# Design Trends and Comparative Analysis of Lightweight Block Ciphers for IoTs
## 3 Classification of Lightweight Block Ciphers  
To illustrate the classification hierarchy of cryptographic algorithms, highlighting where lightweight block ciphers (LWBCs) fit within the broader context of symmetric cryptography. The LWBCs are further divided into five main structural types: SPN, FN, GFN, ARX, and Hybrid, as shown in Figure 3. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Criteria** & **Inclusion** & **Exclusion** \\ \hline
**Year of Publication** & Between 2018 and 2025 & Before 2018 or after 2025 \\ \hline
**Type of Article** & Journal and conference proceedings & Other sources (e.g., theses, patents, slides) \\ \hline
**Language** & English & Non-English (e.g., Chinese, Russian, Japanese) \\ \hline
**Domain** & Cryptographic algorithm & Other than cryptographic algorithms (e.g., attack-only, implementation-only) \\ \hline
**Cryptographic Primitive** & Block cipher & Other primitives (e.g., stream cipher, hash, asymmetric, RNG) \\ \hline
**Block Size** & 64-bit and higher & Less than 64-bit \\ \hline
**Key Size** & 80-bit and higher & Less than 80-bit \\ \hline
**Encryption Type** & Text encryption & Other than text encryption (e.g., image, audio, video) \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Article inclusion and exclusion criteria. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Criteria** & **NESSIE (2000)** & **NIST (2015)** & **NCS (Saudi Arabia, 2020)** \\ \hline
**Key size** & At least 128-bit & At least 112-bit & At least 80-bit \\ \hline
**Block size** & 64-bit & 64-bit & At least 64-bit \\ \hline
**Analysis** & Generic attacks & Fault attacks & Differential and linear cryptanalysis \\  & Side-channel attacks & Side-channel attacks & NIST statistical tests \\  & & Resistance to side-channel attacks \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Security evaluation criteria for lightweight block cipher. Lightweight block ciphers are fundamentally shaped by their internal structural design, which directly influences their cryptographic strength, efficiency, and hardware feasibility. Among the most prominent structures, the **substitution-permutation network (SPN)** architecture achieves strong diffusion and confusion through the layered use of S-boxes and permutations. Ciphers such as NVLC, DoT, and HERMES illustrate the effectiveness of this structure in resisting cryptanalysis. However, SPNs often require more complex key schedules and consume more gate equivalents (GEs), which may lead to higher power consumption and increased silicon area--important considerations for constrained environments [6]. In contrast, **Feistel network (FN)** ciphers, such as GRANULE, TED, and FlexAEAD, split the data into two halves and iteratively process them through round functions. Their simplicity facilitates low-cost hardware implementations and makes encryption and decryption symmetric. Yet, due to the limited diffusion per round, Feistel ciphers typically require more rounds to achieve comparable security to SPN designs, which can impact processing time and energy use [7]. The **generalized Feistel network (GFN)** extends the classical Feistel architecture by allowing more than two branches--typically 4, 8, or more. Each round operates by applying round functions to selected branches and then permuting or rotating the positions of the branches. GFN designs enhance diffusion by enabling more flexible and parallel data mixing while maintaining invertibility. Variants such as Type-1 (alternating rounds), Type-2 (F-functions applied in parallel), and Type-3 (round-dependent permutations) are used to improve performance and security. GFNs are well-suited to lightweight environments because they allow for modular, scalable implementations and can leverage smaller, simpler round functions while still achieving high security margins across fewer rounds [8]. The **addition-rotation-XOR (ARX)** structure, seen in ciphers such as BRIGHT and SAND-2, ARX-based ciphers rely solely on three basic operations: modular addition, bitwise rotation, and XOR.