# Design Trends and Comparative Analysis of Lightweight Block Ciphers for IoTs
## 4 Related Work
### Generalized Feistel Network (GFN)  
The NUX [44] cipher is an ultra-LWBC designed for resource-constrained applications such as IoTs and wireless sensor networks. It employs a generalized Feistel network with 31 rounds and supports 128-bit or 80-bit key lengths, offering significant security while maintaining a minimal hardware footprint. The cipher, as displayed in Figure 33 uses a 16-bit permutation and a compact 4 \(\times\) 4 S-box to reduce gate equivalents (GEs), achieving only 1022 GEs for the 128-bit key version, making it one of the smallest ciphers in its class. NUX is resistant to linear and differential cryptanalysis and advanced attacks such as biclique attacks. Its design features high diffusion and confusion properties, contributing to strong security with low memory and power requirements. The cipher is particularly suitable for IoTs and pervasive computing environments where both security and efficiency are critical. NUX has high computational overhead due to 31 encryption rounds, and while resistant to basic attacks, its resistance to advanced attacks is unverified. The bit permutation layer adds complexity, and reducing gate equivalents (GEs) may impact long-term security. Additionally, its performance on non-ARM platforms and large-scale deployments needs further testing. The NUCLEAR [45] cipher is a lightweight encryption scheme designed to replace AES-128/256 in the 6 LoWPAN protocol stack for IoTs applications, optimizing the system's performance in resource-constrained environments. Using a generalized Feistel structure (GFS), NUCLEAR, according to Figure 34, is highly efficient in terms of power consumption, memory requirements, and footprint area, making it suitable for wireless sensor networks. It operates on 64-bit blocks, with 25 rounds, and features an optimized S-box and circular shift operators for maximum active S-boxes. Compared to other lightweight ciphers, NUCLEAR demonstrates superior performance, requiring fewer gate equivalents (GEs) and memory while providing higher throughput and lower power dissipation. Security analyses confirm its resilience against common attacks like differential, linear, and biclique attacks, as well as its strong avalanche effect. The such as design makes it a promising candidate for implementing lightweight encryption in IoTs systems, reducing overhead while maintaining robust security. The NUCLEAR algorithm has limited cryptanalysis testing, making its resistance to advanced attacks uncertain. Its S-box and shift parameters may pose potential security risks over time. The higher number of rounds (25) increases processing time, and there is  
Figure 33: Block diagram of NUX cipher. a trade-off between security and efficiency. Additionally, its real-world deployment and integration in diverse IoTs environments require further evaluation. The Shadow [46] cipher is an LWBC designed specifically for resource-constrained IoTs environments. It is structured using a generalized Feistel network with four branches as illustrated in Figure 35, deliberately avoiding the substitution-permutation network (SPN) model. Shadow supports two configurations: a 32-bit block size with a 64-bit key using 16 rounds and a 64-bit block size with a 128-bit key using 32 rounds. Its design prioritizes efficiency and low computational overhead while maintaining cryptographic strength suitable for IoTs applications. The Shadow cipher, while efficient for IoTs applications, has several limitations. Its early rounds exhibit a low algebraic degree, making it more vulnerable to algebraic and higher-order differential attacks in reduced-round versions. The theoretical degree growth bounds are not tight, leading to potential underestimations of its security margin. Additionally, distinguishers have been found for reduced rounds (e.g., 4-round versions), suggesting limited resistance when fewer rounds are used. Although no full round attack is currently  
Figure 34: NUCLEAR block cipher. Figure 35: Round function of Shadow.