This paper describes a framework that allows finegrained and flexible access control to connected devices with very limited processing power and memory. We propose a set of security and performance requirements for this setting and derive an authorization framework distributing processing costs between constrained devices and less constrained back-end servers while keeping message exchanges with the constrained devices at a minimum. As a proof of concept we present performance results from a prototype implementing the device part of the framework.

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Abstract-DTLS is becoming the de facto standard for communication security in the Internet of Things. In order to run the DTLS protocol one needs to establish keys between the communicating devices. The default method of key establishment requires X.509 certificates and a Public Key Infrastructure, an approach which is often too resource consuming for small IoT devices. DTLS also supports the use of pre-shared keys and raw public keys. These modes are more lightweight, but they are not scalable to a large number of devices.We present Scalable Security with Symmetric Keys (S3K), a key management architecture for the resource constrained Internet of Things. S3K provides a flexible and scalable way of establishing keys between resource constrained IoT devices. S3K enables devices that have no previous, direct security relation to use DTLS with either pre-shared symmetric keys or raw public keys established and authorized during the DTLS handshake. We implement S3K in the Contiki OS and evaluate it on real IoT hardware. Our evaluation shows that S3K is feasible in constrained environment and at the same time scalable to a large number of devices.Note to Practitioners: Key management is one of the hardest problems in cyber security. It is even more challenging in the Internet of IoT considering that most things are resourceconstrained. Therefore, IoT devices either end-up using the symmetric cryptography with pre-shared key mode or asymmetric cryptography with raw public keys (RPK) mode. These modes either require a pre-provisioning of all expected trusted clients in individual nodes before deployment or requires outof-band validation of RPKs. Also, if the number of clients that a node would communicate with varies dynamically, this would demand frequent re-provisioning of each trusted client to the individual nodes. The approach based on preprovisioning and re-provisioning of trusted keys is certainly not scalable and requires a continuous management of security policies. We therefore propose a solution that is scalable and does not require pre-provisioning or re-provisioning the individual nodes with keys for all future trusted clients. The basic approach is to establish shared keys between resource servers and a trust anchor. When a client wants to establish a trust relationship with a resource server it requests a key from a trust anchor. The trust anchor asserts a secret key or a public key of the client that can be conveyed to the resource server.

The Internet of Things (IoT) is here, more than 10 billion units are already connected and five times more devices are expected to be deployed in the next five years. Technological standarization and the management and fostering of rapid innovation by governments are among the main challenges of the IoT. However, security and privacy are the key to make the IoT reliable and trusted. Security mechanisms for the IoT should provide features such as scalability, interoperability and lightness. This paper adresses authentication and access control in the frame of the IoT. It presents Physical Unclonable Functions (PUF), which can provide cheap, secure, tamper-proof secret keys to authentify constrained M2M devices. To be successfully used in the IoT context, this technology needs to be embedded in a standardized identity and access management framework. On the other hand, Embedded Subscriber Identity Module (eSIM) can provide cellular connectivity with scalability, interoperability and standard compliant security protocols. The paper discusses an authorization scheme for a constrained resource server taking advantage of PUF and eSIM features. Concrete IoT uses cases are discussed (SCADA and building automation).

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