SSZ transactions

Abstract

This EIP defines a migration process of EIP-2718 Recursive-Length Prefix (RLP) transactions to Simple Serialize (SSZ).

Motivation

RLP transactions have a number of shortcomings:

1. Linear hashing: The signing hash (sig_hash) and unique identifier (tx_hash) of an RLP transaction are computed by linear keccak256 hashes across its serialization. Even if only partial data is of interest, linear hashes require the full transaction data to be present, including potentially large calldata or access lists. This also applies when computing the from address of a transaction based on the sig_hash.

2. Inefficient inclusion proofs: The Merkle-Patricia Trie (MPT) backing the execution block header's transactions_root is constructed from the serialized transactions, internally prepending a prefix to the transaction data before it is keccak256 hashed into the MPT. Due to this prefix, there is no on-chain commitment to the tx_hash and inclusion proofs require the full transaction data to be present.

3. Incompatible representation: As part of the consensus ExecutionPayload, the RLP serialization of transactions is hashed using SSZ merkleization. These SSZ hashes are incompatible with both the tx_hash and the MPT transactions_root.

4. Technical debt: All client applications and smart contracts handling RLP transactions have to correctly deal with caveats such as LegacyTransaction lacking a prefix byte, the inconsistent chain_id and v / y_parity semantics, and the introduction of max_priority_fee_per_gas between other fields instead of at the end. As existing transaction types tend to remain valid perpetually, this technical debt builds up over time.

5. Inappropriate opaqueness: The Consensus Layer treats RLP transaction data as opaque, but requires validation of consensus blob_kzg_commitments against transaction blob_versioned_hashes, resulting in a more complex than necessary engine API.

This EIP addresses these by defining a lossless conversion mechanism to normalize transaction representation across both Consensus Layer and Execution Layer while retaining support for processing RLP transaction types.

Specification

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 and RFC 8174.

Existing definitions

Definitions from existing specifications that are used throughout this document are replicated here for reference.

Name	Value
BYTES_PER_FIELD_ELEMENT	uint64(32)
FIELD_ELEMENTS_PER_BLOB	uint64(4096)

Name	SSZ equivalent
Hash32	Bytes32
ExecutionAddress	Bytes20
VersionedHash	Bytes32
KZGCommitment	Bytes48
KZGProof	Bytes48
Blob	ByteVector[BYTES_PER_FIELD_ELEMENT * FIELD_ELEMENTS_PER_BLOB]

Signatures

Transaction signatures are represented by their native, opaque representation. ECDSA signatures are no longer split up into r, s, and y_parity components.

Name	Value	Description
SECP256K1_SIGNATURE_SIZE	32 + 32 + 1 (= 65)	Byte length of a secp256k1 ECDSA signature

class Secp256k1ExecutionSignature(ProgressiveContainer[active_fields=[1]]):
    secp256k1: ByteVector[SECP256K1_SIGNATURE_SIZE]

def secp256k1_pack(r: uint256, s: uint256, y_parity: uint8) -> ByteVector[SECP256K1_SIGNATURE_SIZE]:
    return r.to_bytes(32, 'big') + s.to_bytes(32, 'big') + bytes([y_parity])

def secp256k1_unpack(signature: ByteVector[SECP256K1_SIGNATURE_SIZE]) -> tuple[uint256, uint256, uint8]:
    r = uint256.from_bytes(signature[0:32], 'big')
    s = uint256.from_bytes(signature[32:64], 'big')
    y_parity = signature[64]
    return (r, s, y_parity)

def secp256k1_validate(signature: ByteVector[SECP256K1_SIGNATURE_SIZE]):
    SECP256K1N = 0xfffffffffffffffffffffffffffffffebaaedce6af48a03bbfd25e8cd0364141
    r, s, y_parity = secp256k1_unpack(signature)
    assert 0 < r < SECP256K1N
    assert 0 < s <= SECP256K1N // 2
    assert y_parity in (0, 1)

def secp256k1_recover_signer(signature: ByteVector[SECP256K1_SIGNATURE_SIZE],
                             sig_hash: Hash32) -> ExecutionAddress:
    ecdsa = ECDSA()
    recover_sig = ecdsa.ecdsa_recoverable_deserialize(signature[0:64], signature[64])
    public_key = PublicKey(ecdsa.ecdsa_recover(sig_hash, recover_sig, raw=True))
    uncompressed = public_key.serialize(compressed=False)
    return ExecutionAddress(keccak(uncompressed[1:])[12:])

Gas fees

The different kinds of gas fees are combined into a single structure.

