Security has been considered as a key concern in real world deployments of Low power and Lossy networks (LLNs). It has been challenging to ensure a secure communication between the constrained devices. IPSec and DTLS are known not to be amenable to immediate LLN applications due to their memory footprint and the amount of traffic established during their session. In this paper, we propose to use the so-called Authenticated Encryption (AE) that is based on Poly1305 and XSalsa20 to provide confidentiality, integrity and authenticity over the data packets. Experiments are carried out using Contiki OS, an open source operating system for the Internet of Things. They demonstrate that AE has extremely high speed, requires very little resource, and can be implemented in constrained environments.