An important challenge in networked control systems is to ensure the confidentiality and integrity of the message in order to secure the communication and prevent attackers or intruders from compromising the system. However, security mechanisms may jeopardize the temporal behavior of the network data communication because of the computation and communication overhead. In this paper, we study the effect of adding Hash Based Message Authentication (HMAC) to a time-triggered networked control system. Time Triggered Architectures (TTAs) provide a deterministic and predictable timing behavior that is used to ensure safety, reliability and fault tolerance properties. The paper analyzes the computation and communication overhead of adding HMAC and the impact on the performance of the time-triggered network. Experimental validation and performance evaluation results using a TTEthernet network are also presented.