# Design Trends and Comparative Analysis of Lightweight Block Ciphers for IoTs
## 5 Comparative Analysis  
This diagram, as presented in Figure 43, highlights the inherent trade-offs in the design of lightweight block cyphers concerning three critical factors: security, performance, and low-cost. Enhancing one aspect often diminishes either efficiency or strength in the others:  
* Increasing security typically requires longer key lengths and more rounds, which raises cost and reduces performance. * Improving performance (e.g., by reducing rounds or using parallel architecture) may lower the cipher's resistance to cryptanalysis. * Achieving low-cost (via smaller key sizes or serial processing) is ideal for IoTs devices but might compromise security or throughput. This triangular relationship highlights the balance required in designing optimal lightweight ciphers. Among the analyzed 58 lightweight block ciphers, several distinct trade-offs become evident. Substitution-permutation network (SPN) ciphers such as NVLC, RARE, DoT, and HERMES exhibit high levels of confusion and diffusion by design, using complex S-boxes and structured permutation layers. These designs offer robust resistance to linear and differential cryptanalysis, making them suitable for security-critical IoTs deployments. However, this strength comes at a cost: SPN ciphers tend to consume more gate equivalents and power, with examples such as HERMES and IVLBC showing elevated implementation costs. Their typically more elaborate key schedules also increase resource usage and may introduce latency. Feistel network (FN)-based ciphers such as GRANULE, TED, improved_DLBCA, and FlexAEAD prioritize structural simplicity. Their design facilitates low-overhead encryption and decryption, enabling deployment on minimal resource hardware. These ciphers often achieve high throughput and balanced performance, though they require more rounds to compensate for slower diffusion per round. As a result, their round count and latency may be higher compared to SPN counterparts, but their cost-efficiency and symmetrical structure remain advantages. Generalized Feistel network (GFN) designs such as Shadow, HDLBC, and GFLE provide better data diffusion through multiple branches, which helps reduce the number of rounds needed. Their scalability and parallelism make them appealing for hardware-efficient applications. However, the increased complexity in round function coordination and key mixing can pose challenges in secure and efficient implementation, especially for ultra-constrained devices. ARX ciphers, including BRIGHT, LiARX, and SAND-2, eliminate the need for non-linear S-boxes and instead use simple arithmetic (addition), bitwise rotation, and XOR operations. This makes them highly efficient in software and naturally resistant to timing and simple power analysis attacks. On the downside, maintaining strong non-linearity and diffusion without S-boxes requires careful round function design, and their resistance to certain cryptanalytic attacks may be weaker compared to SPN or GFN models. Hybrid designs such as FeW, SFN, and GFRX attempt to strike a balance by incorporating features from multiple structures (e.g., combining ARX and SPN or SPN and Feistel). These ciphers can achieve versatile trade-offs in terms of speed, area, and security. Nonetheless, their structural complexity and difficulty in analyzing combined effects of different layers can complicate formal verification and hardware optimization. This comparative overview highlights that cipher design often aligns with specific priorities. For example, SPISE and GFRX prioritize minimal gate count and efficient hardware implementation, making them ideal for ultra-constrained environments. In contrast, ciphers such as T-TWINE and Improved_SM4 emphasize robustness and broader cryptographic capabilities, even at the cost of higher hardware complexity. There is no universally optimal design; instead, each cipher reflects specific trade-offs tailored to unique performance, area, or security requirements.