Foreword: I don't really know much about security or encryption or how it works.
I am developing a small server for a game that uses ENet, which doesn't support higher level things such as security (e.g. SSL/TLS in the TCP world) to, as I think they said, maintain simplicity and embeddability. Supposing that this game requires at least some reasonable degree of security and authentication (e.g. logging in and such), what would be a good approach?