## 2 Literature Review on IoT and Lightweight Security Protocols
### Lightweight Solutions for IoT  
Table 2 contains a summary of seven studies that describe or propose lightweight solutions for IoT. Beginning with [PERSON] et al. [21], their study is about the IoT communications model:  
* _Smart devices_ such as sensors and actuators. * _Edge computing_ networks are composed of smartphones, Wi-Fi routers, or LoRa gateways. They support device mobility and process data closer to the smart device. * _Fog computing_ networks have more powerful routers, industrial switches, and servers. * _Cloud computing_ components such as the Amazon Web Services (AWS) cloud platform, where data are stored, visualized, and analyzed. At different levels of this IoT communication model, security protocols must be adapted to support the different capabilities that the communication equipment has. For example, a gateway in the edge computing system has more processing power and energy to run encryption algorithms than a sensor node. They also use the term _constrained networks_, such as a private network inside an organization where smart devices are located. These networks may not require robust security algorithms, whereas security requirements increase as the data become close to the edge, fog, and cloud computing, i.e., towards _unconstrained networks_ and the public internet. Thus, the authors review previous works on the idea of proxy re-encryption, where edge nodes act as proxy nodes and re-encrypt the data with more secure algorithms. They name this an _end-through-nodes-to-end_ security model, where smart devices use lightweight encryption protocols, and then the edge nodes use more robust encryption protocols. This is one important relation to this paper because the authors take into account the issue of device's resource asymmetry (i.e., depending on where the device is located in the IoT communication model) as an essential requirement for IoT security. Additionally, [PERSON] et al. [21] compared several new solutions for lightweight cryptographic algorithms and their advantages, drawbacks, and vulnerabilities, followed by an analysis of their security features in terms of the CIA triad. For example, one of the solutions analyzed uses a mix of encryption algorithms such as the Elliptic Curve Discrete Logarithmic Protocol (ECDLP) to generate the session key, the Diffie-Hellman (DH) public/private key algorithm to distribute the session key, and the Tiny Encryption Algorithm (TEA) as the symmetric algorithm to encrypt the actual data packets with the session key. According to the study by [PERSON] [21], it prevents the issue of key equivalence attack of TEA, but it adds complexity because it requires periodic updates of the private key. [PERSON] et al. [22] reviewed lightweight authentication encryption (AE) algorithms, which provide both confidentiality and integrity by encrypting the data and creating a hash value. This hash value can be called a message authentication code (MAC) or message integrity code (MIC). For traditional computer applications, the standard that provides authenticated encryption is Advanced Encryption Standard-Galois/Counter Mode (AES-GCM) [41]. Although there are standards for lightweight encryption algorithms, such as ISO/IEC 29192's block ciphers--PRESENT, CLEFIA, and LEA--the authors argue that there are no standards for lightweight AE algorithms. Nonetheless, the authors compare a variety of AE schemes dating back to 2010. In particular, these are algorithms proposed at the 2014 Competition for Authenticated Encryption: Security, Applicability, and Robustness (CAESAR). Their types vary: some are based on (a) sponge functions (or key-less algorithms), (b) stream ciphers, (c) block ciphers, and (d) hash functions. Then, the performance of these different AE algorithms is compared in terms of hardware implementation (e.g., gate equivalent area, power consumption) and software speed (e.g., number of CPU cycles per byte). [PERSON] et al.