# SC-SA: Byte-Oriented Lightweight Stream Ciphers Based on S-Box Substitution
### The Proposal of SC-SA  
Linear Feedback Shift Register and Nonlinear Feedback Shift Register are commonly employed structures in stream ciphers. LFSR, rooted in feedback from linear shift registers, updates register bit values through XOR operations between the current state and the linear feedback function output, generating a pseudo-random sequence. Its hardware implementation requires minimal gate circuits, minimizing resource utilization and facilitating high-speed pseudo-random sequence generation. However, LFSR-generated pseudo-random sequences exhibit periodicity, with the period length determined by the register's size. Adjusting the register length caters to cipher design requirements. Despite its efficiency, LFSR's predictability arises from its linear structure, allowing attackers to deduce the initial state and linear feedback function by analyzing the output sequence. Consequently, attackers can reproduce the same pseudo-random sequence. On the contrary, NFSR introduces nonlinearity into the feedback function, enhancing resistance against attacks compared to LFSR. Complex operations like substitution boxes or Boolean functions contribute to the cryptographic security of the stream cipher. NFSR's nonlinearity introduces a higher level of confusion and diffusion, complicating sequence analysis and prediction for potential attackers. The Nonlinear Feedback Shift Register (NFSR) is a cryptographic structure commonly employed in stream cipher designs to augment the complexity and security of cryptographic algorithms. NFSR updates register bit values by executing operations between the current state and a nonlinear function. In contrast to LFSR-based cipher structures, NFSR employs nonlinear feedback functions, rendering the output sequence more challenging to predict. The incorporation of nonlinearity elevates the cipher algorithm's complexity, rendering linear analysis attacks that exploit linear relationships more difficult to execute. NFSR's nonlinear characteristics provide robust resistance against linear analysis, thereby enhancing its ability to withstand various attacks. In stream ciphers, NFSR is frequently utilized to bolster the security of cryptographic algorithms. By introducing nonlinear functions and cascading multiple registers, NFSR enhances cryptographic strength and fortifies the algorithm's resilience against attacks. Cipher algorithms adopting the LFSR and NFSR structures, such as RC4 [22] and Grain, find widespread use in practical applications. These algorithms carefully select register lengths and nonlinear functions to meet diverse security requirements. The Feedback with Carry Shift Register (FCSR) shares similarities with NFSR, utilizing a nonlinear function as the feedback function to map the current register state to the subsequent moment's state. In FCSR, each bit corresponds to a feedback function output, typically a nonlinear Boolean function with arbitrary complexity. The output of each bit undergoes XOR with the corresponding feedback function output, accumulating with a carry bit, serving as the input for the corresponding bit position in the next moment. In practical applications, combining LFSR and NFSR enhances the security and efficiency of stream ciphers. The incorporation of both linear and nonlinear components in the resulting cipher achieves a balance between computational complexity, security, and resource utilization. This balanced approach provides robust encryption capabilities suitable for a variety of applications. Lightweight sequence ciphers based on shift registers are better suited for hardware implementation. In software implementation, using 1-bit shifts in shift registers can significantly reduce the cipher's efficiency. Nonlinear feedback functions in sequence ciphers based on registers also face performance degradation issues in software implementation. Directly porting shift-register-based cipher algorithms to IoT devices is not straightforward due to the underlying heterogeneity in computing architecture and word size.