# Design Trends and Comparative Analysis of Lightweight Block Ciphers for IoTs
## 4 Related Work  
### Substitution-Permutation Network (SPN)  
The NVLC [11] (New Variant Lightweight Cryptographic Algorithm) is a symmetric block cipher specifically designed for IoTs applications with constrained resources. It operates on 64-bit blocks with key sizes of either 80 bits or 128 bits. The cipher utilizes six rounds for basic security, although twenty rounds are implemented to increase encryption complexity. The NVLC algorithm incorporates a substitution-permutation network (SPN) structure, combining mathematical and logical operations for efficiency in IoTs environments. The algorithm's key features include whitening, a mix-column layer, a 4-bit S-box, a key schedule derived from the PRESENT algorithm, and a shifting layer for diffusion as presented in Figure 4. Security evaluations show that NVLC is resilient against various \begin{table}
\begin{tabular}{c c} \hline
**Structure Type** & **Algorithms** \\ \hline
**Substitution-Permutation** & NVLC, RARE, SAT\_Jo, CRAFT, DoT, HERMES, Loong, ACT, ILEA, PriPresent, \\
**(SPN)** & 3D\_RECTANGLE, ULM, LAO-3D, FUTURE, IVLBC, LIBC, INLEC, LELBC, LTLBC \\ \hline
**Feistel Network (FN)** & GRANULE, improved\_DLBCA, MANTRA, TED, improved\_RoadRunner, LCA, \\  & M-XXTEA, improved\_SIMECK, improved\_SM4, SCENERY, FlexAEAD, RBFK, \\  & LiteEncrypt, ANU-II \\ \hline
**Generalized Feistel (GFN)** & NUX, NUCLEAR, Shadow, HDLBC, GFLE, \(\mu\)2, IIoTBC, LPHD, T-TWINE \\ \hline
**ARX** & BRIGHT, LiARX, DABC, SAECHAM, SAND-2 \\ \hline
**Hybrid** & Hybrid PRESENT and Salsa20, GFRX, FeW, ECLBC, SIMECK-T, SFN, NDN, \\  & RAZOR, SPISE, GFSPX, QLW \\ \hline \end{tabular}
\end{table} 
Table 3: Lightweight block ciphers in this study. cryptographic attacks, including differential and linear attacks, while achieving a low hardware cost of just 1134 gate equivalents (GEs). This makes it a highly efficient choice for low-cost IoTs applications such as RFID tags and sensors, ensuring both security and performance with minimal resource consumption. The NVLC algorithm, while optimized for IoTs security, has several limitations. Its increased computational complexity due to 20 encryption rounds may impact processing efficiency in low-power devices. Although designed as a lightweight cipher, its power consumption of 1.83 \(\upmu\)W at 100 kHz could still be significant for ultra-low-power IoTs applications. The use of S-Boxes and key expansion may pose memory constraints for devices with limited storage. Additionally, while it claims resistance to attacks, further cryptanalysis is needed to validate its security against advanced cryptographic threats. Lastly, its implementation complexity may make integration into existing IoTs systems more challenging. RARE [12] (Robust Algorithm for Rapid Encryption) is a lightweight cryptosystem designed specifically for image encryption in IoTs applications. The cipher targets the challenges presented by image data, such as high redundancy, correlation, and voluminosity, which can undermine the security of traditional lightweight ciphers. RARE uses an innovative \"Outer-Inner\" structure, as shown in Figure 5, where the outer phase applies horizontal and vertical diffusion to reduce image redundancy and correlation, while the inner phase divides the image into sub-blocks and processes them using a block cipher in ECB mode. The system ensures high diffusion and confusion properties, essential for secure image encryption. RARE outperforms existing lightweight ciphers in terms of correlation reduction, entropy increase, and enhanced diffusion. It also shows improved resistance to differential and linear cryptanalysis, demonstrating a lower number of rounds required for secure encryption. Furthermore, RARE is faster and more efficient compared to other ciphers such as PRESENT and RoadRunner, making it highly suitable for IoTs applications that require both security and real-time processing. Figure 4: NVLC encryption algorithm.