def keypair(self, i, keypair_class):
        """ Return the keypair that corresponds to the provided sequence number
            and keypair class (BitcoinKeypair, etc.).
        """

        # Make sure keypair_class is a valid cryptocurrency keypair
        if not is_cryptocurrency_keypair_class(keypair_class):
            raise Exception(_messages["INVALID_KEYPAIR_CLASS"])

        currency_name = keypair_class.__name__.lower().replace('keypair', '')

        k = keypair_class.from_passphrase(
            self._passphrase + " " + currency_name + str(i))

        return k