How do I check the checksum of a Bitcoin address?

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?

• If you can choose the option of answering your own question for future queries that'd help! Commented Nov 6, 2014 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
Commented Nov 7, 2014 at 1:07

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.

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! Commented Nov 7, 2014 at 3:05

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)

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` Commented Aug 19, 2017 at 0:29
• If you wish to check the version byte, check that too. Commented Aug 19, 2017 at 0:31