def _calc_checksum(self, secret):
        if isinstance(secret, unicode):
            secret = secret.encode("utf-8")

        # check for truncation (during .hash() calls only)
        if self.use_defaults:
            self._check_truncate_policy(secret)

        # parse salt value
        try:
            salt_value = h64.decode_int12(self.salt.encode("ascii"))
        except ValueError: # pragma: no cover - caught by class
            raise suppress_cause(ValueError("invalid chars in salt"))

        # convert first 8 byts of secret string into an integer,
        key1 = _crypt_secret_to_key(secret)

        # run data through des using input of 0
        result1 = des_encrypt_int_block(key1, 0, salt_value, 20)

        # convert next 8 bytes of secret string into integer (key=0 if secret < 8 chars)
        key2 = _crypt_secret_to_key(secret[8:16])

        # run data through des using input of 0
        result2 = des_encrypt_int_block(key2, 0, salt_value, 5)

        # done
        chk = h64big.encode_int64(result1) + h64big.encode_int64(result2)
        return chk.decode("ascii")