Name	SSZ equivalent	Description
FeePerGas	uint256	Fee per unit of gas

class BasicFeesPerGas(ProgressiveContainer[active_fields=[1]]):
    regular: FeePerGas

class BlobFeesPerGas(ProgressiveContainer[active_fields=[1, 1]]):
    regular: FeePerGas
    blob: FeePerGas

Normalized transactions

RLP transactions are converted to a normalized SSZ representation. Their original RLP TransactionType is retained to enable recovery of their original RLP representation and associated sig_hash and historical tx_hash values.

class Transaction(CompatibleUnion[
    RlpTransaction,
]):
    pass

class RlpTransaction(CompatibleUnion[
    RlpLegacyReplayableBasicTransaction,
    RlpLegacyReplayableCreateTransaction,
    RlpLegacyBasicTransaction,
    RlpLegacyCreateTransaction,
    RlpAccessListBasicTransaction,
    RlpAccessListCreateTransaction,
    RlpBasicTransaction,
    RlpCreateTransaction,
    RlpBlobTransaction,
    RlpSetCodeTransaction,
]):
    pass

Name	SSZ equivalent	Description
TransactionType	uint8	EIP-2718 transaction type, range [0x00, 0x7F]
ChainId	uint256	EIP-155 chain ID
GasAmount	uint64	Amount in units of gas

Replayable legacy transactions

The original RLP representation of these transactions is replayable across networks with different chain ID.

class RlpLegacyReplayableBasicTransactionPayload(
    ProgressiveContainer[active_fields=[1, 0, 1, 1, 1, 1, 1, 1]]
):
    type_: TransactionType  # 0x00
    nonce: uint64
    max_fees_per_gas: BasicFeesPerGas
    gas: GasAmount
    to: ExecutionAddress
    value: uint256
    input_: ProgressiveByteList

class RlpLegacyReplayableBasicTransaction(Container):
    payload: RlpLegacyReplayableBasicTransactionPayload
    signature: Secp256k1ExecutionSignature

class RlpLegacyReplayableCreateTransactionPayload(
    ProgressiveContainer[active_fields=[1, 0, 1, 1, 1, 0, 1, 1]]
):
    type_: TransactionType  # 0x00
    nonce: uint64
    max_fees_per_gas: BasicFeesPerGas
    gas: GasAmount
    value: uint256
    input_: ProgressiveByteList

class RlpLegacyReplayableCreateTransaction(Container):
    payload: RlpLegacyReplayableCreateTransactionPayload
    signature: Secp256k1ExecutionSignature

EIP-155 legacy transactions

These transactions are locked to a single EIP-155 chain ID.

class RlpLegacyBasicTransactionPayload(
    ProgressiveContainer[active_fields=[1, 1, 1, 1, 1, 1, 1, 1]]
):
    type_: TransactionType  # 0x00
    chain_id: ChainId
    nonce: uint64
    max_fees_per_gas: BasicFeesPerGas
    gas: GasAmount
    to: ExecutionAddress
    value: uint256
    input_: ProgressiveByteList

class RlpLegacyBasicTransaction(Container):
    payload: RlpLegacyBasicTransactionPayload
    signature: Secp256k1ExecutionSignature

class RlpLegacyCreateTransactionPayload(
    ProgressiveContainer[active_fields=[1, 1, 1, 1, 1, 0, 1, 1]]
):
    type_: TransactionType  # 0x00
    chain_id: ChainId
    nonce: uint64
    max_fees_per_gas: BasicFeesPerGas
    gas: GasAmount
    value: uint256
    input_: ProgressiveByteList

class RlpLegacyCreateTransaction(Container):
    payload: RlpLegacyCreateTransactionPayload
    signature: Secp256k1ExecutionSignature

EIP-2930 access list transactions

These transactions support specifying an EIP-2930 access list.

class AccessTuple(Container):
    address: ExecutionAddress
    storage_keys: ProgressiveList[Hash32]

class RlpAccessListBasicTransactionPayload(
    ProgressiveContainer[active_fields=[1, 1, 1, 1, 1, 1, 1, 1, 1]]
):
    type_: TransactionType  # 0x01
    chain_id: ChainId
    nonce: uint64
    max_fees_per_gas: BasicFeesPerGas
    gas: GasAmount
    to: ExecutionAddress
    value: uint256
    input_: ProgressiveByteList
    access_list: ProgressiveList[AccessTuple]

class RlpAccessListBasicTransaction(Container):
    payload: RlpAccessListBasicTransactionPayload
    signature: Secp256k1ExecutionSignature

class RlpAccessListCreateTransactionPayload(
    ProgressiveContainer[active_fields=[1, 1, 1, 1, 1, 0, 1, 1, 1]]
):
    type_: TransactionType  # 0x01
    chain_id: ChainId
    nonce: uint64
    max_fees_per_gas: BasicFeesPerGas
    gas: GasAmount
    value: uint256
    input_: ProgressiveByteList
    access_list: ProgressiveList[AccessTuple]

class RlpAccessListCreateTransaction(Container):
    payload: RlpAccessListCreateTransactionPayload
    signature: Secp256k1ExecutionSignature

EIP-1559 fee market transactions

These transactions support specifying EIP-1559 priority fees.

