# Design Trends and Comparative Analysis of Lightweight Block Ciphers for IoTs
## 4 Related Work
### Hybrid
SIMECK-T [63] is an ultra-LWBC tailored for software-oriented, resource-constrained environments such as IoTs and embedded systems. It is a hybrid design combining the efficient ARX (addition, rotation, XOR) structure of SIMECK and TEA ciphers. The core innovation lies in embedding multiple TEA rounds within SIMECK rounds, effectively enhancing randomness and cryptographic strength while minimizing reliance on substitution boxes. This approach allows SIMECK-T to maintain strong resistance to various cryptanalytic attacks--validated by NIST SP800-22, AIS.31, and the Law of Iterated Logarithm (LIL)--and achieve excellent performance on low-power devices such as the Raspberry Pi Pico. The cipher also supports secure key derivation and initialization vector (IV) generation using PBKDF2, making it suitable for secure file encryption. Its design avoids the use of lookup tables, reducing vulnerability to side-channel attacks and memory overhead, thereby offering a fast, compact, and secure solution for modern lightweight encryption needs. While SIMECK-T is highly optimized for lightweight, software-based encryption, it has several limitations. First, its security relies on a relatively small number of rounds (e.g., 3 SIMECK rounds with 5 embedded TEA rounds), which--though empirically sufficient--may offer a narrower security margin compared to more conservative cipher designs. Second, its performance is optimized specifically for 64-bit microcontrollers, potentially limiting efficiency on systems with different architectures. Third, although it avoids substitution boxes to minimize memory usage, this lack of dynamic substitution layers could reduce confusion compared to S-box-based designs. Lastly, while SIMECK-T passes standard randomness tests, its resilience to future cryptanalysis remains to be thoroughly validated over time, especially as it depends on ARX structures that could be vulnerable to emerging attack methods. NDN [64] is a hybrid LWBC combining SPN and Feistel structures, supporting 64-bit blocks with 80- and 128-bit keys over 12 and 18 rounds, respectively. It incorporates dynamic round functions, a high diffusion key schedule, and 4 \(\times\) 4 S-boxes, ensuring strong non-linearity and resistance to known cryptanalytic attacks. Its innovative DEFFT-based permutations and primitive polynomial transformations enhance key diffusion and scalability. Hardware implementation on 0.13 \(\upmu\)m ASIC shows GE of \(\sim\)1068 (NDN-80) and \(\sim\)1272 (NDN-128), while software benchmarks on AVR microcontrollers confirm low memory footprint and fast execution, making NDN well-suited for both low-power and high security IoTs environments. While NDN offers strong theoretical security and efficient performance, its evaluation primarily relies on internal analysis without independent third-party cryptanalysis. The absence of public cryptanalytic challenges or open-source implementations limits reproducibility and peer validation. Moreover, although the cipher demonstrates competitive GE and speed, there is limited discussion on side-channel resistance or fault attack tolerance. The dual-round structure, while efficient, may also introduce complexity in hardware pipeline optimization. Future studies are needed to assess its resilience in real-world adversarial settings and broader platform adaptability. RAZOR [65] is a newly proposed LWBC designed specifically for software-oriented IoTs applications. It combines the Feistel structure and substitution-permutation network (SPN) in a hybrid round function and utilizes 4-bit S-boxes along with a diffusion function based on rotation and XOR operations. This design achieves strong resistance to differential, linear, impossible differential, and related-key attacks. RAZOR demonstrates improved security compared to other lightweight ciphers such as PRESENT, Rectangle, LBlock, GIFT, and SCENERY, notably achieving the highest number of active S-boxes in a 5-round differential analysis (21 S-boxes).