
    assert header_epoch in [get_previous_epoch(state), get_current_epoch(state)]
    # Verify that the shard is valid
    shard_count = get_active_shard_count(state, header_epoch)
    assert shard < shard_count
    # Verify that a committee is able to attest this (slot, shard)
    start_shard = get_start_shard(state, slot)
    committee_index = (shard_count + shard - start_shard) % shard_count    
    committees_per_slot = get_committee_count_per_slot(state, header_epoch)
    assert committee_index <= committees_per_slot

    # Check that this data is still pending
    committee_work = state.shard_buffer[slot % SHARD_STATE_MEMORY_SLOTS][shard]
    assert committee_work.status.selector == SHARD_WORK_PENDING

    # Check that this header is not yet in the pending list
    current_headers: List[PendingShardHeader, MAX_SHARD_HEADERS_PER_SHARD] = committee_work.status.value
    header_root = hash_tree_root(header)
    assert header_root not in [pending_header.attested.root for pending_header in current_headers]

    # Verify proposer matches
    assert header.proposer_index == get_shard_proposer_index(state, slot, shard)

    # Verify builder and proposer aggregate signature
    blob_signing_root = compute_signing_root(header, get_domain(state, DOMAIN_SHARD_BLOB))
    builder_pubkey = state.blob_builders[header.builder_index].pubkey
    proposer_pubkey = state.validators[header.proposer_index].pubkey
    assert bls.FastAggregateVerify([builder_pubkey, proposer_pubkey], blob_signing_root, signed_header.signature)

    # Verify the length by verifying the degree.
    body_summary = header.body_summary
    points_count = body_summary.commitment.samples_count * POINTS_PER_SAMPLE
    if points_count == 0:
        assert body_summary.degree_proof == G1_SETUP[0]
    assert (
        bls.Pairing(body_summary.degree_proof, G2_SETUP[0])
        == bls.Pairing(body_summary.commitment.point, G2_SETUP[-points_count])
    )

    # Charge EIP 1559 fee, builder pays for opportunity, and is responsible for later availability,
    # or fail to publish at their own expense.
    samples = body_summary.commitment.samples_count
    # TODO: overflows, need bigger int type
    max_fee = body_summary.max_fee_per_sample * samples

    # Builder must have sufficient balance, even if max_fee is not completely utilized
    assert state.blob_builder_balances[header.builder_index] >= max_fee

    base_fee = state.shard_sample_price * samples
    # Base fee must be paid
    assert max_fee >= base_fee

    # Remaining fee goes towards proposer for prioritizing, up to a maximum
    max_priority_fee = body_summary.max_priority_fee_per_sample * samples
    priority_fee = min(max_fee - base_fee, max_priority_fee)

    # Burn base fee, take priority fee
    # priority_fee <= max_fee - base_fee, thus priority_fee + base_fee <= max_fee, thus sufficient balance.
    state.blob_builder_balances[header.builder_index] -= base_fee + priority_fee
    # Pay out priority fee
    increase_balance(state, header.proposer_index, priority_fee)

    # Initialize the pending header
    index = compute_committee_index_from_shard(state, slot, shard)
    committee_length = len(get_beacon_committee(state, slot, index))
    initial_votes = Bitlist[MAX_VALIDATORS_PER_COMMITTEE]([0] * committee_length)
    pending_header = PendingShardHeader(
        attested=AttestedDataCommitment(
            commitment=body_summary.commitment,
            root=header_root,
            includer_index=get_beacon_proposer_index(state),
        ),
        votes=initial_votes,
        weight=0,
        update_slot=state.slot,
    )

    # Include it in the pending list
    current_headers.append(pending_header)

The degree proof works as follows. For a block B with length l (so l values in [0...l - 1], seen as a polynomial B(X) which takes these values), the length proof is the commitment to the polynomial B(X) * X**(MAX_DEGREE + 1 - l), where MAX_DEGREE is the maximum power of s available in the setup, which is MAX_DEGREE = len(G2_SETUP) - 1. The goal is to ensure that a proof can only be constructed if deg(B) < l (there are not hidden higher-order terms in the polynomial, which would thwart reconstruction).

process_shard_proposer_slashing
def process_shard_proposer_slashing(state: BeaconState, proposer_slashing: ShardProposerSlashing) -> None:
    slot = proposer_slashing.slot
    shard = proposer_slashing.shard
    proposer_index = proposer_slashing.proposer_index

    reference_1 = ShardBlobReference(slot=slot, shard=shard,
                                     proposer_index=proposer_index,
                                     builder_index=proposer_slashing.builder_index_1,
                                     body_root=proposer_slashing.body_root_1)
    reference_2 = ShardBlobReference(slot=slot, shard=shard,
                                     proposer_index=proposer_index,
                                     builder_index=proposer_slashing.builder_index_2,
                                     body_root=proposer_slashing.body_root_2)

