A growing number of systems are highly connected and distributed over the internet. These systems require innovative software services and infrastructures in order to guarantee security and reliability. This issue has a particular relation to a wide range of topics such as real-time scheduling and encryption with quality of service constraints which are the main interests of this paper. The first contribution is a chaos-based cryptosystem which provides an acceptable level of security while executing at least 41 times faster than the AES algorithm. Secondly, we propose a scheduling scheme known as the deadline mechanism in order to guarantee an acceptable level of security even in computing overload situations. Dynamically choosing the best possible cryptosystem in dependance with processing availability is the basis of our mechanism.