This paper deals with the problem of secure data transmission based on multi-input multi-output delayed chaotic systems. A new multi-input secure data transmission scheme is proposed. Moreover, in order to increase again the robustness of secure data transmission, some delays areintroduced as a second firewall against known plain-text attack. With thismethod, the parameters used as secret keys of the system are not identifiable and, as a result, the proposed scheme is robust to known plain-text attacks.