def METHOD_NAME(parent_fingerprint, private_key, chain, depth, childnr):
    """ Private key b58 serialization format. """
    raw = (b'\x04\x88\xad\xe4' + bytes(chr(depth), 'utf-8') + parent_fingerprint + int(childnr).to_bytes(
        4, byteorder='big') + chain + b'\x00' + private_key)
    return b58encode_check(raw)