# Modified Generalized Feistel Network Block Cipher for the Internet of Things
###### Abstract
Confusion uses a substitution function to obscure the relationship between plaintext and ciphertext, whereas diffusion uses a permutation function to distribute the plaintext statistics throughout the ciphertext. To achieve these objectives, there are two main fundamental structures of lightweight block ciphers: the Feistel network and the substitution-permutation network (SPN). While different techniques can be used to develop algorithms, they are based on either one of these foundational structures. The encryption and decryption operations in the SPN lightweight block cipher are non-identical, requiring additional cycles and codes for execution. In contrast, the encryption and decryption processes in the Feistel lightweight block cipher are similar [10]. Feistel ciphers do not require additional cycles and codes for execution, resulting in a low memory usage and the ability to be implemented in hardware with a low average power consumption. Feistel networks are a popular choice for implementing lightweight block ciphers, with classical Feistel networks and generalized Feistel networks (GFN) being the two main types. While classical Feistel networks divide messages into two sub-blocks, the GFN divides them into k sub-blocks, where k is greater than 2. The GFN of Type-II, in particular, is a popular form where the Feistel transformation is applied to two consecutive sub-blocks, followed by a cyclic shift of sub-blocks. This type of Feistel structure is easy to implement and has been used in ciphers such as RC6 [11], HIGHT [12], and CLEFIA [13]. However, it is important to note that the Type-II GFS has low diffusion properties and requires a larger number of rounds to achieve full diffusion [14; 15], meaning all output sub-blocks are affected by all input sub-blocks. In this research paper, we propose a secure modified GFN lightweight block cipher that provides optimal security for IoT devices while utilizing a minimal number of rounds. To the author's knowledge, there have been no previous studies on the proposal of GFN lightweight block ciphers with fewer than 25 rounds and sufficient security levels. Existing Feistel-based lightweight block cipher algorithms, such as ANU [16], FeW [17], Granule [18],LiCi [19], MANTRA [20], NUCLEAR [21], NUX [22], Piccolo [23], TEA [24], TED [25], T-TWINE [26], and VAYU [27], all use more than 25 rounds. Aligned with Malaysia's National Cryptography Policy (NCP) [28] and National Cyber Security Policy (NCSP) [29], we proposed a modified GFN algorithm. This algorithm will be rigorously tested through various statistical and cryptographic analyses to assess its ability to meet fundamental security requirements, such as confusion, diffusion, and independence, while maintaining a high performance level within the resource constraints of IoT devices. Specifically, we will analyze the algorithm's avalanche effect on the ciphertext and its random properties. The results of these analyses will demonstrate the algorithm's effectiveness as a lightweight block cipher for IoT devices. The paper is structured as follows: The existing Feistel-based lightweight block cipher, its design structure, and its weaknesses are discussed in Section 2. Section 3 describes the proposed modified generalized Feistel network lightweight block cipher's basic structure design. Section 4 then discusses the statistical and cryptographic analyses conducted on the proposed cipher. Section 5 presents the results and analyses' discussions. Finally, Section 6 provides a summary of our research findings.