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