def _int_to_bn(self, num, bn=None):
        """
        Converts a python integer to a BIGNUM. The returned BIGNUM will not
        be garbage collected (to support adding them to structs that take
        ownership of the object). Be sure to register it for GC if it will
        be discarded after use.
        """
        assert bn is None or bn != self._ffi.NULL

        if bn is None:
            bn = self._ffi.NULL

        if not six.PY2:
            # Python 3 has constant time to_bytes, so use that.

            binary = num.to_bytes(int(num.bit_length() / 8.0 + 1), "big")
            bn_ptr = self._lib.BN_bin2bn(binary, len(binary), bn)
            self.openssl_assert(bn_ptr != self._ffi.NULL)
            return bn_ptr

        else:
            # Under Python 2 the best we can do is hex(), [2:] removes the 0x
            # prefix.
            hex_num = hex(num).rstrip("L")[2:].encode("ascii")
            bn_ptr = self._ffi.new("BIGNUM **")
            bn_ptr[0] = bn
            res = self._lib.BN_hex2bn(bn_ptr, hex_num)
            self.openssl_assert(res != 0)
            self.openssl_assert(bn_ptr[0] != self._ffi.NULL)
            return bn_ptr[0]

        Converts a python integer to a BIGNUM. The returned BIGNUM will not
        be garbage collected (to support adding them to structs that take
        ownership of the object). Be sure to register it for GC if it will
        be discarded after use.
        
stackoverflow/venv/lib/python3.6/site-packages/cryptography/hazmat/backends/openssl/backend.py