Clock Frequency Impact on the Performance of High-Security Cryptographic Cipher Suites for Energy-Efficient Resource-Constrained IoT Devices +
Footnote †: journal: Accepted: 15 December 2018; Published: 20 December 2018  
[PERSON]  
1 Department of Computer Engineering, Faculty of Computer Science, Universidade da Coruna, 15071 A Coruna, Spain; [EMAIL_ADDRESS] (M.S.-A.); [EMAIL_ADDRESS] (L.C.)  
[PERSON]  
1 Correspondence: [EMAIL_ADDRESS] (P.F.-L.); [EMAIL_ADDRESS] (T.M.F.-C.); Tel.: +34-98116700 (ext. 6051) (P.F.-L.)  
[PERSON]  
1 Department of Computer Engineering, Faculty of Computer Science, Universidade da Coruna, 15071 A Coruna, Spain; [EMAIL_ADDRESS] (M.S.-A.); [EMAIL_ADDRESS] (L.C.)  
Footnote 1: email: {[EMAIL_ADDRESS] (T.M.F.-C.);  
Footnote 2: email:  
communications [4]. Moreover, some resource-constrained IoT devices might be deployed in large areas where power outlets are not available, so energy harvesting techniques have to be introduced.  
Since IoT networks usually rely on TCP/IP, the use of already proven security protocols seems to be the best approach in terms of reliability and implementation efficiency. In this scenario, Transport Layer Security (TLS) arises as the main alternative, but it has a relevant limitation: its most popular cipher suites were not designed for resource-constrained IoT devices [5]. In the last years, UDP-based solutions such as Datagram Transport Layer Security (DTLS) [6] have been introduced to provide lightweight alternatives, but, when security is added to such solutions, their gains over TLS are diminished, being better to make use of broadly available and optimized TLS implementations [7].  
One important consideration when securing the communications of resource-constrained devices is the possibility of modifying the hardware configuration to improve both performance and energy consumption. In this aspect, IoT System-on-Chip (SoC) clock frequency is a factor that usually presents a relevant impact, since cryptographic algorithms require high computational capabilities. Moreover, it is possible to modify and test the impact of different SoC frequencies on different hardware platforms with minimum modifications. In contrast, other hardware optimizations such as the modification of the memory mapping [8] are platform-specific and therefore more difficult to implement and test.  
This study evaluated the impact of modifying the clock frequency on resource-constraint IoT devices when executing TLS with different cipher suite configurations, thus providing a variety of security levels that ranges from 80 to 192 bits. First, Rivest-Shamir-Adleman (RSA) was evaluated in terms of security, scalability, power consumption and data throughput. The results were then compared with a more suitable approach for resource-constrained devices based on Elliptic Curve Cryptography (ECC). The evaluated ECC key sizes were selected to maintain an acceptable security level for the years to come, unless breakthroughs in ECC cryptography or in certain disruptive technologies (e.g., quantum computing) happen. In fact, IoT node hardware was selected with the intention of being as close as possible in terms of computational power to the nodes that will be deployed in the next generation of IoT networks.  
The rest of this article is organized as follows. Section 2 reviews some of the latest academic publications that deal with IoT security energy efficiency. Moreover, it introduces the basics of TLS and its cipher suites, and analyzes the state of the art related to the performance of RSA and ECC when executed by resource-constrained devices. Section 3 details the design of the proposed system. In Section 4, the experimental setup and the performed tests are described. Finally, Section 5 is devoted to the conclusions.