SSZ Transaction Signature Scheme

Abstract

This EIP defines a signature scheme for native Simple Serialize (SSZ) encoded transactions.

Motivation

EIP-6404 introduces SSZ transactions by converting from RLP transactions. Defining a signature scheme for native SSZ transactions further reduces required conversions and unlocks the forward compatibility benefits of SSZ StableContainer.

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.

Transaction signature scheme

Native SSZ transactions are based on the TransactionPayload and Transaction types defined in EIP-6404 and emit an EIP-6466 Receipt. To distinguish native SSZ transactions from those converted from RLP, native SSZ transactions do not set an RLP TransactionType in their TransactionPayload.

All native SSZ transactions follow an identical scheme based on hash_tree_root to compute their signing hash (sig_hash) and unique identifier (tx_hash).

Additional information is mixed into sig_hash to uniquely identify the underlying specification and avoid hash collisions across different signature kinds. Vendor-defined networks MUST use a different DomainType for signing custom transaction types.

class ExecutionSigningData(Container):
    object_root: Root
    domain_type: DomainType

def compute_ssz_sig_hash(payload: TransactionPayload) -> Hash32:
    return Hash32(ExecutionSigningData(
        object_root=payload.hash_tree_root(),
        domain=DOMAIN_TX_SSZ,
    ).hash_tree_root())

def compute_ssz_tx_hash(tx: Transaction) -> Hash32:
    assert tx.payload.type_ is None
    return Hash32(tx.hash_tree_root())

JSON-RPC

Certain JSON-RPC endpoints such as eth_getTransactionByHash indicate the corresponding EIP-2718 envelope type prefix in a type field.

When representing native SSZ transactions on such endpoints, SSZ_TX_TYPE SHOULD be indicated as their type, as defined in EIP-6404. Omitting the type is NOT RECOMMENDED as certain client applications could confuse the omission with untyped LegacyTransaction.

Transaction profiles

New EIP-7495 Profile definitions are introduced to represent native SSZ transactions:

• BasicTransaction shares the functionality of EIP-1559 fee market transactions
• BlobTransaction shares the functionality of EIP-4844 blob transactions

class BasicTransactionPayload(Profile[TransactionPayload]):
    chain_id: ChainId
    nonce: uint64
    max_fees_per_gas: BasicFeesPerGas
    gas: GasAmount
    to: Optional[ExecutionAddress]
    value: uint256
    input_: ByteList[MAX_CALLDATA_SIZE]
    access_list: List[AccessTuple, MAX_ACCESS_LIST_SIZE]
    max_priority_fees_per_gas: BasicFeesPerGas

class BasicTransaction(Container):
    payload: BasicTransactionPayload
    signature: Secp256k1ExecutionSignature

class BlobTransactionPayload(Profile[TransactionPayload]):
    chain_id: ChainId
    nonce: uint64
    max_fees_per_gas: BlobFeesPerGas
    gas: GasAmount
    to: ExecutionAddress
    value: uint256
    input_: ByteList[MAX_CALLDATA_SIZE]
    access_list: List[AccessTuple, MAX_ACCESS_LIST_SIZE]
    max_priority_fees_per_gas: BlobFeesPerGas
    blob_versioned_hashes: List[VersionedHash, MAX_BLOB_COMMITMENTS_PER_BLOCK]

class BlobTransaction(Container):
    payload: BlobTransactionPayload
    signature: Secp256k1ExecutionSignature

The identify_transaction_profile helper from EIP-6404 is updated to support native SSZ transactions.

def identify_transaction_profile(tx: Transaction) -> Type[Profile]:
    if tx.payload.type_ is None:
        if tx.payload.blob_versioned_hashes is not None:
            return BlobTransaction

        return BasicTransaction

    else:
        if tx.payload.type_ == BLOB_TX_TYPE:
            return RlpBlobTransaction

        if tx.payload.type_ == FEE_MARKET_TX_TYPE:
            return RlpFeeMarketTransaction

        if tx.payload.type_ == ACCESS_LIST_TX_TYPE:
            return RlpAccessListTransaction

        if tx.payload.type_ == LEGACY_TX_TYPE:
            return RlpLegacyTransaction

        raise Exception(f'Unsupported transaction: {tx}')

Rationale

The SSZ signature scheme reduces hashing overhead and ensures that tx_hash commitments are available on-chain. It also provides a flexible basis for future transaction functionality.

Backwards Compatibility

The new transaction signature scheme is solely used for SSZ transactions and is represented using a different EIP-2718 envelope type prefix as existing RLP transactions.

Security Considerations

SSZ signatures MUST NOT collide with RLP transaction and message hashes.

As RLP messages are hashed using keccak256, and all SSZ objects are hashed using SHA256. These two hashing algorithms are both considered cryptographically secure and are based on fundamentally different approaches, minimizing the risk of hash collision between those two hashing algorithms.

Furthermore, RLP messages are hashed linearly across their serialization, while SSZ objects are hashed using a recursive Merkle tree. Having a different mechanism further reduce the risk of hash collisions.

Copyright

Copyright and related rights waived via CC0.