---
eip: 1191
title: Add chain id to mixed-case checksum address encoding
author: Juliano Rizzo (@juli)
status: Last Call
last-call-deadline: 2019-11-18
type: Standards Track
category: ERC
created: 2018-03-18
requires: 55, 155
discussions-to: https://github.com/ethereum/EIPs/issues/1121
---

## Simple Summary
This EIP extends EIP-55 by optionally adding a chain id defined by EIP-155 to the checksum calculation.

## Abstract
The EIP-55 was created to prevent users from losing funds by sending them to invalid addresses. This EIP extends EIP-55 to protect users from losing funds by sending them to addresses that are valid but that where obtained from a client of another network.For example, if this EIP is implemented, a wallet can alert the user that is trying to send funds to an Ethereum Testnet address from an Ethereum Mainnet wallet.  

## Motivation
The motivation of this proposal is to provide a mechanism to allow software to distinguish addresses from different Ethereum based networks. This proposal is necessary because Ethereum addresses are hashes of public keys and do not include any metadata. By extending the EIP-55 checksum algorithm it is possible to achieve this objective.

## Specification
Convert the address using the same algorithm defined by EIP-55 but if a registered chain id is provided, add it to the input of the hash function. If the chain id passed to the function belongs to a network that opted for using this checksum variant, prefix the address with the chain id and the `0x` separator before calculating the hash. Then convert the address to hexadecimal, but if the ith digit is a letter (ie. it's one of `abcdef`) print it in uppercase if the 4*ith bit of the calculated hash is 1 otherwise print it in lowercase.

## Rationale 
 Benefits:
 - By means of a minimal code change on existing libraries, users are protected from losing funds by mixing addresses of different Ethereum based networks.

## Implementation
```python
#!/usr/bin/python3
from sha3 import keccak_256
import random
"""
   addr (str): Hexadecimal address, 40 characters long with 2 characters prefix
   chainid (int): chain id from EIP-155 """
def eth_checksum_encode(addr, chainid=1):
    adopted_eip1191 = [30, 31]
    hash_input = str(chainid) + addr.lower() if chainid in adopted_eip1191 else addr[2:].lower()
    hash_output = keccak_256(hash_input.encode('utf8')).hexdigest()
    aggregate = zip(addr[2:].lower(),hash_output)
    out = addr[:2] + ''.join([c.upper() if int(a,16) >= 8 else c for c,a in aggregate])
    return out
```

## Test Cases
```python
eth_mainnet = [
"0x27b1fdb04752bbc536007a920d24acb045561c26",
"0x3599689E6292b81B2d85451025146515070129Bb",
"0x42712D45473476b98452f434e72461577D686318",
"0x52908400098527886E0F7030069857D2E4169EE7",
"0x5aAeb6053F3E94C9b9A09f33669435E7Ef1BeAed",
"0x6549f4939460DE12611948b3f82b88C3C8975323",
"0x66f9664f97F2b50F62D13eA064982f936dE76657",
"0x8617E340B3D01FA5F11F306F4090FD50E238070D",
"0x88021160C5C792225E4E5452585947470010289D",
"0xD1220A0cf47c7B9Be7A2E6BA89F429762e7b9aDb",
"0xdbF03B407c01E7cD3CBea99509d93f8DDDC8C6FB",
"0xde709f2102306220921060314715629080e2fb77",
"0xfB6916095ca1df60bB79Ce92cE3Ea74c37c5d359",
]
rsk_mainnet = [
"0x27b1FdB04752BBc536007A920D24ACB045561c26",
"0x3599689E6292B81B2D85451025146515070129Bb",
"0x42712D45473476B98452f434E72461577d686318",
"0x52908400098527886E0F7030069857D2E4169ee7",
"0x5aaEB6053f3e94c9b9a09f33669435E7ef1bEAeD",
"0x6549F4939460DE12611948B3F82B88C3C8975323",
"0x66F9664f97f2B50F62d13EA064982F936de76657",
"0x8617E340b3D01Fa5f11f306f4090fd50E238070D",
"0x88021160c5C792225E4E5452585947470010289d",
"0xD1220A0Cf47c7B9BE7a2e6ba89F429762E7B9adB",
"0xDBF03B407c01E7CD3cBea99509D93F8Dddc8C6FB",
"0xDe709F2102306220921060314715629080e2FB77",
"0xFb6916095cA1Df60bb79ce92cE3EA74c37c5d359",
]
rsk_testnet = [
"0x27B1FdB04752BbC536007a920D24acB045561C26",
"0x3599689e6292b81b2D85451025146515070129Bb",
"0x42712D45473476B98452F434E72461577D686318",
"0x52908400098527886E0F7030069857D2e4169EE7",
"0x5aAeb6053F3e94c9b9A09F33669435E7EF1BEaEd",
"0x6549f4939460dE12611948b3f82b88C3c8975323",
"0x66f9664F97F2b50f62d13eA064982F936DE76657",
"0x8617e340b3D01fa5F11f306F4090Fd50e238070d",
"0x88021160c5C792225E4E5452585947470010289d",
"0xd1220a0CF47c7B9Be7A2E6Ba89f429762E7b9adB",
"0xdbF03B407C01E7cd3cbEa99509D93f8dDDc8C6fB",
"0xDE709F2102306220921060314715629080e2Fb77",
"0xFb6916095CA1dF60bb79CE92ce3Ea74C37c5D359",
]
test_cases = {30 : rsk_mainnet, 31 : rsk_testnet, 1 : eth_mainnet}

for chainid, cases in test_cases.items():
    for addr in cases:
        assert ( addr == eth_checksum_encode(addr,chainid) )
```
## Usage

### Usage  Table

| Network      | Chain id | Supports this EIP |
|-|-|-|
| RSK Mainnet  | 30       | Yes               |
| RSK Testnet  | 31       | Yes               |

### Implementation Table
| Project         | EIP Usage        | Implementation |
|-|-|-|
| MyCrypto       | Yes              | [JavaScript](https://github.com/MyCryptoHQ/MyCrypto/blob/develop/common/utils/formatters.ts#L126) |
| MyEtherWallet  | Yes              | [JavaScript](https://github.com/MyEtherWallet/MyEtherWallet/blob/73c4a24f8f67c655749ac990c5b62efd92a2b11a/src/helpers/addressUtils.js#L22) |
| Ledger         | Yes              | [C](https://github.com/LedgerHQ/ledger-app-eth/blob/master/src_common/ethUtils.c#L203) |
| Trezor         | Yes              | [Python](https://github.com/trezor/trezor-core/blob/270bf732121d004a4cd1ab129adaccf7346ff1db/src/apps/ethereum/get_address.py#L32) and [C](https://github.com/trezor/trezor-crypto/blob/4153e662b60a0d83c1be15150f18483a37e9092c/address.c#L62) |
| Web3.js           | Yes              | [JavaScript](https://github.com/ethereum/web3.js/blob/aaf26c8806bc9fb60cf6dcb6658104963c6c7fc7/packages/web3-utils/src/Utils.js#L140) |
| EthereumJS-util   | Yes              | [JavaScript](https://github.com/ethereumjs/ethereumjs-util/pull/204/commits/cdf0b3c996b05ac5b1f758f17ea9f9ed1847c1eb) |
| ENS address-encoder | Yes | [TypeScript](https://github.com/ensdomains/address-encoder/commit/5bf53b13fa014646ea28c9e5f937361dc9b40590) |

## Copyright

Copyright and related rights waived via [CC0](../LICENSE.md).