How to calculate the checksum of an extended public key?

Question

Very difficult to find definitive info on this.

From what I can find on google and looking at BIP32 it seems like the process to calculate a checksum for an extended key is to decode the base58 string to data, then perform a double sha256 on the data and the first 4 bytes from the double hash are the checksum.

I am trying to take in a non xpub (vpubs, upub ypub etc) and convert it to an xpub/tpub programmatically. Currently I have it working but the last 6 digits of the xpub are always incorrect, I am assuming this is the checksum?

In order to correctly calculate the checksum do I remove the prefix first, then remove the existing checksum then do the double sha256 on the stripped down extended key or do I keep the new xpub prefix and then perform the double sha256? Or am I completely wrong in my assumptions here?

Thanks in advanced!

Answer 1

it seems like the process to calculate a checksum for an extended key is to decode the base58 string to data,

If raw_xpub = version + depth + fingerprint + child + chain + DATA than to make the checksum you need to do doublehash of xpriv/xpub (sha256), in python:

hashed_xpub = hashlib.sha256(raw_xpub).digest()
hashed_xpub = hashlib.sha256(hashed_xpub).digest()

and then take the checksum as first 4 characters (0 to 3) and add it at the end of the raw xpub:

raw_xpub += hashed_xpub[:4]

Finally, run it through Base58 to encode it. I think you've might performed base58 operation too early based on your question. It's the last step in the process.