EIPs/EIPS/eip-55.md

eip	title	author	type	category	status	created
55	Mixed-case checksum address encoding	Vitalik Buterin <vitalik.buterin@ethereum.org>, Alex Van de Sande <avsa@ethereum.org>	Standards Track	ERC	Final	2016-01-14

Specification

Code:

import eth_utils


def checksum_encode(addr): # Takes a 20-byte binary address as input
    hex_addr = addr.hex()
    checksummed_buffer = ""

    # Treat the hex address as ascii/utf-8 for keccak256 hashing
    hashed_address = eth_utils.keccak(text=hex_addr).hex()

    # Iterate over each character in the hex address
    for nibble_index, character in enumerate(hex_addr):

        if character in "0123456789":
            # We can't upper-case the decimal digits
            checksummed_buffer += character
        elif character in "abcdef":
            # Check if the corresponding hex digit (nibble) in the hash is 8 or higher
            hashed_address_nibble = int(hashed_address[nibble_index], 16)
            if hashed_address_nibble > 7:
                checksummed_buffer += character.upper()
            else:
                checksummed_buffer += character
        else:
            raise eth_utils.ValidationError(
                f"Unrecognized hex character {character!r} at position {nibble_index}"
            )

    return "0x" + checksummed_buffer


def test(addr_str):
    addr_bytes = eth_utils.to_bytes(hexstr=addr_str)
    checksum_encoded = checksum_encode(addr_bytes)
    assert checksum_encoded == addr_str, f"{checksum_encoded} != expected {addr_str}"


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.

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.

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()

Test Cases

# All caps
0x52908400098527886E0F7030069857D2E4169EE7
0x8617E340B3D01FA5F11F306F4090FD50E238070D
# All Lower
0xde709f2102306220921060314715629080e2fb77
0x27b1fdb04752bbc536007a920d24acb045561c26
# Normal
0x5aAeb6053F3E94C9b9A09f33669435E7Ef1BeAed
0xfB6916095ca1df60bB79Ce92cE3Ea74c37c5d359
0xdbF03B407c01E7cD3CBea99509d93f8DDDC8C6FB
0xD1220A0cf47c7B9Be7A2E6BA89F429762e7b9aDb

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
Jaxx Liberty 2.0.0	Yes	Yes	Yes	Yes
Coinomi 1.10	Yes	Yes	Yes	Yes
Trust Wallet	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)
5. Swift implementation in EthereumKit
6. Kotlin implementation in KEthereum