# Feasibility Assessment of a Fine-Grained Access Control Model on Resource Constrained Sensors
## 1 Introduction
Beyond the traditional producer behavior of CDSs, which publish measurements and events to message brokers as depicted with thick arrows, in more advanced IoT scenarios, CDSs behave as tiny information servers that can be addressed by their IPv6 address that is natively implemented over Low-Power Wireless Personal Area Networks (6 LowPAN) [9]. Specifically, requesting clients directly query the tiny CDS servers, establishing a secure end-to-end (E2E) communication, as depicted with thin arrows. These services that are exposed through the IPv6 network enable the usage, operation, maintenance and manageability of the CDSs over their entire life-cycle and protect the value stream of the connected objects. For example, an end user can utilize direct access to tune personal parameters, such as gender, age, and weight, in a constant health monitoring sensor. In these cases, the use of intermediary proxies is avoided because, on the one hand, they are specific for each protocol or application and are not sufficiently flexible, while, on the other hand, breaking the security association into two or more sub-transmissions might not be considered acceptable from a security point of view. In this context, the accuracy and correctness of the information exchanged with CDSs are crucial. Protecting this information requires the implementation of appropriate security mechanisms that include fine-grained access control mechanisms based on expressive policies and that can guarantee essential security properties such as confidentiality, integrity, availability, authenticity and non-repudiation [10; 11; 12]. However, implementing these appropriate security mechanisms in resource-constrained CDSs is not straightforward. Currently, one of the key challenges for enabling the broader adoption of smart things is the availability of feasible access control solutions. Moreover, due to the extremely dynamic nature and purpose of applications based on services in sensors, policy-based security must be enforced locally in the CDSs, where resources are scarce. That is, besides adherence to the least privilege principle through expressive policies, access control must be able to deal with changes to these policies and application services, and with the fact that it is impossible to know all data, resources and users in advance. Instead of security rules coupled within the applications' logic, policy-driven security management has become the de facto approach for security management in large scale systems [13]. In fact, CDSs integrated in IoT are rapidly growing in scale, and at the same time incorporating various emerging technologies. Facing the resulting complexity, traditional system management (including security management) strategies, which mainly rely on IT professionals' manual work, seem effort-consuming and error-prone for large-scale networks or distributed systems. To resolve these issues, policy-driven security management is proposed to be leveraged, to simplify the administration of the large scale systems. In a policy-driven management system, administrators just need to specify their targets and constraints in the form of security policies, to guide the behaviours of system elements. From this perspective, a policy can be seen as a common intermediate format, to map requirements of the system to specific and implementable operations. A core software engineering principle in the study and design of security in general and access control systems in particular is the separation between policy and mechanism, which has its roots in early systems research [14]. The policy is the set of rules that determine what is allowed in the system. The mechanism is the set of software and/or hardware components that know how to enforce the policy in the system. Specific proposals of access control models for constrained devices from the research community, have addressed authentication in various ways, but authorization has received little consideration.