def sign_tx(self, rawtx, wifs):
        """Sign <rawtx> with  given <wifs> as json data.
        <wifs>: '["privatekey_in_wif_format", ...]'
        """
        tx = deserialize.tx(rawtx)
        keys = deserialize.keys(self.testnet, wifs)
        tx = control.sign_tx(self.service, self.testnet, tx, keys)
        return serialize.tx(tx)