---
eip: 7793
title: Conditional Transactions
description: Transactions that only executes at a specific index and slot
author: Marc Harvey-Hill (@Marchhill), Ahmad Bitar (@smartprogrammer93)
discussions-to: https://ethereum-magicians.org/t/eip-7793-asserttxindex-opcode/21513
status: Stagnant
type: Standards Track
category: Core
created: 2024-10-17
requires: 7843
---

## 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

| Constant | Value |
| - | - |
| `COND_TX_TYPE` | `Bytes1(0x05)` |
| `TXINDEX_OPCODE_BYTE` | `Bytes1(0x4c)` |
| `TXINDEX_OPCODE_GAS` | `2` |

### Conditional Transaction Type

We introduce a new type of [EIP-2718](./eip-2718.md) 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](./eip-4844.md). 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](./eip-7843.md) 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](../LICENSE.md).