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Cumulative Message Authentication Codes for Resource 1 Cumulative Message Authentication Codes for Resource-Constrained IoT Networks He Li, Vireshwar Kumar, Member, IEEE, Jung-Min (Jerry) Park, Fellow, IEEE, and Yaling Yang, Member, IEEE Abstract—In resource-constrained IoT networks, the use of problems: the computational burden on the devices for gen- conventional message authentication codes (MACs) to provide erating/verifying the MAC, and the additional communication message authentication and integrity is not possible due to the overhead incurred due to the inclusion of the MAC in each large size of the MAC output. A straightforward yet naive solution to this problem is to employ a truncated MAC which message frame/packet. The first problem can be addressed by undesirably sacrifices cryptographic strength in exchange for using dedicated hardware and cryptographic accelerators [11], reduced communication overhead. In this paper, we address this [31]. However, the second problem is not as easy to address. problem by proposing a novel approach for message authentica- tion called Cumulative Message Authentication Code (CuMAC), Problem. The cryptographic strength of a MAC depends which consists of two distinctive procedures: aggregation and on the cryptographic strength of the underlying cryptographic accumulation. In aggregation, a sender generates compact au- primitive (e.g. a hash or block cipher), the size and quality of thentication tags from segments of multiple MACs by using a systematic encoding procedure. In accumulation, a receiver the key, and the size of the MAC output. Hence, a conventional accumulates the cryptographic strength of the underlying MAC MAC scheme typically employs at least a few hundred bits by collecting and verifying the authentication tags. Embodied of MAC output to ensure a sufficient level of cryptographic with these two procedures, CuMAC enables the receiver to strength. Unfortunately, in resource-constrained IoT networks achieve an advantageous trade-off between the cryptographic (e.g., energy-constrained low-power wide-area network with strength and the latency in processing of the authentication tags. Furthermore, for some latency-sensitive messages where battery-powered devices and bandwidth-constrained in-vehicle this trade-off may be unacceptable, we propose a variant of controller area network), the payload size of each packet is CuMAC that we refer to as CuMAC with Speculation (CuMAC/S). very short, i.e., less than a hundred bits [24]. As such, not more In addition to the aggregation and accumulation procedures, than a few bits can be spared to include an authentication tag, CuMAC/S enables the sender and receiver to employ a spec- prohibiting the usage of the conventional MAC [26]. ulation procedure for predicting future message values and pre- computing the corresponding MAC segments. For the messages Related Work. The legacy solution for generating a short which can be reliably speculated, CuMAC/S significantly reduces the MAC verification latency without compromising the crypto- authentication tag is to truncate the output of a conventional graphic strength. We have carried out comprehensive evaluation MAC so that it fits a message packet [28], [32], [35]. This of CuMAC and CuMAC/S through simulation and a prototype type of MAC is called a truncated MAC. However, the implementation on a real car. truncated MAC sacrifices cryptographic strength in exchange Index Terms—Message authentication code (MAC); Internet- for reduced communication overhead and energy consumption, of-Things (IoT); Controller area network (CAN). which may be undesirable, or even unacceptable, in some applications. Note that the truncated MAC without sufficient I. INTRODUCTION cryptographic strength renders the application vulnerable to collision attacks [4]. To enable authentication with enhanced arXiv:2001.05211v2 [cs.CR] 15 Oct 2020 In emerging applications, such as intelligent automobiles, industrial control systems and smart city networks, a large cryptographic strength, Katz et al. propose the concept of ag- number of energy-constrained computing devices are getting gregate MAC where conventional MACs of multiple messages closely integrated with the existing computer infrastructure are combined into one aggregate MAC, and transmitted over through bandwidth-constrained networks to form the Internet- successive packets [16]. Similarly, Nilson et al. propose a of-Things (IoT) [26]. The successful adoption of those ap- compound MAC which is calculated on a compound of mul- plications will partially depend on our ability to thwart se- tiple messages, and distributed over successive packets [24]. curity and privacy threats, including message forgery and However, both the aggregate and compound MAC schemes tampering. Today, message authentication