The Cost of OSCORE and EDHOC for Constrained Devices

Many modern IoT applications rely on the Constrained Application Protocol (CoAP) because of its efficiency and seamless integrability in the existing Internet infrastructure. One of the strategies that CoAP leverages to achieve these characteristics is the usage of proxies. Unfortunately, in order for a proxy to operate, it needs to terminate the (D)TLS channels between clients and servers. Therefore, end-to-end confidentiality, integrity and authenticity of the exchanged data cannot be achieved. In order to overcome this problem, an alternative to (D)TLS was recently proposed by the Internet Engineering Task Force (IETF). This alternative consists of two novel protocols: 1) Object Security for Constrained RESTful Environments (OSCORE) providing authenticated encryption for the payload data and 2) Ephemeral Diffie-Hellman Over COSE (EDHOC) providing the symmetric session keys required for OSCORE. In this paper, we present the design of four firmware libraries for these protocols especially targeted for constrained microcontrollers and their detailed evaluation. More precisely, we present the design of uOSCORE and uEDHOC libraries for regular microcontrollers and uOSCORE-TEE and uEDHOC-TEE libraries for microcontrollers with a Trusted Execution Environment (TEE), such as microcontrollers featuring ARM TrustZone-M. Our firmware design for the later class of devices concerns the fact that attackers may exploit common software vulnerabilities, e.g., buffer overflows in the protocol logic, OS or application to compromise the protocol security. uOSCORE-TEE and uEDHOC-TEE achieve separation of the cryptographic operations and keys from the remainder of the firmware, which could be vulnerable. We present an evaluation of our implementations in terms of RAM/FLASH requirements, execution speed and energy on a broad range of microcontrollers.