Conditional Transactions

Abstract

This EIP proposes to add a new transaction format for "conditional transactions", that are only valid at a specified slot and index within the block. A new opcode TXINDEX is introduced to expose the conditional transaction index onchain.

Motivation

The proposal aims to improve support for encrypted mempools. Transactions in an encrypted mempool are ordered while the transactions are encrypted, before being decrypted and included onchain at the top of the block. If the builder does not respect the order when including the decrypted transactions then they could frontrun decrypted transactions. The new transaction type can be used to make this impossible; if a decrypted transaction is not included at the correct index, it will be invalid.

Specification

Parameters

Conditional Transaction Type

We introduce a new type of EIP-2718 transaction, "conditional transactions", where the TransactionType is COND_TX_TYPE and the TransactionPayload is the RLP serialization of the following TransactionPayloadBody:

[chain_id, nonce, max_priority_fee_per_gas, max_fee_per_gas, gas_limit, to, value, data, access_list, max_fee_per_blob_gas, blob_versioned_hashes, conditional_slot, conditional_tx_index, y_parity, r, s]

The fields chain_id, nonce, max_priority_fee_per_gas, max_fee_per_gas, gas_limit, to, value, data, access_list, max_fee_per_blob_gas follow the same semantics as EIP-4844. The field blob_versioned_hashes is the same except that the list may be empty for a conditional transaction.

The fields conditional_slot and conditional_tx_index are both uint64 and specify the slot and transaction index in which this transaction should be considered valid respectively. In order to verify that conditional_slot is correct, EIP-7843 is a dependency, as this adds the slot number to the header. For both fields -1 is used as a sentinel value for which the slot or transaction index check will not take place.

Signature

The signature values y_parity, r, and s are calculated by constructing a secp256k1 signature over the following digest:

keccak256(COND_TX_TYPE || rlp([chain_id, nonce, max_priority_fee_per_gas, max_fee_per_gas, gas_limit, to, value, data, access_list, max_fee_per_blob_gas, blob_versioned_hashes, conditional_slot, conditional_tx_index])).

TXINDEX opcode

A new opcode TXINDEX is introduced at TXINDEX_OPCODE_BYTE.

Output

One element TransactionIndex is added to the stack. TransactionIndex is a uint64 in big endian encoding. It is equal to conditional_tx_index if the current transaction is a conditional transaction, and -1 otherwise.

Gas Cost

The gas cost for TXINDEX is a fixed fee of TXINDEX_OPCODE_GAS.

Rationale

Transaction Type

An alternative design could simply return the current transaction index without adding a new transaction type. Adding a new transaction type is favoured as it means that the expected transaction index must be declared statically upfront, rather than allowing dynamic behaviour based on the returned transaction index. This prevents complex constraints being imposed that makes it difficult to build a block.

Gas Price

The opcode is priced to match similar opcodes in the W_base set.

Backwards Compatibility

No backward compatibility issues found.

Test Cases

N/A

Security Considerations

None.

Copyright

Copyright and related rights waived via CC0.