code (MAC) is the incur significant latency in the verification of the messages most commonly used method for providing message authen- because the receiver needs to receive and process all associated ticity and integrity in wired/wireless network applications. To packets before being able to verify the validity of the MAC. employ MACs in a resource-constrained (i.e., energy and/or Challenges. In the above discussion, we identify three critical bandwidth-constrained) network, we need to consider two challenges in employing MACs for IoT networks: (1) incurring A preliminary version of some portions of this work appeared in [18]. minimal communication overhead so that the MAC can fit in a H. Li, J. Park, and Y. Yang are with the Department of Electrical and packet, (2) ensuring that the cryptographic strength meets the Computer Engineering, Virginia Tech, Blacksburg, Virginia, USA (e-mail: security need of the application, and (3) incurring minimal [email protected], [email protected], [email protected]). V. Kumar is with the Department of Computer Science and Engineering, latency so that the MAC generation and verification processes Indian Institute of Technology, Delhi, India (e-mail: [email protected]). do not cause unacceptable delays in the packet processing. Proposed Solution. In this paper, we addresses the afore- mentioned challenges through a novel approach for message Cloud layer: authentication that we refer to as Cumulative Message Au- resource abundant thentication Code (CuMAC). In CuMAC, a sender utilizes a procedure called aggregation through which the sender first Edge layer: divides the full-sized MAC of each message into multiple short resource sufficient MAC segments, and then “aggregates” the MAC segments of multiple messages using a systematic encoding procedure to form a short authentication tag. This procedure resolves the first challenge of ensuring low communication overhead. Smart meters in Further, the receiver utilizes a procedure called accumula- smart building Sensor networks for smart tion through which it first verifies the MAC segments aggre- In-vehicle controller Device layer: city infrastructure gated into the authentication tag of each received packet, and resource constrained area network (CAN) then “accumulates” the cryptographic strength by collecting the verified MAC segments associated with the target mes- Fig. 1: Architecture of typical IoT networks. sage. In this procedure, the receiver may incur delay that is proportional to the accumulated cryptographic strength since it of two novel concepts that we refer to as aggregation needs to wait for the relevant tags to be received and processed. (which reduces the communication overhead) and accu- Hence, while the accumulation procedure caters to the second mulation (which increases the cryptographic strength). and the third challenge, it brings up a novel and flexible trade- • We propose a variant of CuMAC called CuMAC/S that off between the cryptographic strength and latency. CuMAC meets the security need of delay-sensitive, resource- enables the receiver to authenticate the message in real-time constrained IoT applications. CuMAC/S enables accumu- with the cryptographic strength which is commensurate with lation of cryptographic strength while incurring minimal the size of each tag. Meanwhile, CuMAC also enables the delay by employing the novel idea of speculation. authentication with the highest level of cryptographic strength • We have thoroughly evaluated the effectiveness of after accumulating all segments of the MAC that covers the CuMAC and CuMAC/S through a simulated in-vehicle message in the associated packets. controller area network and a prototype implementation Moreover, in latency-sensitive IoT applications, the receiver on a real car. Our results illustrate that while incurring may be required to immediately authenticate a message with the same communication overhead as the truncated MAC high cryptographic strength as it arrives. In such cases, the scheme, CuMAC achieves the cryptographic strength trade-off made by CuMAC may not be sufficient. To address equivalent to the conventional MAC scheme at the cost this need, we propose a variant of CuMAC called CuMAC with of increased latency. Further, for the messages which can Speculation (CuMAC/S) that enables a receiver to accumulate be accurately speculated, CuMAC/S achieves the cryp- the MAC’s cryptographic strength while incurring minimal tographic strength equivalent to the conventional MAC delay. The core concept of CuMAC/S is motivated by the scheme without any additional latency. technique of speculative execution1 which is widely employed in modern computer systems [7], [23]. CuMAC/S can be II. MOTIVATION FOR SHORT MACS utilized in IoT applications where future messages can be IoT networks
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