EIPs/EIPS/eip-55.md

EIP: 55
Title: Mixed-case checksum address encoding
Author: Vitalik Buterin
Type: Standard Track
Category: ERC
Status: Accepted
Created: 2016-01-14

Specification

Code:

from ethereum import utils

def checksum_encode(addr): # Takes a 20-byte binary address as input
    o = ''
    v = utils.big_endian_to_int(utils.sha3(addr.hex()))
    for i, c in enumerate(addr.hex()):
        if c in '0123456789':
            o += c
        else:
            o += c.upper() if (v & (2**(255 - 4*i))) else c.lower()
    return '0x'+o

def test(addrstr):
    assert(addrstr == checksum_encode2(bytes.fromhex(addrstr[2:])))

test('0x5aAeb6053F3E94C9b9A09f33669435E7Ef1BeAed')
test('0xfB6916095ca1df60bB79Ce92cE3Ea74c37c5d359')
test('0xdbF03B407c01E7cD3CBea99509d93f8DDDC8C6FB')
test('0xD1220A0cf47c7B9Be7A2E6BA89F429762e7b9aDb')

In English, convert the address to hex, 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 hash of the lowercase hexadecimal address is 1 otherwise print it in lowercase.

Implementation

In javascript:

const createKeccakHash = require('keccak')

function toChecksumAddress (address) {
  address = address.toLowerCase().replace('0x','');
  var hash = createKeccakHash('keccak256').update(address).digest('hex')
  var ret = '0x'

  for (var i = 0; i < address.length; i++) {
    if (parseInt(hash[i], 16) >= 8) {
      ret += address[i].toUpperCase()
    } else {
      ret += address[i]
    }
  }

  return ret
}

> toChecksumAddress('0xfb6916095ca1df60bb79ce92ce3ea74c37c5d359')
'0xfB6916095ca1df60bB79Ce92cE3Ea74c37c5d359'

Note that the input to the Keccak256 hash is the lowercase hexadecimal string (i.e. the hex address encoded as ASCII):

    var hash = createKeccakHash('keccak256').update(Buffer.from(address.toLowerCase(), 'ascii')).digest()

Rationale

Benefits:

• Backwards compatible with many hex parsers that accept mixed case, allowing it to be easily introduced over time
• Keeps the length at 40 characters
• On average there will be 15 check bits per address, and the net probability that a randomly generated address if mistyped will accidentally pass a check is 0.0247%. This is a ~50x improvement over ICAP, but not as good as a 4-byte check code.

Adoption

Wallet	displays checksummed addresses	rejects invalid mixed-case	rejects too short	rejects too long
Etherwall 2.0.1	Yes	Yes	Yes	Yes
Jaxx 1.2.17	No	Yes	Yes	Yes
MetaMask 3.7.8	Yes	Yes	Yes	Yes
Mist 0.8.10	Yes	Yes	Yes	Yes
MyEtherWallet v3.9.4	Yes	Yes	Yes	Yes
Parity 1.6.6-beta (UI)	Yes	Yes	Yes	Yes

Exchange support for mixed-case address checksums, as of 2017-05-27:

Exchange	displays checksummed deposit addresses	rejects invalid mixed-case	rejects too short	rejects too long
Bitfinex	No	Yes	Yes	Yes
Coinbase	Yes	No	Yes	Yes
GDAX	Yes	Yes	Yes	Yes
Kraken	No	No	Yes	Yes
Poloniex	No	No	Yes	Yes
Shapeshift	No	No	Yes	Yes

References

1. EIP 55 issue and discussion https://github.com/ethereum/eips/issues/55
2. Python example by @Recmo https://github.com/ethereum/eips/issues/55#issuecomment-261521584
3. Python implementation in ethereum-utils
4. Ethereumjs-util implementation 75f529458b/index.js (L452-L466)