9

I'm working on a Bitcoin ATM. We want to detect addresses that are entered incorrectly.

There's a checksum on Bitcoin addresses - but how can we check it?

  • 1
    ah.found it if(!validateAddress(rcp.address)) { return InvalidAddress; } – Willipm Nov 6 '14 at 3:32
  • If you can choose the option of answering your own question for future queries that'd help! – Wizard Of Ozzie Nov 6 '14 at 8:51
  • @WizardOfOzzie: Right on. Also, if you know that the information is correct, feel free to use any "comment answer" as a starting point to create a full answer. It might be nice to quote the original commenter, but anyone has my blessing to the reputation for the answer if he does that. – Murch Nov 7 '14 at 1:07
9

The way Bitcoin checks a public address is using what is called Base58Check. While the Bitcoin Wiki covers Base58Check, I'll outline how it works in both abstract and technical terms.

Diagram of Base58Check

The whole idea of Base58Check is to prevent invalid addresses. This is done by adding digits to the start (00) and end of the Base58 value (4 bytes of double SHA256 hash).

  1. Add 00 to the start of Base58 value ie 00 + BASE58 ==> 00BASE58
  2. Checksum = 1st 8 digits from SHA256(SHA256(00BASE58))

BASE58Check = (00)(BASE58_SHA256RIPEMD160_HEXADECIMAL_VALUE)(CHECKSUM) (parenthesis for visualisation only, all other data is correct in size, ie CHECKSUM = 8 chars)

The best way to visualize all this is with The Piachu's App

  • Excellent answer and infographic! – Nick ODell Nov 7 '14 at 3:05
6

Rosetta Code has several example implementations in multiple languages.

Python 2 & 3

import codecs
from hashlib import sha256

digits58 = '123456789ABCDEFGHJKLMNPQRSTUVWXYZabcdefghijkmnopqrstuvwxyz'

def to_bytes(n, length):
    s = '%x' % n
    s = s.rjust(length*2, '0')
    s = codecs.decode(s.encode("UTF-8"), 'hex_codec')
    return s

def decode_base58(bc, length):
    n = 0
    for char in bc:
        n = n * 58 + digits58.index(char)
    return to_bytes(n, length)

def check_bc(bc):
    bcbytes = decode_base58(bc, 25)

    if bcbytes[0] not in [0x00, 0x05]:
        return False

    return bcbytes[-4:] == sha256(sha256(bcbytes[:-4]).digest()).digest()[:4]

if __name__ == '__main__':
    print(check_bc('1111111111111111111114oLvT2'))
  • This cannot be used to validate bitcoin addresses. Returns True for the following strings:DPPGKttReEyzL5iQJExKvmS68QWTFvaecJ, MfwMaegJ8qB4QMvtFqgfaD8o2FrYse7LU, mtcrV4Wux9xPapqHz13eAEmynVhyfNTY7u – Max Malysh Aug 19 '17 at 0:29
  • If you wish to check the version byte, check that too. – Nick ODell Aug 19 '17 at 0:31

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