    # Verify the signed messages are different
    assert reference_1 != reference_2

    # Verify the proposer is slashable
    proposer = state.validators[proposer_index]
    assert is_slashable_validator(proposer, get_current_epoch(state))

    # The builders are not slashed, the proposer co-signed with them
    builder_pubkey_1 = state.blob_builders[proposer_slashing.builder_index_1].pubkey
    builder_pubkey_2 = state.blob_builders[proposer_slashing.builder_index_2].pubkey
    domain = get_domain(state, DOMAIN_SHARD_PROPOSER, compute_epoch_at_slot(slot))
    signing_root_1 = compute_signing_root(reference_1, domain)
    signing_root_2 = compute_signing_root(reference_2, domain)
    assert bls.FastAggregateVerify([builder_pubkey_1, proposer.pubkey], signing_root_1, proposer_slashing.signature_1)
    assert bls.FastAggregateVerify([builder_pubkey_2, proposer.pubkey], signing_root_2, proposer_slashing.signature_2)

    slash_validator(state, proposer_index)

Epoch transition

This epoch transition overrides the Merge epoch transition:

def process_epoch(state: BeaconState) -> None:
    # Sharding pre-processing
    process_pending_shard_confirmations(state)
    reset_pending_shard_work(state)

    # Base functionality
    process_justification_and_finalization(state)
    process_inactivity_updates(state)
    process_rewards_and_penalties(state)  # Note: modified, see new TIMELY_SHARD_FLAG_INDEX
    process_registry_updates(state)
    process_slashings(state)
    process_eth1_data_reset(state)
    process_effective_balance_updates(state)
    process_slashings_reset(state)
    process_randao_mixes_reset(state)
    process_historical_roots_update(state)
    process_participation_flag_updates(state)
    process_sync_committee_updates(state)

process_pending_shard_confirmations

def process_pending_shard_confirmations(state: BeaconState) -> None:
    # Pending header processing applies to the previous epoch.
    # Skip if `GENESIS_EPOCH` because no prior epoch to process.
    if get_current_epoch(state) == GENESIS_EPOCH:
        return

    previous_epoch = get_previous_epoch(state)
    previous_epoch_start_slot = compute_start_slot_at_epoch(previous_epoch)

    # Mark stale headers as unconfirmed
    for slot in range(previous_epoch_start_slot, previous_epoch_start_slot + SLOTS_PER_EPOCH):
        buffer_index = slot % SHARD_STATE_MEMORY_SLOTS
        for shard_index in range(len(state.shard_buffer[buffer_index])):
            committee_work = state.shard_buffer[buffer_index][shard_index]
            if committee_work.status.selector == SHARD_WORK_PENDING:
                winning_header = max(committee_work.status.value, key=lambda header: header.weight)
                if winning_header.attested.commitment == DataCommitment():
                    committee_work.status.change(selector=SHARD_WORK_UNCONFIRMED, value=None)
                else:
                    committee_work.status.change(selector=SHARD_WORK_CONFIRMED, value=winning_header.attested)

reset_pending_shard_work

def reset_pending_shard_work(state: BeaconState) -> None:
    # Add dummy "empty" PendingShardHeader (default vote if no shard header is available)
    next_epoch = get_current_epoch(state) + 1
    next_epoch_start_slot = compute_start_slot_at_epoch(next_epoch)
    committees_per_slot = get_committee_count_per_slot(state, next_epoch)
    active_shards = get_active_shard_count(state, next_epoch)

    for slot in range(next_epoch_start_slot, next_epoch_start_slot + SLOTS_PER_EPOCH):
        buffer_index = slot % SHARD_STATE_MEMORY_SLOTS
        
        # Reset the shard work tracking
        state.shard_buffer[buffer_index] = [ShardWork() for _ in range(active_shards)]

        start_shard = get_start_shard(state, slot)
        for committee_index in range(committees_per_slot):
            shard = (start_shard + committee_index) % active_shards
            # a committee is available, initialize a pending shard-header list
            committee_length = len(get_beacon_committee(state, slot, CommitteeIndex(committee_index)))
            state.shard_buffer[buffer_index][shard].status.change(
                selector=SHARD_WORK_PENDING,
                value=List[PendingShardHeader, MAX_SHARD_HEADERS_PER_SHARD](
                    PendingShardHeader(
                        attested=AttestedDataCommitment(),
                        votes=Bitlist[MAX_VALIDATORS_PER_COMMITTEE]([0] * committee_length),
                        weight=0,
                        update_slot=slot,
                    )
                )
            )
        # a shard without committee available defaults to SHARD_WORK_UNCONFIRMED.