class RlpBasicTransactionPayload(
    ProgressiveContainer[active_fields=[1, 1, 1, 1, 1, 1, 1, 1, 1, 1]]
):
    type_: TransactionType  # 0x02
    chain_id: ChainId
    nonce: uint64
    max_fees_per_gas: BasicFeesPerGas
    gas: GasAmount
    to: ExecutionAddress
    value: uint256
    input_: ProgressiveByteList
    access_list: ProgressiveList[AccessTuple]
    max_priority_fees_per_gas: BasicFeesPerGas

class RlpBasicTransaction(Container):
    payload: RlpBasicTransactionPayload
    signature: Secp256k1ExecutionSignature

class RlpCreateTransactionPayload(
    ProgressiveContainer[active_fields=[1, 1, 1, 1, 1, 0, 1, 1, 1, 1]]
):
    type_: TransactionType  # 0x02
    chain_id: ChainId
    nonce: uint64
    max_fees_per_gas: BasicFeesPerGas
    gas: GasAmount
    value: uint256
    input_: ProgressiveByteList
    access_list: ProgressiveList[AccessTuple]
    max_priority_fees_per_gas: BasicFeesPerGas

class RlpCreateTransaction(Container):
    payload: RlpCreateTransactionPayload
    signature: Secp256k1ExecutionSignature

EIP-4844 blob transactions

These transactions support specifying EIP-4844 blobs.

class RlpBlobTransactionPayload(
    ProgressiveContainer[active_fields=[1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1]]
):
    type_: TransactionType  # 0x03
    chain_id: ChainId
    nonce: uint64
    max_fees_per_gas: BlobFeesPerGas
    gas: GasAmount
    to: ExecutionAddress
    value: uint256
    input_: ProgressiveByteList
    access_list: ProgressiveList[AccessTuple]
    max_priority_fees_per_gas: BasicFeesPerGas
    blob_versioned_hashes: ProgressiveList[VersionedHash]

class RlpBlobTransaction(Container):
    payload: RlpBlobTransactionPayload
    signature: Secp256k1ExecutionSignature

EIP-7702 set code transactions

These transactions support specifying an EIP-7702 authorization list.

class RlpAuthorization(CompatibleUnion[
    RlpReplayableBasicAuthorizationPayload,
    RlpBasicAuthorizationPayload,
]):
    pass

class RlpReplayableBasicAuthorizationPayload(ProgressiveContainer[active_fields=[1, 0, 1, 1]]):
    magic: TransactionType  # 0x05
    address: ExecutionAddress
    nonce: uint64

class RlpBasicAuthorizationPayload(ProgressiveContainer[active_fields=[1, 1, 1, 1]]):
    magic: TransactionType  # 0x05
    chain_id: ChainId
    address: ExecutionAddress
    nonce: uint64

class RlpSetCodeTransactionPayload(
    ProgressiveContainer[active_fields=[1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1]]
):
    type_: TransactionType  # 0x04
    chain_id: ChainId
    nonce: uint64
    max_fees_per_gas: BasicFeesPerGas
    gas: GasAmount
    to: ExecutionAddress
    value: uint256
    input_: ProgressiveByteList
    access_list: ProgressiveList[AccessTuple]
    max_priority_fees_per_gas: BasicFeesPerGas
    authorization_list: ProgressiveList[RlpSetCodeAuthorization]

class RlpSetCodeTransaction(Container):
    payload: RlpAuthorization
    signature: Secp256k1ExecutionSignature

Execution block header changes

The execution block header's txs-root is transitioned from MPT to SSZ.

transactions = ProgressiveList[Transaction](
    tx_0, tx_1, tx_2, ...)

block_header.transactions_root = transactions.hash_tree_root()

Engine API

In the engine API, the semantics of the transactions field in ExecutionPayload versions adopting this EIP are changed to emit transactions using SSZ serialization.

• transactions - Array of DATA - Array of transaction objects, each object is a byte list (DATA) representing ssz.serialize(tx)

Consensus ExecutionPayload changes

When building a consensus ExecutionPayload, the transactions list is no longer opaque and uses the new Transaction type, aligning the transactions_root across execution blocks and execution payloads.

class ExecutionPayload(...):
    ...
    transactions: ProgressiveList[Transaction]
    ...

Rationale

Forward compatibility

The proposed transaction design is extensible with new fee types, new signature types, and entirely new transaction features (e.g., CREATE2), while retaining compatibility with the proposed transactions.

Verifier improvements

Future RPC could expose an SSZ based tx_root on top of the tx_hash, against which proofs can be validated. The transactions_root can now be reconstructed from the list of tx_root. Further, partial data becomes provable, such as destination / amount without requiring the full calldata. This can reduce gas cost or zk proving cost when verifying L2 chain data in an L1 smart contract.

Consensus client improvements

Consensus Layer implementations may drop invalid blocks early if consensus blob_kzg_commitments do not validate against transaction blob_versioned_hashes and no longer need to query the Execution Layer for that validation. Future versions of the engine API could be simplified to drop the transfers of blob_kzg_commitments to the EL.

Backwards Compatibility

Applications that rely on the replaced MPT transactions_root in the block header require migration to the SSZ transactions_root.

While there is no on-chain commitment of the tx_hash, it is widely used in JSON-RPC and the Ethereum Wire Protocol to uniquely identify transactions. The conversion from RLP transactions to SSZ is lossless. The original RLP sig_hash and tx_hash can be recovered from the SSZ representation.

RLP and SSZ transactions may clash when encoded. It is essential to use only a single format within one channel.

Security Considerations

None

Copyright

Copyright and related rights waived via CC0.