---
eip: 1344
title: ChainID opcode
author: Richard Meissner (@rmeissner), Bryant Eisenbach (@fubuloubu)
discussions-to: https://ethereum-magicians.org/t/add-chain-id-opcode-for-replay-protection-when-handling-signed-messages-in-contracts/1131
category: Core
type: Standards Track
status: Last Call
created: 2018-08-22
review-period-end: 2019-04-30
---

## Abstract
This EIP adds an opcode that returns the current chain's EIP-155 unique identifier.

## Motivation
[EIP-155](https://github.com/ethereum/EIPs/blob/master/EIPS/eip-155.md) proposes to use the chain ID to prevent replay attacks between different chains. It would be a great benefit to have the same possibility inside smart contracts when handling signatures, especially for Layer 2 signature schemes using [EIP-712](https://github.com/ethereum/EIPs/blob/master/EIPS/eip-712.md).

## Specification
Adds a new opcode `CHAINID` at 0x46, which uses 0 stack arguments. It will push the current chain ID onto the stack. The operation costs `G_base` to execute.

## Rationale
The current approach proposed by EIP-712 is to specify the chain ID at compile time. Using this approach will result in problems after a hardfork, as well as human error that may lead to loss of funds or replay attacks on signed messages. 
By adding the proposed opcode it will be possible to access the current chain ID and validate signatures based on that.

## Backwards Compatibility
This EIP is fully backwards compatible with all chains which implement EIP-155 chain ID domain separator for transaction signing.

## References
This was previously suggested as part of [EIP901](https://github.com/ethereum/EIPs/issues/901).

## Test Cases
TBD

## Implementation
A sample implementation was attempted here: https://github.com/fubuloubu/py-evm/commit/eaab5ffa2164d4cc06ae5c855a49d030965be828

## Copyright
Copyright and related rights waived via [CC0](https://creativecommons.org/publicdomain/zero/1.0/).