The purpose of this thesis is to develop a novel objective function that ensures secure links between all nodes in an Internet of Things network when using the Routing Protocol for Low-Power and Lossy Networks (RPL) and only allow nodes in the network that share a key to join the network. We propose the Shared Identifier Secure Link Objective Function (SISLOF) to allow only nodes that share a key to join the network and therefore ensuring that all links between the nodes in the network are secure. SISLOF will look at a route that includes all nodes in the network and if a node shares a key with more than one node, it will then choose the node that has a shorter pathway to the root. We evaluate the overhead of the security keys on the Internet of Things nodes and the routing metrics by measuring the overhead when using first ETX and OF0 objective functions when using either the probabilistic scheme or the deterministic scheme. We then identified that the use of ETX or OF0 with both schemes is not appropriate because of the large overhead it adds on the devices and the link. We show that both ETX and OF0 add a large overhead and they are not suitable to be used with the security schemes. The secure objective function was needed as the existing objective functions add a large overhead on the Internet of Things devices when using two different key distribution schemes to distribute and provide keys between nodes and to create a link. We develop an objective function that only adds nodes that share a key to the routing table without the overhead cost the other objective functions added. We also identify that the probabilistic key distribution scheme outperforms the deterministic key distribution scheme for all objective functions. The significance of this study is that it has identified the need for an objective function that incorporates the security key distributions for the Routing Protocol for Low-Power and Lossy Networks (RPL) in the Internet of Things networks and the Shared Identifier Secure Link Objective Function (SISLOF) was developed to solve this problem.