For the past few years, a number of new wireless technologies have emerged, allowing users to access the Internet and a myriad of services through mobile devices such as smartphones, tablets, wearables..., Among others, body area networks is one of the applications of such technologies. These networks are based on the deployment of several sensor nodes over a body that get information about parameters such as glucose level or temperature and send it to another device, such as a smartphone. Body area networks can be very valuable for athletes, since they can monitor and track their performance but also in the medical field. In this case, a patient can get information about vital signs and report them to a medical expert/center remotely. Body area networks deal with sensitive data and therefore it is of paramount importance to provide a given level of reliability and security to their data communications. In this work we have implemented and evaluated a simple body area network that provides secure and robust communications by means of cryptographic tools and applies the idea of cognitive radio, which allows selecting in an opportunistic manner the optimal channel for data transmission.