# Design Trends and Comparative Analysis of Lightweight Block Ciphers for IoTs
## 4 Related Work
### Substitution-Permutation Network (SPN)
The cipher as depicted in Figure 18 consists of multiple rounds (64 or 76 depending on key size), each applying a bitwise XOR with the round key, a 4-bit involutive S-box, and a bit permutation layer to ensure confusion and diffusion. Its lightweight key schedule uses a Linear Feedback Shift Register (LFSR) mechanism. By using minimal logic and avoiding complex operations, IVLBC achieves a strong balance between security and implementation efficiency, making it ideal for secure communication in embedded and IoTs systems. Figure 6.0 FUTURE round function. Figure 6.1 One full round function of IVLBC. Despite its efficient design, IVLBC has several limitations. Its use of a single small 4-bit S-box, though beneficial for low-cost hardware, may offer lower non-linearity and reduced resistance to algebraic attacks compared to larger or more complex S-boxes. Additionally, its security margin is relatively tight, especially in the 64-bit key variant, which could become vulnerable as cryptanalytic techniques evolve. The bit permutation layer, while simple and hardware-friendly, may not provide the strongest diffusion compared to more advanced methods. Finally, the algorithm is highly hardware-centric, meaning its performance and suitability in software-based or hybrid environments may be limited. These factors suggest that while IVLBC is well-suited for current low resource IoTs systems, its long-term cryptographic strength and adaptability may be constrained. LIBC [26] is an LWBC designed to provide efficient cryptographic protection for IoTs devices with limited computational resources. It features a 64-bit block size and 128-bit keys, utilizing a 4-round structure to generate optimized 4-bit S-boxes. The cipher employs an almost MDS (Maximum Distance Separable) matrix for diffusion and optimal involutive nibble-based permutations, as demonstrated by Figure 19, ensuring full diffusion within minimal rounds while maintaining high security against differential and linear cryptanalysis. The cipher is highly efficient in hardware, requiring only 10.05 gate equivalents (GEs) per bit per round, making it one of the most cost-effective options compared to other lightweight block ciphers such as PRESENT, RECTANGLE, and Midori. The decryption process is nearly identical to encryption, simplifying hardware implementation. Security tests show that LIBC provides robust protection against a variety of cryptographic attacks, including related-key and impossible differential cryptanalysis, with low hardware and energy consumption, making it ideal for resource-constrained IoTs environments. Despite its efficiency, LIBC has some limitations. Its reliance on 4-bit S-boxes, while beneficial for low-cost hardware implementation, offers weaker cryptographic properties compared to 8-bit S-boxes, potentially making it more susceptible to future attacks. Additionally, its design is heavily optimized for hardware, which may reduce its performance or adaptability in software-based environments. The algorithm also achieves full diffusion in 4 rounds, but with only 31 rounds in total, the security margin--the difference between the number of secure rounds and total rounds--is relatively tight, leaving less room for future-proofing against more advanced cryptanalysis techniques. These factors could limit LIBC's suitability for applications requiring long-term or high-assurance cryptographic security. LELBC [27] is a low energy LWBC designed for IoTs devices in smart agriculture, ensuring secure data transmission while minimizing energy consumption. It has designed  
Figure 6.2 The encryption process of LIBC. a PSP structure as presented in Figure 20 based on the SPN structure, with highly consistent encryption and decryption, making it ideal for resource-constrained environments. The cipher features a 4-bit involutive S-box generated using a genetic algorithm, reducing energy consumption and latency. Experimental results demonstrate that LELBC reduces energy and area consumption by 24% compared to similar ciphers such as Midori.