eth2.0-specs/specs/eip4844/beacon-chain.md

EIP-4844 -- The Beacon Chain

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

Table of contents

• Introduction
• Custom types
• Constants
  • Domain types
• Preset
  • Trusted setup
• Configuration
• Containers
  • Extended containers
    • BeaconBlockBody
• Helper functions
  • KZG core
    • blob_to_kzg
    • kzg_to_versioned_hash
  • Misc
    • tx_peek_blob_versioned_hashes
    • verify_kzgs_against_transactions
• Beacon chain state transition function
  • Block processing
    • Blob KZGs
• Testing

Introduction

This upgrade adds blobs to the beacon chain as part of EIP-4844.

Custom types

Name	SSZ equivalent	Description
BLSFieldElement	uint256	x < BLS_MODULUS
Blob	Vector[BLSFieldElement, FIELD_ELEMENTS_PER_BLOB]
VersionedHash	Bytes32
KZGCommitment	Bytes48	Same as BLS standard "is valid pubkey" check but also allows 0x00..00 for point-at-infinity

Constants

Name	Value
BLOB_TX_TYPE	uint8(0x05)
FIELD_ELEMENTS_PER_BLOB	4096
BLS_MODULUS	52435875175126190479447740508185965837690552500527637822603658699938581184513

Domain types

Name	Value
DOMAIN_BLOBS_SIDECAR	DomainType('0x0a000000')

Preset

Trusted setup

The trusted setup is part of the preset: during testing a minimal insecure variant may be used, but reusing the mainnet settings in public networks is a critical security requirement.

Name	Value
KZG_SETUP_G2	Vector[G2Point, FIELD_ELEMENTS_PER_BLOB], contents TBD
KZG_SETUP_LAGRANGE	Vector[KZGCommitment, FIELD_ELEMENTS_PER_BLOB], contents TBD

Configuration

Containers

Extended containers

BeaconBlockBody

Note: BeaconBlock and SignedBeaconBlock types are updated indirectly.

class BeaconBlockBody(Container):
    randao_reveal: BLSSignature
    eth1_data: Eth1Data  # Eth1 data vote
    graffiti: Bytes32  # Arbitrary data
    # Operations
    proposer_slashings: List[ProposerSlashing, MAX_PROPOSER_SLASHINGS]
    attester_slashings: List[AttesterSlashing, MAX_ATTESTER_SLASHINGS]
    attestations: List[Attestation, MAX_ATTESTATIONS]
    deposits: List[Deposit, MAX_DEPOSITS]
    voluntary_exits: List[SignedVoluntaryExit, MAX_VOLUNTARY_EXITS]
    sync_aggregate: SyncAggregate
    # Execution
    execution_payload: ExecutionPayload 
    blob_kzgs: List[KZGCommitment, MAX_BLOBS_PER_BLOCK]  # [New in EIP-4844]

Helper functions

KZG core

KZG core functions. These are also defined in EIP-4844 execution specs.

blob_to_kzg

def blob_to_kzg(blob: Blob) -> KZGCommitment:
    computed_kzg = bls.Z1
    for value, point_kzg in zip(blob, KZG_SETUP_LAGRANGE):
        assert value < BLS_MODULUS
        computed_kzg = bls.add(
            computed_kzg,
            bls.multiply(point_kzg, value)
        )
    return computed_kzg

kzg_to_versioned_hash

def kzg_to_versioned_hash(kzg: KZGCommitment) -> VersionedHash:
    return BLOB_COMMITMENT_VERSION_KZG + hash(kzg)[1:]

Misc

tx_peek_blob_versioned_hashes

This function retrieves the hashes from the SignedBlobTransaction as defined in EIP-4844, using SSZ offsets. Offsets are little-endian uint32 values, as defined in the SSZ specification.

def tx_peek_blob_versioned_hashes(opaque_tx: Transaction) -> Sequence[VersionedHash]:
    assert opaque_tx[0] == BLOB_TX_TYPE
    message_offset = 1 + uint32.decode_bytes(opaque_tx[1:5])
    # field offset: 32 + 8 + 32 + 32 + 8 + 4 + 32 + 4 + 4 = 156
    blob_versioned_hashes_offset = uint32.decode_bytes(opaque_tx[message_offset+156:message_offset+160])
    return [VersionedHash(opaque_tx[x:x+32]) for x in range(blob_versioned_hashes_offset, len(opaque_tx), 32)]

verify_kzgs_against_transactions

def verify_kzgs_against_transactions(transactions: Sequence[Transaction], blob_kzgs: Sequence[KZGCommitment]) -> bool:
   all_versioned_hashes = []
   for tx in transactions:
      if tx[0] == BLOB_TX_TYPE:
         all_versioned_hashes.extend(tx_peek_blob_versioned_hashes(tx))
   return all_versioned_hashes == [ksg_to_version_hash(kzg) for kzg in blob_kzgs]

Beacon chain state transition function

Block processing

def process_block(state: BeaconState, block: BeaconBlock) -> None:
    process_block_header(state, block)
    if is_execution_enabled(state, block.body):
        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)
    process_sync_aggregate(state, block.body.sync_aggregate)
    process_blob_kzgs(state, block.body)  # [New in EIP-4844]

Blob KZGs

def process_blob_kzgs(state: BeaconState, body: BeaconBlockBody):
    assert verify_kzgs_against_transactions(body.execution_payload.transactions, body.blob_kzgs)

Testing

Note: The function initialize_beacon_state_from_eth1 is modified for pure EIP-4844 testing only.

The BeaconState initialization is unchanged, except for the use of the updated eip4844.BeaconBlockBody type when initializing the first body-root:

state.latest_block_header=BeaconBlockHeader(body_root=hash_tree_root(BeaconBlockBody())),