# Design Trends and Comparative Analysis of Lightweight Block Ciphers for IoTs
## 4 Related Work
### Substitution-Permutation Network (SPN)
The cipher operates using a substitution-permutation (SP) network and employs a simple but effective design with a single S-box, bit permutations, block shuffling, and circular shifting to enhance security and performance, as demonstrated in Figure 8. DoT requires only 993 gate equivalents (GEs) for hardware implementation and consumes only 2464 bytes of flash memory, making it highly compact. Its throughput of 53.77 Kbps outperforms many existing ciphers, and it offers strong resistance to linear, differential, and biclique attacks. The key scheduling mechanism is inspired by the PRESENT cipher, ensuring both efficiency and security. Overall, DoT's design makes it an ideal candidate for low-power and memory-constrained applications, offering high encryption performance with minimal hardware and memory requirements. The DoT algorithm has limitations, including the need for further validation against advanced cryptanalysis, a high number of rounds (31) that may impact execution speed, and potential inefficiency in ultra-low-power IoTs devices despite its low hardware footprint (993 GEs). Its complex key scheduling and reliance on bit permutation could make integration into existing frameworks challenging. Additionally, while it claims resistance to biclique attacks, its real-world security effectiveness remains uncertain. Figure 7: Structure of CRAFT. HERMES [16] is an LWBC designed to secure resource-constrained devices such as IoTs devices, microcontrollers, and RFID chips. It operates with 64-bit plaintext blocks and 128-bit keys, making it suitable for environments where memory, processing power, and energy are limited. Hermes features a simple yet effective structure using an \(8\times 8\) matrix of transformations, including bitwise operations and an S-box for non-linearity as depicted in Figure 9. With 30 rounds of encryption, it strikes a balance between strong security and minimal resource requirements. The cipher is optimized for low-power consumption, small code size, and efficient memory use. Security analyses show that Hermes is resistant to common cryptographic attacks, including differential, linear, and other advanced attacks, while also providing high diffusion and confusion properties. Hermes' efficiency in both hardware and software implementations makes it an ideal candidate for deployment in IoTs and similar applications where traditional cryptographic methods may be too resource-intensive. Figure 8: Design of DoT cipher. Figure 9: The initialization stage of HERMES. The HERMES algorithm has limitations, including high computational complexity due to its 30-round structure, which may impact efficiency in low-power devices. Its memory and power consumption could be a concern for ultra-constrained environments, and further cryptanalysis is needed to confirm its resistance to advanced attacks. The key scheduling and substitution mechanisms add implementation complexity, making integration more challenging. Additionally, its real-world performance in terms of speed and execution efficiency remains unverified through practical hardware testing. The Loong [17] cipher is an involutional LWBC based on a novel SPN (substitution-permutation network) structure, designed for efficient use in IoTs devices with resource constraints. Unlike traditional SPN ciphers, Loong ensures that both the encryption and decryption processes are identical, allowing for the same hardware and software implementation for both operations, thus reducing the required resources. It uses a 64-bit block with key sizes of 64, 80, and 128 bits, supporting 16, 20, and 32 rounds, respectively. The round function in Loong, according to Figure 10, includes AddRoundKey, SubCells (involving a 4-bit S-box), MixRows, and MixColumns, with involutional components that improve security and efficiency. Loong demonstrates strong resistance to differential and linear attacks, with high security derived from its unique round function and the use of a small, low latency S-box.