Decrypt the data. Also, update the cipher iv. This is needed for SSLv3
        and TLS 1.0. For TLS 1.1/1.2, it is overwritten in TLS.pre_dissect().
        If we lack the key, we raise a CipherError which contains the input.

def decrypt(self, data):
        """
        Decrypt the data. Also, update the cipher iv. This is needed for SSLv3
        and TLS 1.0. For TLS 1.1/1.2, it is overwritten in TLS.pre_dissect().
        If we lack the key, we raise a CipherError which contains the input.
        """
        if False in six.itervalues(self.ready):
            raise CipherError(data)
        decryptor = self._cipher.decryptor()
        tmp = decryptor.update(data) + decryptor.finalize()
        self.iv = data[-self.block_size:]
        return tmp