Python script to combine two Neo private keys into a third private key

combineprivkeys.py

#!/usr/bin/env python3

"""
combineprivkeys.py - by hal0x2328

This tool combines two private keys into a new private key, as part of
the "split-key" scheme used to generate vanity addresses without sharing
the private key with a third party generating the vanity address for you.

The party generating the vanity address only needs your public key. With it,
they can generate a private key that, when combined with your private
key, will unlock the vanity address. The system is secure in that it is
impossible to calculate the private key of the vanity address without having
both private keys.

Usage: copy this script into an existing neo-python installation and run:

pip3 install pyopenssl
python3 ./combineprivkeys.py

When prompted, enter your private key, then the key given to you by the
person who generated the vanity address, then the script will combine the
keys to generate the private key for the vanity address which you can then
import into any Neo 2.x wallet

"""
import binascii
from neo.Core.KeyPair import KeyPair
from prompt_toolkit import prompt
from OpenSSL._util import(
    lib as _lib,
    ffi as _ffi
)

modulus =  b'FFFFFFFF00000000FFFFFFFFFFFFFFFFBCE6FAADA7179E84F3B9CAC2FC632551'

in1 = prompt('Private key WIF 1: ')
key1 = KeyPair.PrivateKeyFromWIF(in1)
in2 = prompt('Private key WIF 2: ')
key2 = KeyPair.PrivateKeyFromWIF(in2)

ctx = _ffi.gc(_lib.BN_CTX_new(), _lib.BN_CTX_free)

m = _ffi.gc(_lib.BN_new(), _lib.BN_free)
m_ptr = _ffi.new("BIGNUM**")
m_ptr[0] = m
_lib.BN_hex2bn(m_ptr, modulus)

k1 = _ffi.gc(_lib.BN_new(), _lib.BN_free)
k1_ptr = _ffi.new("BIGNUM**")
k1_ptr[0] = k1
_lib.BN_hex2bn(k1_ptr, binascii.hexlify(key1))

k2 = _ffi.gc(_lib.BN_new(), _lib.BN_free)
k2_ptr = _ffi.new("BIGNUM**")
k2_ptr[0] = k2
_lib.BN_hex2bn(k2_ptr, binascii.hexlify(key2))

r = _ffi.gc(_lib.BN_new(), _lib.BN_free)
r_ptr = _ffi.new("BIGNUM**")
r_ptr[0] = r

_lib.BN_mod_add(r, k1, k2, m, ctx);

r_c = _lib.BN_bn2hex(r)
final = _ffi.string(r_c).decode()
_lib.OPENSSL_free(r_c)

kp = KeyPair(binascii.unhexlify(final))
print("\n=== Resulting private key ===");
print("Hex: {}".format(final))
print("Address: {}".format(kp.GetAddress()))
print("WIF: {}".format(kp.Export()))